COPPER – Cu – 63·54

CHAPTER

8

COPPER - Cu - 63-54 is frequently determined in natural copper, copper metal or its alloys (brass, bronze), as well as the ores of its cuprite, sulphide (chalcopyrite) and carbonate (malachite, azurite). Copper samples containing arsenic, antimony and phosphorus, or salts containing its silicate, phosphate and sulphate, are frequently analysed. The determination of copper is often required in slags, industrial wastes and metallurgical substances. Often the determination of small amounts of copper in foods, antibiotics, culturemedia or other natural organic substances is undertaken. Dissolution of the sample. Metallic copper, or copper alloys, can only be dissolved in hydrochloric and sulphuric acid in the presence of oxidizing agents (Br2, NaC103, H 2 0 2 , HN0 3 ). Nitric acid dissolves most copper alloys and copper compounds. To dissolve sulphides a 1:1 mixture of saturated bromine water and concentrated hydrochloric acid can be used. To dissolve slags containing sulphide, oxide or silicate, and for metallurgical products (matte smelting), it is advisable to use a 1 : 1:1 mixture of concentrated hydrochloric, nitric and sulphuric acids, from which silica precipitates in the dehydrated form after evaporation when sulphuric acid fumes appear. For sulphidic ores the Freiberg-fusion (S + Na 2 C0 3 , see Chapter 2.5.7.) can also be used, but when the smelt is leached out a considerable amount of copper(II) sulphide goes into solution with the sulpho-acids. The dissolving power of sodium and ammonium polysulphides on the copper sulphide is not negligible, and is increased in the presence of thioarsenates. Samples, from which copper is deposited electrolytically, are often dissolved directly in concentrated sulphuric acid, without previous conversion to chloride or nitrate. Potassium pyrosulphate can be used for the fusion of ignited copper oxides. The forms of determination and interfering ions. The most frequent methods used for the gravimetric determination of copper are shown in Table 8.1. Of these the most accurate and reliable is the electroanalytical determination of copper, which at the same time effects a separation from many other metal ions. The determinations in the form of copper (I) iodide and copper(I) thiocyanate are also very rapid and fairly selective, but precipitation and washing must be carried out cautiously because of the slight solubility of these precipitates. For both methods filter crucibles or filter funnels must be used. The determination in the form of copper(I) sulphide is not as selective as the former methods, but filter crucibles are COPPER

74

75

DETERMINATION BY ELECTROLYSIS

not required. If the copper(II) sulphide precipitate is ignited to copper(II) oxide at 840-940 °C satisfactory results can be obtained, provided that the temperature is accurately controlled. The precipitation and weighing in the form of copper (II) oxide is not selective, the precipitate is difficult to filter and it is always contaminated with alkali metal ions. T A B L E 8.1. F o r m s of d e t e r m i n a t i o n of c o p p e r (for R e f e r e n c e s see p . 114) Ref. number

F o r m of precipitation

Precipitant

1.

Cu

electrolysis

2.

CuSCN

KSCN+Na^Og or F e S 0 4

acidic reduction

3.

CuS

H 2 S or Na 2 S 2 0 3

acidic (HN0 3 -free)

KOH K I + NajSOg

4. 5.

Cu(OH) 2 ->CuO Cul

Reaction of t h e medium H^SO^HNOa, N H 3 , KCN

Weigh- ForHeat ing mula treatment form weight °c Cu

63-54

CuSCN 121-63

in cold

<260

Cu 2 S

159-15

400-600

CuO

79-54

700-900

basic

CuO

79-54

700-900

acidic

Cul

190-45

<260

Seldom used forms of determination: 6. Copper (II) iodate [Cu(I0 3 ) 2 ], 7.copper (II) sulphate [CuSOJ, 8. copper (II)-sulphate m o n o h y d r a t e [CuS0 4 · Η „ 0 ] , 9. copper (II) hydrazinesulphate [CuS0 4 «(N 2 H 4 ) 2 S0 4 ], 10. copper(II) anthranilate [Cu(C 7 H 6 0 2 N) 2 ], 11. copper(II)-cupferronate [Cu(C 6 H 5 0 2 N 2 ) 2 ], 12. copper(II) neocupferronate [Cu(C 1 0 H 7 O 2 N 2 ) 2 ], 13. copper(II) quinaldinate [Cu(C 1 0 H 6 O 2 N) 2 · Η 2 0 ] , 14. copper (II)-5-6-benzoquinaldinate [Cu(C 14 Hg0 2 N) 2 ], 15. copper(II)salicylaldoxime [Cu(C 7 H 6 0 2 N) 2 ] 16. copper(II) benzoinoxime [ C u ( C 1 4 H n 0 2 N ) ] , 17.copper (II) oxinate tCu(C 9 H 6 ON) 2 ], 18. copper(II) thionalide [Cu(C 1 2 H 1 0 ONS) 2 .H 2 O], 19. c o p p e r ( Π ) mercaptobenzthiazole [Cu(C 7 H 4 NS 2 ) 2 ], 2 0 . copper(II) pyridine dichromate {[Cu(C 5 H 5 N) 4 ]Cr 2 0 7 }, 2 1 . copper (II) pyridine thiocyanate {[Cu(C 5 H 5 N) 2 ](CNS) 2 }, a n d a n u m b e r of other forms. OF

8.1. D E T E R M I N A T I O N I N T H E F O R M METALLIC COPPER BY ELECTROLYSIS

Copper can be deposited from many simple and complex salts by electrolysis, although electrolysis is usually carried out in a nitric acid or sulphuric acid medium. The free acid content of the solution should not be too high because the deposition of copper is not complete in a strongly acidic solution, and the simultaneous liberation of hydrogen makes the coating spongy. Two reactions may occur at the cathode: Cu2++2e-=:Cu 2H++2e-=H 2

E0=+0MY E0=±0-0 V

pH = 0

76

COPPER

On the surface of copper, hydrogen shows a considerable overvoltage. Thus, the evolution of hydrogen can be considerable if the concentration of copper(II) ions in the solution is low, or if the hydrogen ion concentration is high. In the presence of nitric acid the latter process is repressed and a well-adhering, metallic coating of copper can be obtained. Nitric acid acts as a depolarizer: NO3- + 10 H+ + 8 e - = NH4+ + 3 H 2 0

E0 = + 0 8 7 V

The standard redox potential of nitrate ions is more positive than that of the hydrogen ions, and therefore the latter cannot be deposited in the gaseous form. This process causes the hydrogen ion concentration of the solution to decrease, and if the electrolysis is continued for too long other metals (Ni) may also be deposited on the cathode. The nitric acid used must be free from nitrous acid, because the latter catalyses the dissolution of copper in nitric acid. In the presence of nitrous acid, therefore, copper may not be deposited at all, or the coating may dissolve during the electrolysis. The error can be even higher in the presence of large amounts of iron. Iron can be easily reduced on the cathode, and in turn reduces nitric acid to nitrous acid. Thus the presence of nitric acid is advantageous, but the presence of nitrous acid must be prevented. This can be done by the following methods: (a) A part of the nitric acid can be replaced by ammonium nitrate, and the solution must previously have been boiled out. A similar effect can be obtained by the use of urea or amidosulphonic acid. These compounds decompose nitrites in the same way as ammonium salts, with the formation of N2 according to the following equations: 2 HN0 2 + (NH2)2CO = 3 H 2 0 + 2 N 2 | + C 0 2 | HN0 2 + (NH 2 )HS0 3 = H 2 S0 4 + N 2 | + H 2 0 It is essential that the solution should be heated to boiling in the presence of these substances, otherwise the removal of nitrites is not complete, (b) Traces of chloride, bromide or chlorate ions can also effect the d ecomposition of nitrites. Large amounts of these substances, however, may corrode the platinum anode, and then the dissolved platinum may also be deposited on the cathode. Nitrite can also be formed when the electrolysis is interrupted, the deposited copper begins to dissolve and nitrous acid is formed during this process. The same error can occur if the copper coating is not fully immersed in the electrolyte. Owing to nitric acid fumes, nitrite-formation occurs, especially if the electrolysis is carried out in hot solution with vigorous stirring. The washing of the cathode must therefore be carried out while the voltage is applied, taking care that the coating is not in contact with air and nitric acid at the same time. In the presence of sulphuric acid these errors do not occur, but the depolarization effect of nitric acid is also absent and therefore copper coatings obtained under these conditions are never faultless. It is advisable, however, to carry out the electrolyses from electrolytes containing sulphuric acid if accurate results are required.

DETERMINATION BY ELECTROLYSIS

77

At the anode, if nitric acid or sulphuric acid is present, oxygen is evolved, together with small amounts of ozone. 2 H 2 0 = O a t + 4H+ + 4 e + overvoltage

E0= +123 V r?0 = about +0-45 V + 1-68 V At this electrode, on the other hand, those anions which have a more negative deposition potential than oxygen, [Cl~, Br~, etc. as well as large amounts of Fe(III) ions] may have a depolarization effect. This results in the dissolution of platinum, and therefore the presence of large amounts of these ions must be avoided. Chlorides can be removed by repeated evaporation with sulphuric acid, and iron can be removed by previous separation, or by the addition of fluoride, so that the [FeF 6 ] 3 ~ complex is formed. Traces of chloride1 or small amounts of iron, however, do not interfere. In an ammoniacal solution chloride ions do not interfere. If an attempt to increase the current-density by too large an increase of voltage is made, and not by heating or stirring, a spongy or dark coating can easily be obtained. A dark coating indicates that it is contaminated by other metals which have a more negative deposition potential. Copper can be separated easily from Ni, Co, Pb, Cd and Zn by electrolysis, as well as from those metals whose deposition potential (see Table 3.27.) is more negative than that of copper. Metals whose deposition potential is close to, or more positive than copper (Au, Pt, Ag, Hg, Bi, Sb, As, Sn), are deposited partly or completely together with copper. These must therefore be removed from the copper before electrolysis, or copper must be precipitated and separated in the form of copper (I) thiocyanate. The washed copper (I) thiocyanate precipitate can be dissolved on boiling in a mixture of concentrated nitric and sulphuric acid, the solution evaporated until sulphuric acid fumes appear, and, after dilution, copper can be deposited by electrolysis. In the analysis of bronze or brass, tin can be removed in the form of metastannic acid, while lead can be removed in the form of lead sulphate, or by anodic deposition (in the form ofPb0 2 ). The fine crystalline copper coating which is deposited on the electrode is very sensitive to the oxygen of the air because of its large surface, especially if the deposition is carried out in an ammoniacal solution. The colour of the coating, when heated to 60-70 °C, becomes darker owing to oxidation and its weight increases. It is advisable to dry the coating in an air-stream after washing with water and alcohol or acetone (using a fan or hair-drier). 8.1.1. The determination of copper by electrolysis from a sulphuric acid medium (W. Gibbs, F. Förster) Add to the solution, which is nearly neutral, free from interfering ions and contains 0-20-0-50 g of copper, 2-3 ml of concentrated sulphuric acid, and 1 According t o J . A. Scherrer, R . K. Bell a n d W. D . Morgan [J.Res. Nat. Bur. Standards 22, 697 (1939)] traces of chloride help t h e formation of a well-adhering copper coating, a n d therefore it is advisable t o a d d 1 drop of 0-1 N hydrochloric acid to chloride-free electrolytes.

78

COPPER

dilute to 100 ml in a 200-ml beaker. Dip the platinum (or copper) net electrode, which has been washed with alcohol or acetone and dried and weighed, into the solution after fastening it into an insulated holder, and connect it to the negative pole of an accumulator. If slow electrolysis is carried out, a thick platinum spiral wire electrode may be used as an anode, and for rapid electrolyses a Fischer-type net anode can be used. The cathode net should project several millimetres above the electrolyte level. (a) Slow electrolysis. The electrolysis should be carried out overnight at 2 v with one lead accumulator by the short circuit method, using a current of about 0-5 A in a cold solution. A watch glass should be cut into two pieces and placed over the mouth of the beaker. If the electrolysis is carried out in a solution heated to 60-80°C, at 1-5 A current (2-3 accumulators connected in series, and connected to the cell through a slide resistance), the deposition is finished within 3-4 hr. (b) Bapid electrolysis. If the deposition is carried out in a solution heated to 60-80 °C, with vigorous stirring (magnetic stirrer), and by connecting suitable cell units in series so that the current in the electrolysis cell is 3-4 A regulated by a slide resistance, the deposition of copper is complete within 20-40 min. As the solution becomes colourless, owing to the deposition of copper(II) ions, the bottom of the watch glass sections and the wall of the beaker are rinsed and the solution is stirred cautiously. The electrolysis is complete if after 30 min no copper deposition is noted on the freshly immersed surface of

TABLE 8.2. Copper determination with electrolysis in sulphuric acid solution

The way of electrolysis

Volt

Number of measurements

Mean of weight of Cudeposit mg

S t a n d a r d deviation mg

%

Slow electrolysis a t 20 °C; 11·5 hours

2-0

6

102-1

±0-1

±0-10

Rapid electrolysis a t 60-70°C; about 1 hour

2-0

6

100-2

±0-1

±0-10

Slow electrolysis a t 60-70°C; about 3 hours

2-0

6

521-9

±0-8

±0-17

the cathode. The absence of copper in the electrolyte can be confirmed by testing one drop of the solution with a suitable reagent [H 2 S, K 4 Fe(CN) G ]. The net must be removed cautiously from the electrolyte, either by lowering the beaker or removing the electrolyte from the beaker by suction, and washing the electrode with water at the same time using a fine jet from a wash-bottle.

DETERMINATION BY ELECTROLYSIS

79

Another m e t h o d is t o change t h e electrolyte quickly, using water placed in another beaker, and t o continue t h e electrolysis for several minutes. A wellproven m e t h o d is t o pour carefully a layer of water several c m thick on t o the electrolyte which is of higher density, t h e n t o slowly remove t h e electrodes through this layer, and afterwards rinse t h e electrodes. After rinsing w i t h water, t h e electrodes should be rinsed thoroughly w i t h alcohol or acetone from a wash-bottle. The cathode m u s t t h e n be dried in a desiccator or in a n airstream, and weighed. The copper deposited is light red, has a microcrystalline structure and therefore yields a less glossy, velvet-like coating. Copper can be removed from t h e platinum electrode b y placing it into a beaker containing a small volume of boiling concentrated nitric acid. The hot acid, which condenses on the electrode and refluxes, dissolves all the copper from the electrode. Note. I n a solution which is heated above 80°C, the deposition is incomplete, because copper(II) ions dissolve the coating with the formation of copper(I) ions. Copper (I) ions are later oxidized back to copper(II) ions on the anode (similar errors also occur if iron is present). A fairly good copper coating can also be obtained if 1 ml of boiled-out 2 N nitric acid is added to the solution. The electrolysis time can be calculated, for the short-circuited method, from the following data: Electrolyte :CuS0 4 + 2 ml of cone. Ή.βΟΑ in about 100 ml. 100 mg copper is deposited: at 20 °C at 2 V overnight at 70°C at 2 V in 1-3 hr at 20 °C at 2 V with stirring, in 20 min Fig. 8.1. Vessel for at 70°C at 2 V with stirring, in 5-10 min. the dissolution of The standard deviation of the method can be judged from the data of Table 8.2., obtained by I. Buzâs and S. electrolyte copper Gâl. The determinations were carried out using different according t o standard solutions. The results are so accurate that this Hertelendi method could be used for the determination of the copper content of the standard solutions. I t is characteristic o f t h e a c c u r a c y o f t h e m e t h o d t h a t e v e n t h e a n a l y s i s o f * e l e c t r o l y t i c c o p p e r " c o n t a i n i n g 9 9 * 9 5 - 9 9 - 9 9 % o f copper c a n b e carried o u t b y t h i s m e t h o d w i t h a n error o f 0-01 % .l I n t h e s e d e t e r m i n a t i o n s i t is a d v i s a b l e t o w e i g h m o r e t h a n 10 g o f copper, a n d t o a v o i d t h e calibration error o f t h e w e i g h t s a n d o f t h e b a l a n c e b y t a r i n g t h e s a m p l e w i t h t h e s a m p l i n g v e s s e l a n d t h e n e t - e l e c t r o d e . W h e n t h e electrode is re w e i g h e d w i t h t h e copper coating, o n l y t h e rider o f t h e b a l a n c e n e e d b e m o v e d t o restore t h e equilibrium. T h e copper w e i g h e d o u t m u s t b e d i s s o l v e d i n 5 0 m l o f nitric a c i d (Sp. gr. = 1-20) a n d 8 m l o f c o n c e n t r a t e d sulphuric a c i d i n a tall beaker, w h i c h c a n b e c l o s e d b y a s t o p p e r w h i c h incorporates a s m a l l t r a p (see F i g . 8 . I . ) . T h e t r a p r e t a i n s o n l y drops o f s o l u t i o n carried o v e r w i t h t h e gases, a n d t h e s e c a n b e w a s h e d b a c k i n t o t h e s o l u t i o n after 1

L. HERTELENDI, Magyar Kém. Folyôirat.,

45, 156 (1939).

80

COPPER

complete dissolution. Copper can be deposited b y slow electrolysis over about 24 hr (2-3-2-6 V, 0-8 A) in t h e same vessel from 200 ml of solution using a Winkler-type cathode. Washing can be done with water acidified slightly with sulphuric acid, with continued electrolysis as previously described, and finally rinsing with alcohol. The copper content of t h e sample can be determined within ± 0-5 mg (0*005%). Among t h e possible contaminations of electrolytic copper (Pb, Fe, Ni, Co, Cd, Zn, Mn, As, Sb, Sn, Si0 2 , Bi, Se, Te, P , Au, Ag) only Au and Ag are deposited with t h e copper. 8.1.2. Determination

of copper by electrolysis in a nitric acid

medium

Acidify the nearly neutral solution, which should be free of chlorides and other interfering substances and contains 0-1-0-6 g of copper, with 2-5 ml of concentrated nitric acid, add not more than 2 g of urea (or 0-15 g of potassium chlorate) to avoid the interference of nitrite, and boil for several minutes. The electrolysis should be carried out in the cold or at 60-80°C at 2-32-6 V. Stirring greatly accelerates the deposition. The procedure is then similar to t h a t used in a sulphuric acid medium, but special care must be taken when the electrodes are removed from the electrolyte. The time of the electrolysis can be calculated from the following data: 400 mg of copper is deposited: at at at at

20°C using 2-4-2-5 V and 0-2-0-8 A 70°C „ ,, „ ,, „ „ 20°C » » »» » 0-1-0-3 » 20°C » 2-0-2-5 » » 0-6-0-7 »

in 4-5 hr „ 2-3 hr » 1 night with stirring in about 15 min.

TABLE 8.3. Copper determination with electrolysis in nitric acid solution

The t y p e of electrolysis

Volt

Number of measurements

Mean of weight of Cudeposit mg

Standard deviation mg

%

low electrolysis a t 20°C; 11 hours

2-4

6

1021

±0-03

±0-03

R a p i d electrolysis a t 65-70°C; about 1 hour

2-0

6

100-2

±0-20

±0-20

R a p i d electrolysis a t 65-70°C; about 3 hours

2-0

6

521-9

±0-41

±0-08

Notes. (1) The excess urea may cause interference on further treatment of the electrolyte. (2) The determination is somewhat less accurate than the electrolysis in the presence of sulphuric acid. (See results of I. Buzâs and S. Gal in Table 8.3.)

DETERMINATION IN THE FORM OF COPPER(I) THIOCYANATE

81

8.2. D E T E R M I N A T I O N IN T H E F O R M OF C O P P E R ( I ) T H I O C Y A N A T E , CuSCN (Rivot, 1854) Copper(II) ions, when treated with potassium thiocyanate in a sulphurous acid (reducing) solution, give a precipitate of copper (I) thiocyanate which is fairly insoluble (2>2oo — 10 ~ 13 ' 5 ), and which when dry can be weighed in the form of CuSCN. Alternatively, after ignition at a sufficiently high temperature, it may be weighed in the form of CuO. 2 CuS04 + 2 KSCN + H 2 S0 3 + H 2 0 = 2 CuSCN J + 2 KHS0 4 + H 2 S0 4 Although excess precipitant (KSCN) must be used in order to decrease the solubility of the precipitate (common-ion effect), too great an excess of it must be avoided, because the precipitate may be partly dissolved owing to the formation of the K3[Cu(SCN)4] complex. Therefore the thiocyanate concentration of the solution should not exceed 0-05 mole/litre after the precipitation. The solubility of the precipitate increases in strongly acidic media, and therefore the solution should not be more concentrated than 0-5 mole/litre with respect to hydrochloric or sulphuric acid. Nitric acid should never be present because it oxidizes copper(I) ions to copper(II), and the thiocyanate precipitate of the latter is much more soluble. The precipitation can be carried out from cold or warm solutions, but complete precipitation takes a long time, especially in solutions which contain mineral acids. It is advisable, therefore, to allow the mixture to stand overnight, and never less than 6 hr. The precipitation is complete within 3 hr, when the solution contains no mineral acids, but here also the precipitate becomes more easily filterable when allowed to stand for a longer period. In neutral solution the precipitate is formed as very fine crystals, which will very often pass through the filter. If the acid concentration is increased, however, the grain size of the precipitate becomes larger, especially if the precipitation is carried out in hot solution. Sulphurous acid, i.e. an aqueous solution of sulphur dioxide, can be replaced by a solution of sodium or ammonium hydrogen sulphite as reductant (I. M. Kolthoff, G. H. Meene, 1927). This is often used in solutions which contain hydrochloric acid. Recently, iron(II) sulphate solution has also been recommended for the reduction (R. Belcher, T. S. West, 1952), but in the presence of excess of iron(III) there is a danger of oxidation of the copper(I) ions in the precipitate, and thus a negative error may occur. It is not advisable to use concentrated alcohol for washing, because the precipitate may then pass through the filter. It can be established by thermogravimetric measurements (see Fig. 8.2. measurements of F. Paulik), that copper(I) thiocyanate is stable up to 200°C. Above 300°C sulphur and cyanogen gas is liberated, and the precipitate is completely converted to copper(I) sulphide at 440°C. Above this temperature an increase of weight occurs owing to the formation of CuO + CuS04, while at about 900°C black CuO remains behind. The precipitate can therefore also be weighed in this form after ignition at the temperature corresponding to a light red glow. Above 1000°C Cu 2 0 is formed with further loss of oxygen.

82

COPPER

The method is suitable for the separation of copper from almost all common metal ions. Silver, mercury(I) and lead(II) ions also form precipitates under t h e experimental conditions used, a n d therefore t h e y should be separated beforehand. Silver a n d mercury can be precipitated in t h e form of their chlorides, while the lead can be precipitated as its sulphate. I n t h e presence of iron(III) ions the precipitation should be carried out in a hot solution, using a suitable q u a n t i t y of reducing agent. Bismuth, Sb and Sn ions are inclined t o coprecipitate in t h e form of their basic salts.

200

400

600

600

Fig. 8.2. Thermoanalytical curves of copper(I) thiocyanate precipitate If these ions are present, precipitation should be carried out in t h e presence of 1-1*5 g of tartaric acid. No interference occurs in the presence of Co, Ni, Mn, Zn, Fe(II), Cd, As or t h e alkali metals, a n d if t h e solution is sulphatefree the alkaline earth metals do not interfere. Procedure. To the nearly neutral solution, which is free of nitric acid and contains 0-1-0-5 g of copper, add 25 ml of 10% sulphurous acid solution (a saturated aqueous solution of sulphur dioxide), or an equivalent amount of ammonium hydrogen sulphite, NH 4 HS0 3 , or sodium hydrogen sulphite, NaHS0 3 . Acidify with 20 ml of 2 N hydrochloric or sulphuric acid, dilute to about 150 ml and heat to boiling. Then add dropwise with constant stirring a slight excess of 2 N potassium or ammonium thiocyanate solution (for the precipitation of 100 mg of copper, 1 ml of 2 N thiocyanate solution is sufficient). Cover the beaker, boil the solution for half a minute, and allow to stand for at least six hr. Collect the precipitate in a G4 glass or A l porcelain filter crucible, wash 10-15 times with 0-1% ammonium thiocyanate solution and then with 20% alcohol. Dry at 110°C for 2-4 hr. Cool and weigh the precipitate. Stoichiometric factors: Cu/CuSCN = 0-52243, CuO/CuSCN = 0-65398.

83

PRECIPITATION IN THE FORM OF COPPER SULPHIDE

Notes. (1) The precipitate can be dissolved from t h e filter with w a r m 1 : 1 nitric acid. (2) The precipitate can also be collected on a filter p a p e r a n d weighed in t h e form of CuO after ignition a t 900 °C to constant weight. The m e t h o d yields accurate results, as can be seen from t h e d a t a obtained b y I . Buzâs a n d S. Gâl (Table 8.4.). T A B L E 8.4. Copper d e t e r m i n a t i o n i n f o r m of CuSCN Number of measurements

Mean CuSCN mg

6 6

191-0 400-7

Cu m g calculated from precipitate weight 99-8 209-4

True value mg

100-2 209-2

Deviation from t r u e value

-0-4 +0-2

Standard deviation mg

%

±0-3 ±0-4

±0-3 ±0-2

8.3. T H E D E T E R M I N A T I O N O F C O P P E R B Y P R E C I P I T A T I O N IN T H E F O R M OF I T S SULPHIDE

Copper(II) ions can be precipitated in t h e form of CuS using hydrogen sulphide or thioacetamide, a n d as Cu 2 S mixed with elementary sulphur if sodium thiosulphate is used. These precipitates, however, cannot be weighed directly. The wet copper(II) sulphide precipitate is easily oxidized in air a n d decomposes also on gentle heating with t h e separation of elementary sulphur. The precipitate obtained with sodium thiosulphate contains variable a m o u n t s of free sulphur. According t o H . Rose, it is advisable t o convert t h e sulphide precipitates t o copper(I) sulphide b y heating in a n atmosphere of hydrogen and t h e n t o remove sulphur from the copper(I) sulphide b y distillation. The solubility of t h e copper(II) sulphide precipitate obtained with hydrogen sulphide is very low (L2QO — about 1 0 - 4 0 ) , and therefore t h e precipitation can be carried out even from solutions which contain large amounts of hydrochloric or sulphuric acid. I n highly acidic solutions, however, t h e precipitate dissolves. Thus, complete precipitation occurs in t h e cold when a 4 N hydrochloric acid solution is used, b u t in hot solution a considerable a m o u n t of copper remains unprecipitated. If t h e solution is only 2 N in hydrochloric acid, t h e precipitation is complete even a t 90°C. I n exceptional cases t h e precipitation can also be carried out in the presence of a small a m o u n t of nitric acid, b u t in such cases a cold solution must be used. Copper sulphide m a y be precipitated from solutions which contain t h e complexes of copper with ammonia or tartaric acid using alkaline ammonium or hydrogen sulphide, b u t precipitation from t h e cyanide complex can only be carried out after its decomposition with acid. The precipitated copper(II) sulphide is easily oxidized to water-soluble copper(II) sulphate when exposed to air, and therefore when filtering care must be t a k e n t o ensure t h a t the precipitate

84

COPPER

is always immersed in the solution. A solution containing hydrogen sulphide should be used for washing the precipitate. The precipitate tends to peptize during washing and so block the filter, and therefore the washing solution of hydrogen sulphide should be slightly acidified with hydrochloric acid or acetic acid. The filtrate must remain colourless. A pale green colour indicates that the precipitate has oxidized and copper(II) sulphate is present in the filtrate. Data for the changes which take place in the precipitate during heating is given in the thermogravimetric and derivative thermogravimetric curves of Fig. 8.3. (measurements of F. Paulik and G. Liptay). The precipitate

CÜZS,CUQ-QQ

200

400

600

Fig. 8.3. Thermoanalytical curves of copper(II) sulphide precipitate loses adsorbed water from its surface at about 100°C, and above 130°C sulphur is lost at an increasing rate. The composition of the precipitate approximates to the formula Cu2S, but oxidation begins at about 350°C, before this composition is attained. The oxidation proceeds in two stages. The largest increase in weight corresponds to the conversion of about 12% of the CuS to copper(II) sulphate. The TG maximum which occurs at 640°C, corresponds to the composition CuO · CuS04. This compound decomposes slowly up to 710°C. The maximum rate of decomposition occursat 740, and at 820 °C the precipitate is completely converted into copperll) oxide. At 940°C this also begins to decompose and form Cu 2 0, but ths composition is only reached by longer heating at 1050°C. According to the TG curve, only the horizontal part of the curve, between 820-940 °C, represents a suitable temperature range in which the precipitate can be converted to a weighing form of constant composition. Roasting to copper(II) oxide, however, requires fairly accurate control of the heating temperature, and in practice, if the weight of precipitate is greater than 0-15 g, the

PRECIPITATION IN THE FORM OF COPPER SULPHIDE

85

ignition must be continued for a longer period. Another disadvantage is that the reducing combustion products of a gas-flame may easily reduce copper(II) oxide to metallic copper. However, if the heat-treatment of the copper(II) sulphide precipitate is carried out in an atmosphere of hydrogen sulphide gas (H2 + S), the oxidation which occurs above 350°C can be avoided. The boiling point of sulphur is 445°C, while the sulphur vapour pressure in the 2 CuS = Cu2S + S reaction is at 450°C 31 mm/Hg

at 475°C 170 mm/Hg

at 490°C 510 mm/Hg

The partial pressure of the sulphur vapour above the precipitate is greater than 0-5 atmosphere even at 490°C, and is essentially higher than the partial pressure of the sulphur obtained from H2S at this temperature. According to Rose, a suitable atmosphere can be maintained by using excess sulphur and gaseous hydrogen. Under these circumstances, however, care must be taken over the correct control of the temperature, because after the evaporation of sulphur, above 630°C, the copper(I) sulphide which is formed may easily be reduced to metallic copper. The most favourable temperature range for the heat treatment is therefore 480—600°C. I t is safer to use an atmosphere of gaseous hydrogen sulphide and no error is then caused by heating above 630°C. Under these circumstances the cooling must be carried out in a hydrogen gas atmosphere, because sulphur which originates from the thermal decomposition of hydrogen sulphide may condense on the hot crucible wall. The precipitation of copper in the form of copper(II) sulphide is not very specific. When the acid concentration is low all the metals of the hydrogen sulphide group are coprecipitated. Copper(II) sulphide can be precipitated quantitatively in the presence of lead ions from a solution which is 2 N in hydrochloric acid and has been heated to 80 °C, but the separation from other metal sulphides is incomplete, even when higher acid concentrations are used. In the presence of As, Sb and Sn it is advisable first of all to oxidise the latter two to their higher valency states, and to carry out the precipitation from an alkaline solution (perhaps in the presence of tartarate ions), using hydrogen sulphide, ammonium sulphide or sodium sulphide. The solution should not contain poly sulphide, for this dissolves a little of the copper(II) sulphide precipitate in the form of a complex compound. Sodium thioarsenate also has a similar solvent effect, and thus care must be taken during the separation of large amounts of arsenic from small amounts of copper. Of the metals of the ammonium sulphide group (group III), iron(II) and particularly zinc may contaminate the precipitate by post-precipitation. In the presence of these ions, the separation can only be carried out from a strongly acidic solution. The concentration of hydrochloric or sulphuric acid must be between 1-7-3 N, and hydrogen sulphide must be passed with stirring into the hot solution for 30 min. The solution must then be diluted with water to twice its volume, and hydrogen sulphide passed once more until the solution is completely cool. In this method the washing is best carried out using a strongly acidified hydrogen sulphide

86

COPPER

solution. Nickel, Co, Mn, Al, Cr, alkali and alkaline earth metals do n o t interfere in this determination. (a) Determination of copper by precipitation with hydrogen sulphide. To the neutral or neutralized solution, which contains 0-10-0-50 g of copper a n d not more t h a n 1% of nitrate, add 10 ml of concentrated hydrochloric acid (or 5 ml of concentrated sulphuric acid) in a 300-ml thick-necked Erlenmeyer flask, dilute to 100-150 ml and heat to about 80°C. Pass hydrogen sulphide into the hot solution by introducing a glass tube with a fine-jet, through which the gas is already passing, and continue to pass the hydrogen sulphide until the solution cools to room temperature (30-60 min) and t h e supernatant liquid has become completely colourless. The gas-flow rate should be slow enough for the bubbles to be countable. Collect the precipitate on a dry, ash-free, coarse filter paper, taking care t h a t the precipitate is continually immersed in the solution. Wash with a hydrogen sulphide solution containing 4% of acetic acid (or when separations are required, hydrochloric or sulphuric acid of a concentration equal to t h a t of the mother liquor), until t h e disappearance of the chloride or sulphate reaction. (Before testing for chloride with silver nitrate, hydrogen sulphide m u s t be removed from t h e test solution b y boiling.) Because of the sensitivity of the copper(II) sulphide precipitate to air, the funnel must be covered with a watch glass during filtration and washing. The bottom of the watch glass should be moistened with one drop of hydrogen sulphide solution, and care should be t a k e n t h a t the precipitate is always covered with washing solution. Weighing of the precipitate in the form of copper (I) sulphide, Cu2S. Transfer the bulk of the precipitate, which has been dried at 100°C, without loss on to a glossy paper, fold the paper and combust it over an enamel-free Rose crucible so that the ash falls into the crucible. Carbon which remains behind in the ash must be completely combusted in an open crucible. The colour of the copper(II) oxide which is formed makes it rather difficult to judge when the combustion is complete. Pour on to the rest 1-2 g of powdered sulphur (of such purity that it leaves no residue on evaporation), and introduce the bulk of the precipitate into the crucible. Sulphur can be purified by dissolving it in carbon disulphide and evaporating the latter after filtration (danger of explosion !). Cover the crucible with a perforated Rose porcelain cover, and fit this with a pipe for the introduction of gas. Hydrogen mustbe generated in a Kipp J s apparatus from zinc and sulphuric acid. Place the crucible on a porcelain triangle mounted on an iron tripod. Introduce the hydrogen gas into the Rose crucible, after washing it with alkaline potassium permanganate in a washing bottle and then passing it through a U-tube filled with sodium hydroxide pellets. When the apparatus is free of air the gas current should be adjusted so that 3 bubbles/second pass through the washing bottle.Heat the crucible with a small flame. During the heating the sulphur evaporates and combusts with the hydrogen at the openings of the cover. The cover of the crucible must also be heated with a second gas-burner to avoid the condensation of sulphur on it.

PRECIPITATION IN THE FORM OF COPPER SULPHIDE

87

A d j u s t t h e b u r n e r u n d e r t h e crucible so t h a t after c o m p l e t e e v a p o r a t i o n of t h e s u l p h u r , i.e. after t h e d i s a p p e a r a n c e of t h e b l u e flame a n d s u l p h u r d i o x i d e smell, t h e b a s e of t h e crucible r e m a i n s a t a j u s t visible d a r k - r e d glow for 2 5 - 4 0 m i n . T h e n a d j u s t t h e h y d r o g e n flow t o t w i c e i t s p r e v i o u s r a t e a n d r e m o v e t h e g a s b u r n e r . I f t h e crucible h a s cooled sufficiently, ext i n g u i s h t h e flame b y closing t h e t o p of t h e K i p p ' s a p p a r a t u s w h i c h t h u s i n t e r r u p t s t h e g a s c u r r e n t for a m o m e n t . Allow t h e c r u c i b l e t o cool c o m p l e t e l y while t h e h y d r o g e n is still flowing. Close t h e t o p of t h e g a s g e n e r a t i n g a p p a r a t u s , p l a c e t h e crucible in a d e s i c c a t o r for 45 m i n , a n d w e i g h . Stoichiom e t r i c f a c t o r : 2 Cu/Cu 2 S = 0-79851. Notes. (1) The precipitate in the crucible should be bluish-black after h e a t t r e a t m e n t . R e d spots (Cu) indicate t h a t t h e precipitate was h e a t e d more strongly t h a n required. Any white colour indicates oxidation (CuS0 4 ). I n either case the h e a t t r e a t m e n t m u s t be repeated more cautiously (in a hydrogen atmosphere) after pouring on a small a m o u n t of sulphur. Test for constant weight also b y repeated heating with sulphur. (2) I n the presence of P b , Zn or Fe t h e precipitation m u s t be carried out in a cold solution which contains larger a m o u n t s of acid of 2-3 N . (3) I t is advisable to clean t h e gas inlet t u b e occasionally with alkaline potassium p e r m a n g a n a t e , otherwise t h e precipitate m a y adhere to it. The hydrogen sulphide current m u s t be s t a r t e d before immersion of the inlet t u b e into t h e solution, a n d it should be stopped only after removing it from t h e solution, otherwise the internal walls of the t u b e m a y be contaminated with precipitate. (4) The accuracy of the m e t h o d can be judged from t h e d a t a of Table 8.5. (measurements of I . Buzâs a n d S. Gal). The results are r a t h e r scattered, a n d after longer reduction times the results are lower t h a n calculated. T A B L E 8.5. Copper d e t e r m i n a t i o n i n form of Cu 2 S Number of measurements

6

Weight of Cu 2 S precipitate mg 244-6 245-0 247-1 242-5 244-0 242-0

Mean mg

244-2

True value mg

244-6

Deviation from true value

-0-16

Standard deviation

%

mg

±1-68

±0-68

(b) Determination of copper by precipitation with thiosulphate (G. Vortman, 1881). Copper(II) ions, when treated with sodium thiosulphate, form a colourless complex which decomposes easily on boiling to form copper(I) sulphide. 2 CuS0 4 + 4 Na 2 S 2 0 3 = Na2[Cu2(S203)2] + Na2S40G + 2 Na 2 S0 4 Na2[Cu2(S203)2] -» Cu2S + S + S0 2 + Na 2 S0 4

88

COPPER

The precipitate m a y be p a r t l y dissolved b y t h e excess of sodium thiosulphate, therefore t h e excess of precipitant m u s t be decomposed by boiling with sulphuric acid: Na 2 S 2 0 3 + H 2 S 0 4 = N a 2 S 0 4 + S + S 0 2 + H 2 0 The reaction consumes acid, a n d excess should therefore be present. The large a m o u n t of sulphur which is precipitated coagulates the copper (I) sulphide, a n d a n extremely easily filtered precipitate is obtained, which has n o t e n d e n c y to peptization. The precipitate does n o t oxidize even after the excess sulphur dioxide has been washed away, a n d therefore it can also be washed with water. The precipitation is suitable for the separation of copper from Cd, Ni, Co, Zn and also large amounts of Fe(III). Because of this, and because it does not need gaseous hydrogen sulphide, this method of precipitation is preferred whenever possible. Procedure. To the neutral, and if possible nitrate-free solution, containing 0-10-0-50 g of copper, add 5 ml of concentrated sulphuric acid in a thick necked 300-ml Erlenmeyer flask, and dilute with water to 100 ml. Add 20 ml of 10% sodium thiosulphate solution, place a small funnel on the mouth of the flask, and heat the solution to boiling. The solution, which is initially blue, becomes yellow, and on boiling a yellow precipitate forms which rapidly turns black. The boiling should be continued until the precipitate settles and the mother-liquid becomes colourless (about 15 min). Filter the precipitate while the solution is hot on a medium grade ash-free filter paper (5892-white band), and wash with hot water until the acidic reaction disappears in the filtrate. Treat the precipitate, after drying at 100°C, in a hydrogen gas current in a Rose crucible according to the instructions of method (a), and weigh. (c) Ignition of copper (II) sulphide, CuS, and copper (I) sulphide, Cu2S, precipitates to the oxide. According to t h e thermogravimetric curves, copper sulphide precipitates can only be converted to copper(II) oxide within a relatively narrow temperature range (840-940 °C), b u t in a well controlled and ventilated electric furnace, even larger amounts of copper sulphide (0-7 g) can be ignited t o constant weight using a suitably long ignition time (1-3 hr). If a n electric furnace is n o t available and a gas flame is used, it is n o t advisable to ignite amounts of precipitate larger t h a n 0-1 g. Although the conversion of copper (I) sulphide to copper(II) oxide m a y be incomplete, the results can still be used because copper(I) sulphide and copper(II) oxide contain almost t h e same percentage of copper. CuS0 4 or CuO.CuS0 4 , which is seen as white spots, is formed by insufficient heating and can be easily converted into copper(II) oxide b y stronger heating (above 840 °C). Procedure. Dry the sulphide precipitate, obtained by method (a) or (b), together with the filter paper, roast it until the paper is combusted, and finally ignite to constant weight in a well ventilated electric furnace at 850-940 °C (about 2 hr). From the data of Table 8.6. (results of I. Buzâs and S. Gàl) it can be seen that fairly accurate results may be obtained by this method. If the ignition temperature and time is accurately controlled, this method can be regarded as superior to method (a).

DETERMINATION IN THE FORM OF COPPER(I) IODIDE

89

Notes. (1) The product which is obtained on roasting t h e sulphide precipitates (CuO, C u 2 0 , Cu, CuO · CuS0 4 ) can be m a d e soluble in water b y fusion a t 350°C with a 5- to 6-fold excess of potassium pyrosulphate : CuO + K 2 S 2 0 7 = CuS0 4 + K 2 S 0 4 During t h e fusion care m u s t be t a k e n t h a t only small a m o u n t s of s u l p h u r trioxide escape from t h e covered crucible, because this acts as the fusing substance. The cooled smelt can be dissolved b y pouring several millilitres of h o t water into the crucible a n d placing the crucible on a water b a t h for 1-2 hr. T A B L E 8.6. P r e c i p i t a t i o n of c o p p e r i n form of Cu^S + S a n d m e a s u r e d a s CuO Number of measurements

Mean CuO mg

6 6

128-0 654-3

Cu m g calculated from t h e weight of precipitate

True value Cu mg

102-2 522-8

102-1 522-4

Deviation from true value

Standard deviation mg

+0-16 +0-06

±0-20 ±0-44

% ±0-15 ±0-07

(2) Copper can also be determined in this solution b y electrolysis or other methods (e.g. iodometrically). If a gravimetric determination is n o t required t h e precipitate can be destroyed with the filter p a p e r in t h e presence of 1-2 ml of concentrated sulphur acid, using concentrated nitric acid a n d 3 0 % hydrogen peroxide, 1 a n d t h e residue can be freed from nitric acid b y evaporation until sulphuric acid fumes appear. 8.4. D E T E R M I N A T I O N O F C O P P E R I N T H E F O R M OF C O P P E R ( I ) IODIDE (according t o t h e precision m e t h o d of L . W . W i n k l e r )

If a slight excess of a solution of an alkali iodide is added to a hot solution of copper(II) chloride or sulphate, and then sulphurous acid is added, copper is precipitated quantitatively in the form of Cul on cooling: 2 C u 2 + + 4 I - = 2 C u I | + I2 H 2 S0 3 + I 2 + H 2 0 = H 2 S0 4 + 2 HI According to thermogravimetric and derivative thermogravimetric measurements (Fig. 8.4., measurements of I. Markovits) the weight of the precipitate increases up to 280°C by about 0-2%, and this can probably be explained by the formation of iodate. At temperatures between 280-490 °C iodine is liberated causing rapid loss of weight, and the precipitate is converted to coppes(H) oxide. The weight of the copper(II) oxide is constant 1 I . SARUDI, Szervetlen mennyiségi I . Szeged, 1947, p . 82.

analizis

(Quantitative

Inorganic

Analysis)

90

COPPER

u p to 920°C. If the temperature is raised further, loss of weight occurs again and copper(I) oxide is formed. Procedure. The neutral or slightly acidic copper(II) chloride or copper(II) sulphate solution should be free of nitrate and contain not more than 0·1 g of copper. Transfer 100 ml of the solution to a 200-ml beaker, add 1 g of ammonium chloride and heat to boiling. Add to the hot solution a solution of 1 g of sodium or potassium iodide in 5 ml of water dropwise with stirring. Then add dropwise a 5% sodium sulphite solution (5-0 g of anhydrous Na 2 S0 3 dissolved in 100 ml water) until the iodine colour disappears and the precipitate becomes white. Add a further 10 drops of sodium sulphite in excess. Heat the scale 0-

f

v

500

DTQ

V

960 weight 9

w-

\380

0,80,6CuO

TG

0A-

Λ 200

400

600

800 °C

Fig. 8.4. Thermoanalytical curves of copper (I) iodide precipitate solution, which smells of sulphur dioxide, to boiling again and boil for 1-2 min. The higher the temperature of the solution during precipitation, the more compact the precipitate becomes. Leave overnight and collect the precipitate in a G4 glass or A2 porcelain filter crucible, wash with 50 ml of cold water, dry for 2 hr at 130°C and weigh in the form of Cul. The weight of precipitate must be corrected by the following factors: Weight of precipitate g 0-40 0-20 0-10 0-01

Correction mg + 1-3 + 1.1 + 1.0 +0-8

Corresponding deviation Δ % -0-32 -0-55 -LOO -1-3

Stoichiometric factor: Cu/Cul = 0-33363 (the weight of copper is about one third of that of the precipitate). The filter can be cleaned with concentrated nitric acid.

91

DETERMINATION I N THE FORM OF COPPER(II) 0N1DE

Notes. (1) The relatively high systematic error is caused by the slight solubility of the precipitate in iodide solutions because of complex formation. (2) Alkali chlorides do not affect the accuracy of the method. (3) The method also gives good results in the presence of Mg, Zn and small amounts of Cd, Mn, Fe, Co and Ni. In the presence of iron(III) ions small amounts of hydrochloric acid and sufficient sodium sulphite must be added to the cold solution so that the smell of sulphur dioxide can be noticed. (4) Lead interferes and therefore it must be removed initially in the form of sulphate. Even traces of lead coprecipitate and make the precipitate slightly reddish yellow. 8.5. D E T E R M I N A T I O N O F C O P P E R I N T H E F O R M OF C O P P E R ( n ) O X I D E , CuO I n solutions which a r e free of a m m o n i u m salts, organic compounds with several - O H groups and other complex forming substances, copper(II) ions form a fairly insoluble precipitate of copper(II) hydroxide (2>20o = = approximately 10 - 20 ) when treated with potassium or sodium h y 7Ô4 droxide . The Cu ( OH ) 2 precipitate loses ttu(0H)J water easily i n h o t solution a n d is converted into copper(II) oxide: ,\

Cu(OH) 2 1 -> CuO | + H 2 0

i Ά

Precipitation begins a t about p H 5 10' (see Fig. 8.5.), b u t t h e precipitate '//{////////// dissolves i n a strongly alkaline me10 12 14 PH dium owing t o t h e formation of 2_ Fig. 8.5. Solubility of copper(II) cuprite [Cu(OH) 4 ] ions. The precihydroxide as a function of p H pitate can only be separated from t h e adhering mother liquid b y repeated décantation with h o t water; t h e last traces of alkali d o n o t volatilize until a b o u t 900°C when ignited. The precipitate is always contaminated with considerable a m o u n t s of basic salts (e.g. Cu(OH) 2 -CuSÖ 4 ). Figure 8.6. shows t h e thermoanalytical curves for a precipitate obtained from a solution containing sulphate (measurements of I . Markovits). After t h e departure of chemically bound water, a t 250°C, a material consisting of CuS0 4 a n d CuO remains behind. W h e n t h e temperature is increased S 0 3 is removed and in t h e range 780-940°C t h e residue consists of copper(II) oxide, a n d this decomposes o n further ignition with t h e formation of copper(I) oxide. I f t h e heating is done on a gas flame t h e reducing combustion products m a y cause errors due t o t h e formation of copper(I) oxide or metallic copper. A p a r t from t h e interference of complex forming agents, a n y metal ions which form insoluble precipitates under t h e same conditions also interfere. I n practice this means t h a t before t h e precipitation, all metal ions, except those of t h e alkali metals, m u s t be removed.

92

COPPER

Procedure. Dilute the solution, which must be free of interfering substances and contains 0-1-0· 6 g of copper, to 100 ml in a 200-ml beaker. Boil and add dropwise enough2 % potassium hydroxide solution to make the solution alkaline to litmus paper. The precipitate becomes brown and then settles. Filter the mixture with a compact (Schleicher and Schiill 5892 white band or 5893 blue scale

DT6

0weight 9

60

\ / ! 97°

350

750 !

10-

uj^6

Cu20(OHi09-.

CuO Cu20

!

Qd

~

0,7-

200 400

600 Ô00 °C

Fig. 8.6. Thermoanalytical curves of copper(II) hydroxide precipitate band) filter paper and wash by décantation with hot water until the alkaline reaction disappears. Finally transfer the precipitate on to the filter paper. Traces of precipitate adhering to the wall of the beaker can be re-dissolved in a few drops of concentrated nitric acid and reprecipitated with potassium hydroxide. This precipitate must be collected on a second filter and washed TABLE 8.7. Precipitation of copper in form of copper(II) oxide Number of measurements

Mean CuO mg

6 6

128-0 654-2

Cu mg calculated from the weight of the precipitate 102-3 522-8

True value Cu mg

102-1 522-4

Deviation from true value

+0-16 +0-05

Standard deviation

mg

±0-08 ±0-39

%

±0-06 ±0-06

thoroughly. Dry at 100 °C, transfer the bulk of the precipitate without loss into a weighed porcelain crucible, and combust the filter paper above the crucible so that the ash falls on to the precipitate. Ignite the precipitate in an

DETERMINATION IN THE FORM OF COPPER(II) QUINALDINATE

93

electrical furnace at 850-940 °C for 3-5 hr (for less accurate determinations 30-60 min ignition is sufficient). Test for constant weight by repeating the ignition for 30 min. Stoichiometric factor: Cu/CuO = 0-79884. Note. The precipitate can also be ignited in a porcelain filter crucible. By using the correct experimental conditions accurate results can be obtained (see Table 8.7., measurements of I. Buzâs and S. Gâl). 8.6. D E T E R M I N A T I O N O F C O P P E R I N T H E F O R M C O P P E R ( H ) Q U I N A L D I N A T E (C 10 H 6 NO 2 ) 2 Cu. H 2 0 (P. Bay and M. K. Bose, 1933)

OF

Copper(II) ions in a solution containing dilute sulphuric or acetic acid form a n insoluble precipitate of a chelate complex with quinaldic acid, t h e structure of which is probably as shown:

o

o

/ \ COO OOC The precipitate is practically insoluble between p H 1-5-6-9. The solubility is least a t about p H 5-7. The green, crystalline precipitate contains 1 molecule of water of crystallization. According t o R â y a n d Bose t h e precipitate can be dried a t 125°C without danger of t h e loss of this water of crystallization. According to t h e thermoanalytical curves of Fig. 8.7. (measurements of G. L i p t a y and I. Markovits), the precipitate loses its water of crystallization between 120-200 °C, a n d decomposes on further heating with t h e evolution of gas. I n t h e range 660-940°C t h e precipitate has a constant weight, a n d its composition corresponds t o basic copper(II) carbonate CuO · CuC0 3 . A t higher temperatures further decomposition occurs. I t is therefore preferable to dry t h e precipitate a t a b o u t 100 °C to constant weight and t o weigh in t h e form of t h e monohydrate. Quinaldic acid gives chelate precipitates in a neutral or acetic acid solution with P b , Ag, Cd, Zn, Mn, Co, Ni a n d Fe(II) ions as well as with copper(II) ions. The following ions are precipitated in t h e form of their basic salts: Fe(III), U(VI), Al, Cr(III), Be and Ti. Quinaldic acid is a selective reagent for copper(II) ions, when iron a n d zinc are n o t present, because in a solution containing diluted sulphuric acid t h e quinaldinates of t h e other mentioned cations are easily soluble. I n t h e presence of sufficient acetic acid (7-15 ml of glacial acetic acid in 200 ml of t h e solution to be tested) copper(II) quinaldinate can be precipitated even in t h e presence of P b , POf", A s O f a n d A s O | " ions. Precipitant. An aqueous solution of the sodium salt equivalent to 3-3% quinaldic acid. This can be prepared by dissolving 5-64 g of sodium quinaldinate in 150 ml of water.

94

COPPER

Procedure. To 150-160 ml of the nearly neutral solution containing 10-150 mg of copper, add 2-5 ml of 2 N sulphuric acid, heat to boiling, and precipitate the copper with a slight excess of the quinaldic acid reagent. Allow the mixture to settle for some minutes, and filter through a G4 glass or A l porcelain filter crucible by décantation. Wash with hot water. The washing must be continued until the traces of excess reagent have been removed from the filtrate [i.e. until the filtrate does not give a red precipitate with iron(II) sulphate]. Dry the precipitate to constant weight at 100°C. Weighing form: (C10H6NO2)2Cu · •H 2 0. Stoichiometric factor: Cu/(C10H6NO2)2Cu -H 2 0 = 0-14919. scale 0-

ore

~v\?\r~y700

■ weight CuiCMO^O

1Q.

η

\

\1

240

QÔ 0,6-

\

04-

T6

CuO.CuCO* no« Λ J 0,iCuO

^J 200

400

600

800 °C

Fig. 8.7. Thermoanalytical curves of copper(II) quinaldinate precipitate Notes. (1) The weights of the precipitates differ by not more than -j-0-1 mg from the true value. (2) The precipitation can also be carried out from an acetic acid medium. Particularly accurate results can be obtained if 0-05 ml of glacial acetic acid is added to the neutral solution instead of sulphuric acid (this corresponds to pH 5-7). In this procedure, however, the method is less selective. 8.7. P R E C I P I T A T I O N OF C O P P E R ( I I ) I O N S WITH 8-HYDROXYQUINOLINE (according to R. Berg, 1927) Copper(II) ions form a green, fairly insoluble, crystalline precipitate with 8-hydroxyquinoline between p H 2-7-14-6. This precipitate has t h e composition Cu(C 9 H 6 ON) 2 . The precipitate dissolves in strongly acidic medium (pH << 2-2), b u t can be precipitated quantitatively even from a strongly alkaline solution containing sodium t a r t r a t e . Copper(II) oxinate has a constant weight between 60-270 °C, and its composition is strictly t h a t expressed b y its formula (see thermoanalytical curves of Fig. 8.8., measurements of S. Gâl). Between 270-834°C t h e organic

PRECIPITATION WITH 8-HYDR0XYQUINOLINE

95

material decomposes a n d above 834°C copper(II) oxide remains behind. The precipitate can be weighed after drying a t 105-110°C. The bromatometric titration of t h e precipitate dissolved in hydrochloric acid is interfered with b y the copper ions. The precipitate can also be t i t r a t e d iodometrically (see note below). The greatest advantage of t h e method is t h a t it achieves a fairly good separation from a number of ions. (a) Precipitation from an acetic acid medium. The precipitation from a n acetic acid medium effects a good separation from Be, Mg, Ca, Cd, P b , As, Mn a n d P O | ~ ions. scale 0-

270

\\ [\/S^ V590

weight 9 CufCaH-ONL muun-giiQUuj2 J (J

|

360

; !

\

0,60.6-

m

\\480

Λ

t

! XJ^fi

!

\J J

0,4CuO

DTQ

02._

720

200

400

600 800 °C

Fig. 8.8. Thermoanalytical curves of copper(II) oxinate precipitate Procedure. To the neutral or slightly acidic solution add 10-12 ml of glacial acetic acid and 3 g of crystalline sodium or ammonium acetate, and dilute t o 100 ml. Heat the solution to 60 °C and add dropwise 3% alcoholic 8-hydroxyquinoline solution with constant stirring, until a slight excess is present. The presence of excess precipitant is indicated by the yellow colour of the supernatant liquid above the settled precipitate. Heat the mixture for a short time a t 80-90°C, and filter through a G4 glass filter crucible. Wash with hot water, and dry the precipitate at 105-110°C to constant weight. Stoichiometric factor: Cu/Cu(C 9 H 6 ON) 2 = 0-18059. Note. Mg, Cd, Pb and Mn ions can be precipitated in the filtrate in the form of their oxinates, while Be ions may be precipitated in the form of the hydroxide. Before the determination of phosphate ions the filtrate must be evaporated todryness, and excess of 8-hydroxyquinoline destroyed by repeated evaporation with concentrated nitric acid and hydrogen peroxide. (b) Precipitation from an alkaline sodium tartrate solution. The precipitation of copper(II) oxinate m u s t be carried out from a n alkaline t a r t r a t e medium, if the solution contains Al, P b , Sn(IV), As(V), Sb(V), Bi, Cr(III) or Fe(III) ions as well as copper.

96

COPPER

Procedure. To the solution, containing 3-100 mg of copper and 50 mg of each of the accompanying ions, add 3-5 g of pure tartaric acid, neutralize with sodium hydroxide in the presence of phenolphthalein, and add a further 20 ml of 2 N sodium hydroxide in excess. Dilute with water to 100 ml. Precipitate at room temperature with a 3% alcoholic or acetone solution of 8-hy droxyquinoline. The presence of excess reagent is indicated in the presence of iron(III) ions by the appearance of a dark brown colour. Heat the mixture to 60-70°C, allow to cool and filter through a G4 glass filter crucible. Wash with a warm 1% solution of sodium tartrate and then with small amounts of water. Dry at 105-110°C to constant weight. Weighing form: Cu(Ox)2. Al, Bi or Fe(III) ions can be determined in the filtrate in the form of their oxinates. Note. The copper ion content of the copper(II) oxinate precipitate can also be determined iodometrically after dissolving in 2 N hydrochloric acid and the addition of potassium iodide, according to the method of de Haën using 0*1 N sodium thiosulphate as the standard solution. One millilitre of 0· 1 N sodium thiosulphate solution is equivalent to 6-354 mg copper(II) ions.

The Separation of Cu 8.8. Cu-Ag See the Ag-Cu separation (Chapter 5.11.). 8.9. Cu — from other metal ions 1 Copper(II) ions form a precipitate with 2(o-hydroxyphenyl-)benzoxazole in the range p H 3-12. If precipitation is carried out a t p H 11 in t h e presence of disodiumethylenediaminetetraacetate (Na 2 EDTA), Ni, Fe(II), Co, P b , Mn(II), Mn(III), Ca, Sr, Ba, Li, Na, K, Cr(III), Hg(II), Sn(II), Cd and Zn ions do not interfere. If the solution contains Bi, Fe(III), a n d Al ions, their hydroxides precipitate a t p H 11. This interference can be avoided by filtration of the solution containing Na 2 EDTA after adjusting the p H to 11, a n d b y precipitating copper from t h e filtrate. If Fe(III) and Al ions are present, 5-15 g sodium t a r t r a t e are added t o t h e solution as well as Na 2 EDTA instead of filtering off the hydroxides. 2(o-hydroxyphenyl-)benzoxazole reagent. Recrystallize the reagent from ethanol, and use a cold saturated ethanolic solution. Procedure. To the solution containing 1-100 mg of copper add a 2- to 5-fold excess of disodiumethylenediaminetetraacetate, Na 2 EDTA, over the amount required to keep the metal ions in solution (about 2-5 g). For 1 mole of metal ions, therefore, 2-5 moles of Na 2 EDTA should be used. Adjust the p H of the solution to 11·5 with 5% sodium hydroxide. If a precipitate forms the mixture must be filtered. In the presence of aluminium or iron(III) ions the precipitate can be dissolved by the addition of 5-15 g of sodium tartrate. 1

E. E. BYRN and J. H. ROBERTSON, Anal. Ghent., 26, 1605 (1954).

SEPARATIONS

97

Add the 2(o-hydroxyphenyl-)benzoxazole reagent dropwise with constant stirring. The presence of excess reagent is indicated by the blue fluorescent colour of the mother liquid. Cover the mixture, heat on a steam bath for 15-30 min, and filter while hot through a medium pore-sized filter crucible. Wash with a 50% mixture of ether and alcohol. Dry at 130-140 °C to constant weight. Composition of the precipitate: Cu(C 13 H 8 0 2 N) 2 . Stoichiometric factor: Cu/Cu(C 13 H 8 0 2 N) 2 = 0-13129. Note, By this method the copper content of copper-nickel, aluminium alloys, cast iron and bronzes can be determined fairly accurately. For 1-70 mg of copper the deviation from the true value is 0-06-0-13 mg. 8.10. Cu-Pb, Bi The separation is based on t h e fact t h a t b y adding a n excess of potassium cyanide to t h e solution which has been neutralized with potassium carbonate, lead a n d bismuth are precipitated in t h e form of basic carbonates, while copper remains in solution in t h e form of t h e complex

K3[Cu(CN)J.

Procedure. Remove the excess nitric acid from the solution by evaporation, dilute to 150 ml and add dropwise 2 N potassium carbonate (not sodium carbonate, because this dissolves the basic bismuth carbonate precipitate to some extent), until the phenolphthalein indicator shows a slightly alkaline reaction. Then add a slight excess of 5% thiocyanate-free potassium cyanide solution, and heat on a water bath in a fume-cupboard for 1-2 hr, until the eventually dissolved bismuth re-precipitates. Filter the precipitate and wash with cold water. The precipitate can be further treated according to the methods for Pb, Bi-Ag, Hg(II), Cu, Cd separations (Chapter 6.9.). Determination of copper in the filtrate. Add to the cyanide-containing nitrate, in a well ventilated fume-cupboard, 7-10% of its volume of concentrated nitric acid and heat for 30 min near to the boiling point (HCN !). Copper (I) cyanide oxidizes and the precipitate dissolves. Add 10 ml of concentrated sulphuric acid, boil the solution until sulphuric acid fumes appear, dilute the solution and precipitate copper electrolytically or as copper(II) sulphide with hydrogen sulphide. If the latter method is employed, ignite the precipitate to copper(II) oxide at 850-950 °C for 1-3 hr. 8.11. Cu-Pb (a) Copper(II) ions can be precipitated with sodium quinaldinate in t h e presence of excess acetic acid. The filtrate can be evaporated to sulphuric acid fumes after t h e addition of concentrated sulphuric acid and hydrogen peroxide, and lead can be determined in t h e form of P b S 0 4 . Procedure. To the nearly neutral solution, containing 50-150 mg of cop per and the same amount of lead in 200 ml, add 7-15 ml of glacial acetic acid heat to boiling, and add a 3-76% solution of sodium quinaldinate dropwise to the hot solution until precipitation is complete, and then add 1 ml of the precipitant in excess. Allow the precipitate to settle and filter the clear solution by décantation into a G4 glass or A l porcelain filter crucible. Wash with a mix-

98

COPPER

ture of 50 ml of water, 1 ml of glacial acetic acid and a few drops of sodium quinaldinate solution. The precipitate should be transferred to the filter after decanting 3-4 times, and must then be washed with hot water. Dry at 100°C. Weighing form: (C l0 H 6 NO 2 ) 2 Cu · Η 2 0 . Note. The deviation of the weight of the precipitate from the true value is ±0-1 mg. Determination of lead in the filtrate. Add 10 ml of 50% sulphuric acid to the filtrate and boil until sulphuric acid fumes appear. If the solution turns to brown because of charring, the organic material can be destroyed by the addition of several millilitres of concentrated nitric acid and a small amount of 30% hydrogen peroxide, and the evaporation must be repeated after the addition of 10 ml of water. Dilute the sulphuric acid mixture with 50 ml of water, add 50 ml of alcohol, leave overnight and filter on an A1 porcelain filter crucible. Wash with 50% alcohol, to which several drops of sulphuric acid have been added. Ignite at 500-600°C. Weighing form: PbS0 4 . (b) See the separation of P b - C u (Chapter 6.11.). 8.12. Cu-Hg(II) See the separation of Hg(II) from other metal ions and the separation of Hg-Cu (Chapter 7.13.). 8.13. Cu-Bi Procedure. To the slightly acidic solution, which contains nitric acid, add excess ammonium carbonate and ammonium hydroxide, and heat almost to boiling. The precipitated basic bismuth carbonate also contains a considerable amount of copper(II) carbonate, and it must be dissolved in nitric acid after filtration and washing and then re-precipitated. Dissolve the precipitate in nitric acid and precipitate bismuth in the form of bismuth phosphate (see Chapter 9.6.). Weighing form: BiP0 4 . Determination of Cu in the filtrate. Slightly acidify the ammoniacal filtrate with hydrochloric acid, and precipitate copper in the form of copper sulphide using hydrogen sulphide or sodium thiosulphate. Ignite the precipitate to the oxide at 850-950°C (1-3 hr, see Chapter 8.3.b.). Weighing form: CuO. See also the separation of Cu—Pb, Bi (Chapter 8.10.). 8.14. Cu-Cd (a) By electrolysis. Copper can be deposited from a solution containing sulphuric acid, nitric acid or ammonia. 1. Deposition of copper from a sulphuric acid solution: Copper can be deposited according to t h e procedure described in Chapter 8.1.1. b y slow or rapid electrolysis. Weighing form: Cu. 2. Deposition of copper from a nitric acid solution: according to the instructions of Chapter 8.1.2. Weighing form: Cu. 3. Deposition of copper from an ammoniacal solution: This procedure must be used if the original solution contains chloride ions.

SEPARATIONS

99

Procedure. Add 15 ml of concentrated ammonium hydroxide and 5 g of ammonium chloride to the neutral solution and dilute to about 100 ml. Electrolyze at room temperature while stirring, with a platinum net cathode and a platinum spiral anode, at 1-8-2-0 V and 0-5-0-1 A current. Wash the cathode with water while the potential is applied, rinse with alcohol and acetone, and dry in a cold air-stream. Weighing form: Cu. The determination of cadmium in the filtrate can be carried out by one of the simple gravimetric methods (see Chapter 10.). (b) According to G. Vortmann and I. Sarudi. Copper can be precipitated in t h e form of copper(I) sulphide b y boiling with sodium thiosulphate in a solution which contains sulphuric acid. However, if the a m o u n t of t h e metals t o be separated is higher t h a n 50 mg, t h e precipitate also contains a considerable a m o u n t of cadmium. B y repeated precipitation copper(1) sulphide can be obtained in a pure, cadmium-free form. Determination of copper. The mixture should contain not more than 120 mg of copper and 220 mg of cadmium, and be slightly acidic with sulphuric acid. Add a 10-20% solution of sodium thiosulphate until the solution decolourizes, and then add 12 ml of 1 : 1 sulphuric acid. Boil the solution until the precipitate settles and the supernatant liquid becomes clear (about 15 min). Collect the precipitate on a filter paper and wash with hot water. Combust the filter paper, ignite the precipitate to the oxide in a porcelain crucible a t 850-950 °C, and fuse it with a 10-fold amount of potassium pyrosulphate. Dissolve the smelt in hot water which contains a small amount of sulphuric acid. Dilute the solution to 100 ml and repeat the precipitation of copper (I) sulphide with 10-20% sodium thiosulphate and 12 ml of 1 : 1 sulphuric acid. Collect the precipitate on an ash-free filter paper, wash with hot water until the sulphate reaction disappears in the filtrate, and ignite to tb« oxide in a weighed porcelain crucible. Heat in an electric furnace at 850-950°C for 1-3 hr. Weighing form: CuO. Determination of cadmium in the filtrate. Evaporate the combined filtrates and washing solutions to a syrup on a water bath, add 8-10 ml of saturated bromine water and evaporate the excess bromine. Dilute the solution and remove the precipitated sulphur by filtration. Dilute the filtrate to at least 400 ml so t h a t it does not contain more than 3 ml of concentrated sulphuric acid in each 100 ml. Precipitate cadmium sulphide from the boiling solution with hydrogen sulphide. Filter, and wash the precipitate with hydrogen sulphide solution to which 1-2 drops of acetic acid have been added. Rinse the bulk of the precipitate into a porcelain evaporating dish with a fine water-jet, and dissolve it by heating with 125 ml of diluted hydrochloric acid (1 : 3). Dissolve the precipitate which remains on the filter, add 15 ml of 2 N sulphuric acid, and evaporate the combined solutions. Rinse the solution into a weighed platinum or porcelain crucible, evaporate on an air bath, and heat for 30-60 min at 500-600°C. Cool and weigh the residue. Weighing form: CdS0 4 . Notes. (1) By this method copper can be determined to an accuracy of within ±0-4%, and cadmium to within ±0-8% (according to the measurements of I·. Sarudi).

100

COPPER

(2) From a solution which contains less than 50 mg of copper and cadmium, Cu2S is precipitated practically free of cadmium by a single precipitation. The separation should therefore not be repeated. (3) Cadmium can be precipitated from a hydrochloric acid solution of cadmium sulphide in the form of CdNH 4 P0 4 · Η 2 0 (see Chapter 10.3.). (c) If copper and cadmium are present in t h e solution in t h e form of complex cyanides, e.g. after separation from lead and bismuth, cadmium sulphide can be precipitated with sodium sulphide. Weighing form: CdS0 4 . Determination of copper in the filtrate. Acidify the filtrate with sulphuric acid and evaporate in a fume-cupboard (HCN + H 2 S !). The evaporation must be repeated after the addition of several mis of concentrated nitric acid to decompose the copper(I) cyanide formed. Finally evaporate the solution to sulphuric acid fumes. Dilute with water and determine the copper by one of the simple gravimetric methods. Weighing forms: CuS, CuO, Cu, CuSCN. (d) I n a nitrate-free solution copper can be determined in t h e form of copper(I) thiocyanate, according to Rivot (Chapter 8.2.). Weighing form : CuCNS. Procedure. The filtrate should be evaporated to one third of its original volume ; during this time sulphur dioxide is completely removed. Cadmium sulphide is precipitated from the solution with gaseous hydrogen sulphide. The precipitate should be filtered on filter paper and washed with water containing acetic acid. The precipitate must be dissolved in dilute hydrochloric acid (1 : 3), converted to the sulphate, and ignited at 500-600°C (see Chapter 10.2.2.). Weighing form: CdS0 4 . (e) Salicylaldoxime ( H O - C 6 H 4 - C H = N - O H ) is a specific reagent for copper (and palladium). 1 Preparation of the reagent. Dissolve 1 g of salicylaldoxime in 5 ml of alcohol and dilute with 95 ml of hot water (80°C). The emulsion and oily drops dissolve instantaneously without shaking. The solution must be filtered. Procedure. The solution should contain about 100 mg copper. Neutralize the solution with 2 N sodium hydroxide until precipitation begins. Dissolve the precipitate in several drops of 2 N acetic acid, and dilute the solution to 200 ml. Add to the cold solution, with constant stirring, a slight excess of 1 % salicylaldoxime reagent, (test for complete precipitation by adding one drop of the reagent to the clear solution above the precipitate). Large excess of precipitant must be avoided. Collect the precipitate in a G 4 glass or A 2 porcelain filter, and wash with cold water until the filtrate no longer gives a blue colour with iron(III) chloride. Dry at 100-105°C. Weighing form: Cu(C 7 H 6 0 2 N) 2 . Stoichiometric factor: Cu/Cu(C 7 H 6 0 2 N) 2 = 0-18922. Note. Cadmium can be precipitated from the filtrate in the form of CdS with hydrogen sulphide (see Chapter 10.2.2.). Weighing form: CdS0 4 . (f) Separation with sodium quinaldinate (P. Ray and M. Bose, 1933). Copper(II) quinaldinate can be precipitated from both acetic acid or sulphuric acid solutions, b u t cadmium quinaldinate can only be precipitated from a strictly neutral medium. 1

F. EPHRAIM, Ber., 63, 1928 (1930).

SEPARATIONS

101

Determination of copper. Dilute the neutral solution, which contains 50 to 150 mg of copper and the same amount of cadmium, to 150-160 ml. Add 2-10 ml of 2 N sulphuric acid, and precipitate copper from the hot solution with a slight excess of 3-76% sodium quinaldinate solution (see Chapter 8.6.). Filter on a G4 glass or A l porcelain filter crucible. Wash with hot water. Dry: at 100°C to constant weight. Weighing form: (C l0 H 6 NO 2 ) 2 Cu · Η 2 0 . Determination of cadmium. Evaporate the combined filtrate and wash solution down to 160 ml, and neutralize the solution with ammonium hydroxide while stirring. Part of the cadmium is precipitated. Add a slight excess of the sodium quinaldinate precipitant to the hot solution. Cool the solution to room temperature. Filter in the cold, on a G4 glass or A l porcelain filter crucible. Wash with cold water. Dry at 125°C to constant weight. Weighing form: (C10HeNO2)2Cd. Note. The copper values approach the true value to within ±0-1 mg, and those for cadmium are within ±0*3 mg. Other methods for t h e separation of Cu-Cd: (g) W i t h cupferron, 1 (h) with 8-hydroxyquinoline, 2 (i) with mercaptobenzthiazole. 3 8.15. Cu-As(V) Copper(II) arsenate dissolves with complex formation in a n ammoniacal solution containing 1-2 g of a m m o n i u m t a r t r a t e , a n d arsenic can be precipitated from this solution in t h e form of MgNH 4 As0 4 · 6 H 2 0 using magnesia mixture. The precipitate m u s t be dissolved in a small a m o u n t of hot nitric acid a n d re-precipitated. The arsenic m u s t always be present in t h e pentavalent form. F o r this reason t h e solution should be evaporated several times to dryness with concentrated nitric acid. The residue m u s t be dissolved in ammonia. The oxidation is more easily carried out with bromine water. Magnesium ammonium arsenate m u s t be precipitated b y t h e procedure of Chapter 11.4. Owing t o t h e contamination of t h e precipitate, it should always be reprecipitated. Weighing form: Mg 2 As 2 0 7 . Determination of copper in the filtrate. Evaporate the combined filtrate to remove ammonia, dissolve the residue in hydrochloric acid, and dilute with water to about 150 nil. Precipitate copper(II) sulphide from the hot solution with hydrogen sulphide, collect it on a filter, wash with hydrogen sulphide solution acidified with acetic acid, and ignite to the oxide at 950°C (1-3 hr). Weighing form: CuO. Notes. (1) According to Table 8.8., the results are somewhat higher than calculated for arsenic using a double precipitation (measurements of G. Liptay). (2) If the determination of copper in the form of copper(I) thiocyanate is required, nitrates must first be removed from the solution. Add 10 ml of 1 : 1 sulphuric acid to the residue from the evaporation and evaporate on an air-bath. The evaporation must be repeated 3 times with 10 ml of water. 1

H. J. 2 R. 8 G.

BILTZ and O. HÖDTKE, Z. anorg. Chem., 66, 426 (1910). HANUS and A. SOTJKUP, Z. anorg. Chem., 68, 52 (1910). BEUG, Das o-Oxychinolin "Oxin". Enke, Stuttgart, 1935, p. 29. SPACTJ and M. KUBAS, Z. anal. Chem., 102, 24 (1935).

102

COPPER

8.16. Cu-Sb Antimony(III) sulphide is easily soluble in alkali sulphides, b u t copper(II) sulphide is insoluble. T h e separation can be carried o u t b y a Freiberg fusion (Chapter 2.5.7.), b y fusion of t h e sulphides with crystalline sodium sulphide a n d sulphur (Chapter 2.5.8.), or b y extraction of t h e sulphides with a n alkaline sodium sulphide solution. TABLE 8.8. Cu-As(V) separation

according t o Chapter 8.15

As (Mg 2 As 2 0 7 )

mean:

As mg found

True value As mg

103-9 104-0 103-6

103-1

+0-67

103-8 21-3 21-4 21-0

mean:

Deviation from true value Δ%

20-62

+2-0

21-2

If the last method is used the sulphides must be precipitated from a solution containing hydrochloric acid, according to the instructions given for the precipitation of Sb 2 S 3 (see Chapter 12.1.). Wash the precipitate with water containing hydrochloric acid and hydrogen sulphide, and replace the filter paper and the precipitate in the beaker. Pour 50 ml of 10% sodium sulphide and 10 ml of 30% sodium hydroxide on to the precipitate, heat on a water bath for 30 min, dilute with 50 ml of water and heat for a further 30 min. Filter the mixture, and wash with 2% sodium sulphide. Remove any alkali which adheres t o t h e precipitate with 0-5 N hydrochloric acid saturated with hydrogen sulphide, ignite to the oxide a t 950°C (1-3 hr) and weigh. Weighing form: CuO. Determination of antimony in the filtrate. Precipitate antimony(III) sulphide from the filtrate with hydrogen sulphide after acidifying with sulphuric acid, collect on a porcelain filter crucible, wash with cold water, oxidize the precipitate in a desiccator with bromine and nitrous fumes, and ignite to antimony tetroxide at 850°C (see Chapter 12.2.). Weighing form: Sb 2 0 4 . 8.17. Cu-Sn(IV) (a) T h e separation of Cu-Sn(IV) can be achieved b y t h e method described for t h e separation of Cu-Sb, because tin(IV) sulphide dissolves in sodium sulphide, a n d copper(II) sulphide does n o t . Copper can b e weighed in t h e form of CuO after igniting t h e sulphide (Chapter 8.3.).

SEPARATIONS

103

Tin(IV) sulphide can be precipitated from t h e filtrate b y acidification with acetic acid, and can be weighed after roasting t o tin dioxide (see Chapter 13.2.). (b) Metastannic acid can be precipitated quantitatively from a solution which is free of chloride a n d sulphate b y evaporating with nitric acid. (See Chapter 13.1.) The precipitate m u s t be ignited and weighed. Tin can be vaporized as t h e iodide using ammonium iodide, t h e residue is t h e n evaporated with nitric acid, ignited a n d weighed. The loss of weight is equal to the a m o u n t of t i n dioxide. The residue, which contains copper, m u s t be dissolved in nitric acid a n d combined with t h e filtrate. Copper can be determined in t h e filtrate b y electrolysis from a nitric acid medium according t o t h e instructions of Chapter 8.1.2. 8.18. Cu-Sn, Pb, Fe, Zn Analysis of bronze Tin. Dissolve the sample in nitric acid according to the procedure of Chapter 13.1., and weigh the metastannic acid after ignition to the oxide. After vaporization with ammonium iodide, convert the residue first to the nitrate by evaporating with nitric acid, then to the oxide by ignition, and weigh. The weight loss corresponds to Sn0 2 . Lead. Dissolve the residue in nitric acid and combine this with the filtrate. Add 3-5 ml of concentrated sulphuric acid to the solution, and boil to fumes of sulphuric acid. Repeat the evaporation three times after the addition of 10 ml of water. Boil the residue with 100 ml of water, and allow to stand for 3-5 hr. Collect the precipitate in the cold on a porcelain filter crucible, and wash with 50% alcohol containing a small amount of sulphuric acid. Ignite the precipitate at 500-600°C to constant weight. Weighing form: PbS0 4 . Copper. The filtrate contains sulphuric acid. Copper can now be, (1) deposited by electrolysis (Chapter 8.1.1.) and weighed in the metallic form, or (2) it can be precipitated with hydrogen sulphide and can be weighed in the form of the oxide after ignition at 850-950°C for 1-3 hr (see Chapter 8.3.). Iron. I n the copper-free filtrate (after vaporization of the hydrogen sulphide) iron must be oxidized with bromine water or hydrogen peroxide, and precipitated with ammonia in the form of Fe(OH) 3 (see Chapter 20.1.). The precipitate must be ignited to Fe 2 0 3 . Zinc. Zinc can be precipitated from the filtrate either (1) in the form of its sulphide with hydrogen sulphide in the presence of acetic acid, and weighed in the form of ZnO after roasting and ignition (see Chapter 24.1.4.) or (2) by precipitation in the form of ZnNH 4 P0 4 and weighing in the form of Zn 2 P 2 0 7 (see Chapter 24.2.). 8.19. Cu-Fe(in) (a) If copper(II) sulphide is precipitated with hydrogen sulphide from a strongly acidic sulphuric acid solution, t h e precipitate is obtained in a pure, iron-free form (see Chapter 8.3.). Iron can be precipitated from the filtrate with ammonia in the form of Fe(OH) 3 after oxidation (Chapter 20.1.). Determination of copper. Add 3 ml of concentrated sulphuric acid to the nearly neutral solution, dilute to 150 ml and heat to boiling. Remove from

104

COPPER

the flame, and pass gaseous hydrogen sulphide into the solution until it has cooled. Collect the precipitate on an ash-free filter paper, and wash with 2% acetic acid saturated with hydrogen sulphide, ensuring that the precipitate is always covered with solution. Cover the funnel with a watch-glass during the filtration. Combust the paper, roast the precipitate, and finally ignite at 850950°C in an electric furnace for 1-3 hr. Cool and weigh the precipitate as CuO. Determination of iron. Evaporate the filtrate to 150 ml, remove hydrogen sulphide and oxidize the iron with saturated bromine water. Cool slightly and nearly neutralize the excess acid with concentrated ammonium hydroxide. Heat the slightly acidic solution to boiling, remove the source of heat, and precipitate iron(III) hydroxide by the addition of a small excess of 2 N ammonium hydroxide. Collect the precipitate on a filter paper, wash with hot water, dry in a drying oven, combust the filter paper and ignite at about 800°C to constant weight. Weighing form: Fe 2 0 3 . Note. The separation can be carried out rapidly and gives good results according to the data of Table 8.9. (measurements of I. Buzâs). TABLE 8.9. Cu-Fe separation according to Chapter 8.19.a

Cu (CuO)

mean:

Weight of CuO precipitate mg

True value CuO mg

497-8 497-8 498-1

498-0

mean:

mean:

245-6

mean:

123-0

122-7

Weight of F e 2 0 3 precipitate mg

True value Fe203 mg

Deviation from true value Δ%

116-6 116-4 116-6

1165

±0-0

291-5

-0-12

582-5

-0-05

116-5 291-0 291-3 291-0

-0-12

245-3 123-2 123-0 122-8

Fe (Fe 2 0 3 )

-0-02

497-9 245-2 245-6 245-2

mean:

Deviation from true value

291-1 582-0 582-6 582-1

+0-2

mean:

582-2

(b) By electrolysis. I r o n can be precipitated from t h e solution with ammonia, and copper can t h e n be deposited electrolytically from t h e filtrate. Iron. Add 3-4 ml of concentrated sulphuric acid to the solution containing chloride or nitrate, and evaporate the solution until sulphuric acid fumes

105

SEPARATIONS

appear. Repeat this evaporation several times after the addition of 5-10 ml of water each time. Cool, dilute the solution carefully to 150 ml, and after the basic salt has dissolved, neutralize the bulk of the acid with concentrated ammonium hydroxide. Finally precipitate iron(III) hydroxide from the boiling solution with 2 N ammonium hydroxide. Filter the precipitate by décantation, wash with small volumes of hot water, and dissolve it in 10 ml of 2 N sulphuric acid. Dilute the solution to its original volume and repeat the precipitation. Re-dissolve the precipitate in sulphuric acid and repeat the precipitation of iron(III) hydroxide for the third time. After the third precipitation the supernatant solution will usually be devoid of a blue colouration. Wash the precipitate with hot water, ignite to the oxide at 800°C and weigh as Fe 2 0 3 . Copper. Acidify the filtrate with 5 ml of 1 : 1 sulphuric acid, evaporate to about 150 ml, and deposit copper by electrolysis at 80°C on to a platinum net electrode while the solution is stirred (potential of 2 V current of 0-2 A should be used). The time required for the electrolysis is 1-4 hr, according to the amount of copper present. Weighing form: Cu. TABLE 8.10. Cu-Fe separation according to Chapter 8.19.b

Fe (Fe 2 0 3 )

mean:

Weight of F e 2 0 3 precipitate mg

True value Fe203 mg

Deviation from true value

585-9 587-0 586-5

589-4

-0-5

mean:

148-7

1473

+ 1-0

1

mean:

True value Cu mg

Deviation from true value

à%

98-0

-0-2

196-2

-0-4

490-1

-0-18

97-8 1953 1951 196-0

-0-6

293-3 148-2 149-0 149-0

mean:

294-9

Weight of Cu coating mg

97-6 97-8 98-1 mean:

586-5 294-0 293-2 292-8

mean:

Δ%

Cu (with electrolysis)

195-5 489-0 490-0 488-7 489-2

Notes. (1) Owing to the necessity of repeated precipitation and evaporation the separation is time-consuming and is not as accurate as the former method (see data of Table 8.10., measurements of I. Buzâs). (2) The separation can also be carried out by the following method: The solution should be free of nitrate and chloride, and iron should be reduced with 0-5-1-0 g of

106

COPPER

hydrazine sulphate. Copper can then be deposited by electrolysis of the cold solution. After oxidation with bromine water, iron can be precipitated with ammonia from the copper-free electrolyte in the form of Fe(OH) 3 ,and can be weighed after ignition to the oxide. This method also does not yield very accurate results. (3) If electrolysis is made with adequately switched accumulators, or by rectified mains power and current is adjusted to 3-4 A, by a resistor, the deposition of copper is complete within 20-40 min. Other methods for Cu—Fe separation: (c) with cupferron, 1 a n d (d) with salicylaldoxime. 2 W i t h these organic reagents, when large amounts of iron and copper are present, satisfactory results are not obtained. 8.20. Cu-Ni (a) By electrolysis. Copper can be deposited cathodically from a sulphuric acid solution of the metal sulphates, and after making the solution alkaline with ammonia and t h e addition of ammonium sulphate as a conducting electrolyte, nickel can also be deposited on the cathode. The deposition of copper can be carried out b y slow or rapid electrolysis according to procedure of Chapter 8.1.1. Weighing form: Cu. Deposition of nickel. Neutralize the combined solution of electrolyte, which remains behind after the deposition of copper, and the washing solution with concentrated ammonium hydroxide in the presence of phenolphthalein indicator. Add 30 ml of concentrated ammonium hydroxide in excess. Dissolve 5 g of ammonium sulphate in the electrolyte, and dilute with water to 150 ml. Electrolyze at 75-80°C, using a weighed platinum net cathode and a platinum net anode. A voltage of about 3-5 V and a current of about 4-5 A should be used; 25-40 min electrolysis time is required, according to the amount of nickel present. A prolonged electrolysis time may cause slight dissolution of the platinum anode. Test for complete deposition in 1 drop of the electrolyte using 1% alcoholic dimethylglyoxime solution. The cathode must be rinsed with distilled water while the voltage is still applied, and after the heating and stirring have been stopped. The current can then be switched off. Rinse the platinum cathode with alcohol, dry in a cold air stream at room temperature, and weigh. Weighing form: Ni. Note. The separation gives good results according to the data of Table 8.11. (measurements of G. Siposs). The method can also be used for the analysis of constantan a n d monel alloys: Dissolve 1 g of the metal sample in 20 ml of dilute nitric acid (1 : 1), evaporate off the nitrous fumes, and dilute to 100 ml in a volumetric flask. Take an aliquot part of the solution, acidify with 10 ml of dilute sulphuric acid (1 : 4), dilute to about 100 ml and deposit copper electrolytically according to the former procedure. If a small amount of lead is present it will be deposited on the anode in the form of P b 0 2 . After the deposition of copper, evaporate the solution until sulphuric acid fumes appear, add 10 ml of dilute sulphuric acid 1 2

H. BILTZ and O. HÖDTKB, Z. anorg. Chem., 66, 426 (1910). F. EPHBAIM, Ber.f 63, 1928 (1930).

107

SEPARATIONS

(1 : 4), dilute to 50 ml, make slightly alkaline with concentrated ammonium hydroxide, and determine the nickel by electrolysis according to the above procedure. Iron and manganese can be separated in the filtrate by means of acetatehydrolysis (Chapter 20.2.1.).

TABLE 8.11. Cu-Ni separation with electrolysis according to Chapter 8.20.a Cu (with electrolysis)

mean:

Amount of Cu precipitated mg

True value Cu mg

314-7 315-0 315-1

3145

mean:

156-3

20-6

mean:

Deviation from true value Δ%

31-9

+0-3

161-0

-0-4

161-0

+0-2

160-3 161-2 161-7 160-9

+1-0

True value Ni mg

32-0 161-3 159-6 159-9

+0-1

mean:

20-4

Amount of Ni precipitated mg 32-0 31-9 32-2

mean:

156-5 20-6 20-7 20-6

Ni (with electrolysis)

+0-13

314-9 156-6 1561 156-8

mean:

Deviation from true value Δ%

161-3

(b) Copper can be precipitated in the form of t h e sulphide with hydrogen sulphide or sodium thiosulphate, and after t h e evaporation of hydrogen sulphide, nickel can be determined in t h e filtrate b y one of t h e simple gravimetric methods. Determination of copper. Dilute the slightly acidic nitrate-free solution of the two metal ions to about 80 ml. According to the amount of copper present, add 10-50 ml of 10% sodium thiosulphate solution, acidify with 8 ml of 6 N sulphuric acid and heat to boiling, stirring continually with a glass rod. Cover the beaker with a watch glass, and boil the solution for 15 min. Filter the precipitate rapidly on an ash-free filter paper. Wash with hot water until an acidic reaction is no longer obtained. Combust the filter paper, roast the precipitate in a porcelain crucible, and finally ignite it in an electric furnace at 950°C for 1-3 hr. Cool and weigh as CuO.

108

COPPER

Determination of nickel. Evaporate the filtrate on a water-bath and then on a sand bath until sulphuric acid fumes appear. Mix the residue with 10 ml of water, oxidize colloidal sulphur with 10 ml of saturated bromine water, evaporate the excess bromine, and filter the solution into a 400-ml beaker. Wash the filter paper with large amounts of hot water. Dilute the solution to about 150 ml, neutralize with 1 : 1 ammonium hydroxide in the presence of phenolphthalein indicator, add a further 2 ml of ammonia in excess, and then make just acid with acetic acid. Heat the solution to boiling, and precipitate TABLE 8.12. Cu-Ni separation according to Chapter 8.20.b

Cu (CuO)

mean:

Amount of Cu determined mg

True value Cu mg

Deviation from true value Δ%

314-8 315-5 315-0

3145

+0-2

315-1 105-1 103-9 104-1

mean:

mean:

104-2

10-3

mean:

Deviation from true value Δ%

31-9

+0*6

96-2

-0-1

96-2

±0-0

96-1 96-2 96-2 96-1

+2-0

True value Ni mg

32-1 96-1 96-0 96-2

+0-2

mean:

10-1

Amount of Ni determined mg

32-0 32-1 321 mean:

104-4 10-3 10-3 10-4

Ni (Ni-dimethylgiyoxime)

96-2

nickel by the addition of excess 1% alcoholic dimethylglyoxime solution. Cover the mixture with a watch glass, allow to stand for 30 min on a water-bath, and then for 2 hr in the cold, and collect the precipitate on a glass or porcelain filter. Wash with hot water. Dry the precipitate for 2 hr at 120°C, cool and weigh. Weighing form: Ni(C4H7N"202)2. Note. The determination gives excellent results as seen in the data of Table 8.12. (measurements of G. Siposs).

SEPARATIONS

109

Other methods for Cu—Ni separation: (c) with benzoin oxime from a solution containing Seignette-salt, 1 (d) with quinaldic acid, 2 (e) with salicylaldoxime, 3 (f) with thioanalide, 4 (g) with mercaptobenzthiazole, 5 (h) copper in t h e form of CuSCN a n d nickel in t h e form of nickel dimethylglyoxime. 6 8.21. Cu—Zn (a) I n general copper can be separated from t h e cations of group(III) b y precipitation with hydrogen sulphide. I n t h e presence of zinc, however, the precipitate becomes contaminated with zinc sulphide owing t o post-precipitation (see Chapter 2.7.3.4.). I n t h e presence of Co, Ni a n d F e t h e danger of post-precipitation is much less. The post-precipitation of zinc sulphide can be avoided b y precipitating copper(II) sulphide from a strongly acidic, hydrochloric acid solution. The precipitation of copper(II) sulphide can be made complete b y dilution and repeated saturation of t h e solution with hydrogen sulphide. B y this method only copper(II) sulphide, which is last to precipitate, becomes contaminated with zinc sulphide. Procedure. Add to the solution, containing copper and zinc, enough hydrochloric acid to make the overall concentration of hydrochloric acid about 5%. Heat the solution to boiling, remove the source of heat, and pass gaseous hydrogen sulphide for 30 min. Most of the copper is precipitated in the form of copper(II) sulphide. Dilute the solution with hot water to twice its volume, stir continually, and pass hydrogen sulphide for a further 30 min. During this period even the last traces of copper precipitate. Collect the precipitate on filter paper, wash with 2 N hydrochloric acid saturated with hydrogen sulphide until most of the zinc is removed from the precipitate, and continue washing with 2% acetic acid solution saturated with hydrogen sulphide. Where particularly accurate analyses are required the copper(II) sulphide should be reprecipitated. Rinse the bulk of the copper(II) sulphide precipitate into a porcelain evaporating dish with a fine water-jet. Dissolve the remainder of the precipitate from the filter paper with 10 ml of hot, dilute nitric acid (1 : 1), and transfer the filtrate to the dish into which the main part of the precipitate has been placed. Combust the filter paper, dissolve the residue with several drops of concentrated nitric acid and rinse this into the porcelain dish. Evaporate the contents of the porcelain dish to dryness on a water-bath, and repeat the evaporation several times with concentrated nitric acid. Then convert the nitrate to chloride by three evaporations with diluted hydrochloric acid (1 : 1), and dissolve the residue in 25-30 ml of diluted hydrochloric acid ( 1 : 1 ) and 70 ml of hot water. Repeat the precipitation of copper(II) sulphide from this solution using hydrogen sulphide according to the former procedure. Wash, roast the copper(II) sulphide precipitate on a gas flame, and ignite at 850-950°C for 1-3 hr. Weighing form: CuO. 1 2 3

4

F. FEIGL, Ber., 56, 2083 (1923). P. RAY and M. K. BÖSE, Z. anal. Chem., 95, 400 (1933). F. EPHRAIM, Ber., 63, 1928 (1930).

R. BERG and W. ROEBUNG, Z. angew. Chem., 48, 597 (1935). G. SPACU and M. KUBAS Z. anal. Chem., 102, 24 (1935). 6 M. L. RIVOT, Compt. rend., 38, 868 (1854).

5

110

COPPER

Precipitation of zinc from the filtrate. Evaporate the combined filtrate to remove excess hydrochloric acid, and determine zinc by one of the simple gravimetric methods recommended in Chapter 24. Weighing forms: ZnS -*· ZnO, ZnNH 4 P0 4 , Zn 2 P 2 0 7 . (b) The precipitation of copper sulphide can also be carried out with sodium thiosulphate from a strongly acidic, sulphuric acid solution. After t h e oxidation of colloidal sulphur, zinc ammonium phosphate can be precipitated from the filtrate with diammonium hydrogen phosphate. Determination of copper. Dilute the slightly acidic solution, which contains copper and zinc ions, to 100 ml and add sodium thiosulphate solution until the solution is de-colourized. Acidify the solution with 8 ml of 6 N sulphuric acid and heat to boiling, stirring continually with a glass rod. Cover the mixture with a watch glass, boil for 15 min, and collect the precipitate from the hot solution on a filter paper. Wash with hot water until an acid reaction can no longer be obtained. Dry the precipitate at 100°C, combust the filter paper TABLE 8.13. Cu-Zn separation according to Chapter 8.21.b

Cu (CuO)

mean:

Amount of Cu determined mg

True value Cu mg

315-4 315-6 3145

314-5

20-9

mean:

20-4

mean:

35-4

Deviation from true value Δ%

+0-3

143-2

+0-06

2153

+0-05

143-3 215-2 2155 215-4

+2-5

True value Zn mg

35« 5 1433 143-2 1433

+0-5

157· 1 20-8 21-4 20-5

mean:

mean:

1563

Amount of Zn determined mg 35-5 35.4 35-6

+0-2

315«2 156-6 157-8 156-9

mean:

Devia- 1 tion from Zn true (ZnNH4P04) value Δ%

215-4

alone in a porcelain crucible, introduce the bulk of the precipitate into the crucible, and roast carefully on a gas flame. Finally ignite the oxide at 850 to 950°C for 1-3 hr. Cool and weigh as CuO. Determination of zinc. Evaporate the filtrate on an air-bath until sulphuric acid fumes appear, cool and add 10 ml of water and 10 ml of saturated bromine water, mix thoroughly and evaporate the excess bromine on a water-bath. Filter the solution, wash the sulphur on the filter with hot water until the

111

SEPARATIONS

acidic reaction disappears in the filtrate, and dilute the filtrate to 90 ml. Cool the solution, neutralize with concentrated ammonium hydroxide in the presence of methyl orange, and then acidify with 1 drop of 6 N sulphuric acid. Add 2 g of ammonium chloride, heat to boiling, remove the source of heat and precipitate zinc ammonium phosphate by the addition of a fine stream of 20% diammonium hydrogen phosphate solution with constant stirring (for the precipitation of 0-1 g of Zn, 10 ml of 20% diammonium hydrogen phosphate solution should be used). Allow the mixture to settle on a water-bath for 30 min, and then allow it to cool for 3-4 hr. Filter in the cold through a G3 glass or A2 porcelain filter crucible. Wash with 1% diammonium hydrogen phosphate solution until the chloride reaction disappears and finally wash 3 times with cold water. Dry at 100 °C for 2 hr and weigh as Zn(NH 4 )P0 4 . TABLE 8.14. Cu-Zn separation according to Chapter 8.21.c Cu (with electrolysis)

mean:

Amount of Cu measured mg

True value Cu mg

Deviation from true value Δ%

315-4 313-9 314-0

314-5

-0-03

mean:

20-5

mean:

20-4

mean:

35-4

Deviation from true value Δ%

+2-2

143-2

+0-43

143-2

+0-49

143-8 1437 144-0 144-1

+0-5

True value Zn mg

36-2 1441 1438 1435

+0-45

157-0 20-4 20-6 20-5

mean:

156-3

Amount of Zn measured mg 35-9 36-6 36-2

mean:

314-4 156-8 157-2 157-0

Zn (with electrolysis)

143-9

Note. The method is most suitable for the separation of large amounts of copper from smaller amounts of zinc, as can be seen from the data of Table 8.13. (measurements of G. Siposs). If the amount of copper is large, the copper(II) oxide must be fused with potassium pyrosulphate and the precipitation of copper(I) sulphide repeated from this solution. (c) By electrolysis. Copper can be deposited electrolytically from t h e chloride-free sulphuric acid solution of t h e metal sulphates (Chapter 8.1.1.). Zinc can t h e n be deposited cathodicalty after making the solution strongly alkaline with sodium hydroxide (Chapter 24.3.).

112

COPPER

Note, For the accuracy of this method see the data in Table 8.14. (measurements of G. Siposs). Other methods for the Cu-Zn separation: (d) CuCNS-Zn (Chapter 8.2.,), (e) C u I - Z n ( N H 4 ) P 0 4 (Chapter 8.4.), 1 (f) in t h e form of their pyridine thiocyanate complexes, 2 (g) b y precipitating zinc selectively in t h e form of its quinaldinate, a n d copper chelated in t h e form of its thiourea complex (see Chapter 24.11.), (h) b y t h e evaporation of metallic zinc a n d cadmium in a vacuum from t h e alloy. 3 8.22. Cu-Mn (a) Separation with hydrogen sulphide. Procedure. Add 5 ml of concentrated sulphuric acid to the nearly neutral solution, dilute t o about 200 ml, heat t o boiling, remove the heat source and pass hydrogen sulphide gas until the solution becomes clear and the copper(II) sulphide precipitate settles. Collect the precipitate on an ash-free filter paper, and wash with 2% acetic acid saturated with hydrogen sulphide, until the sulphate reaction disappears in the filtrate. Ensure t h a t the precipitate is always covered with the solution during filtration and washing. Dry the precipitate a t 100°C, and transfer it without loss t o a weighed porcelain crucible. Combust the filter paper above the crucible, roast the precipitate on a gasflame, and finally ignite in an electric furnace at 850-950 °C for 1-3 hr. Cool and weigh as CuO. Determination of manganese. Evaporate the filtrate to one quarter of its original volume to remove hydrogen sulphide, and dilute t o about 150 ml. Add 5 g of ammonium chloride and neutralize the solution with 1 : 2 ammonium hydroxide using methyl red, and finally make just acid with several drops of 2 N sulphuric acid. Add 20 ml of 10% diammonium hydrogen phosphate to the hot solution and neutralize again with 2 N ammonium hydroxide. Allow the mixture to stand for 1 hr on a water-bath and then over-night on a cold place. Filter in the cold through a G4 glass or A2 porcelain filter crucible. Wash with 50 ml of a saturated aqueous or 60% alcoholic solution of Μ η Ν Η 4 Ρ 0 4 · Η 2 0 and finally with 10-12 ml of alcohol. Dry at 100°C for 2 hr. Weighing form: ΜηΝΗ 4 Ρ0 4 · Η 2 0 . Note. The separation gives good results, as seen from the data of Table 8.15. (measurements of F. Szabadvâry). (b) Separation with 8-hydroxyquinoline. Procedure. Add 20 ml of glacial acetic acid and 4 g of sodium or ammonium acetate t o the nearly neutral solution of the metal ions, and dilute to about 100 ml. Heat the solution t o 50-60 °C, add a 3% alcoholic solution of 8-hydroxyquinoline with constant stirring until the colour of the supernatant liquid becomes yellow. Heat the solution to 80-90 °C and allow it to stand on a water-bath for 15 min. Filter while hot, through a G3 1 2 3

H. BRINTZINGER, Z. anal. Chem., 86, 157 (1931). G. SPACU and J. DICK, Bulet. Soc. Stiinte Cluj, 1, 284 (1922); Z. anal. Chem. 64, 330 (1924).

H . BOGDÀNDY and M. POLANYI, Z. f. Metallkunde., 19, 164 (1927).

113

SEPARATIONS

glass or A2 porcelain filter crucible. Wash with hot or cold water, until the filtrate becomes colourless. Dry at 110°C for 2 hr. Weighing form: Cu(C 9 H e ON) 2 . Stoichiometric factor: Cu/Cu(C 9 H 6 ON) 2 = 0-18059. TABLE 8.15. Cu-Mn separation according to Chapter 8.22.a

Cu (CuO)

mean:

Weight of CuO precipitates mg

True value CuO mg

125-0 124-8 125-4

125-6

mean:

mean:

25-1

mean:

126-1

125-6

mean:

Deviation from true value Δ%

504-2

-0-6

504-2

-0-34

100-8

-2-2

502-5 98-3 97-6 99-9

+0-4

True value mg

501-2 506-1 500-4 501-1

+3-2

25-9 126-3 128-4 125-6

502-4 500-4 500-9

-0-25

125-0 23-7 26-4 27-8

mean:

Weight Deviaof tion from (MnNH4P04. Mn •H 2 0) true (MnNH4P04 •HaO) precipivalue t ates Δ% mg

98-6

Determination of manganese in the filtrate. Add 0-1-0-2 g of hydroxylamine hydrochloride to the filtrate, neutralize the solution with 2 N ammonium hydroxide in the presence of phenolphthalein and finally acidify with 2 N acetic acid until the red colour just disappears. Add to the warm solution (50-60°C) 20 ml of a 3% alcoholic solution of 8-hydroxyquinoline. Test for complete precipitation. Heat the mixture to 80-90 °C and allow it to stand on a water-bath for 15 min. Filter on a G3 glass or A2 porcelain filter crucible. Wash with hot water. Dry at 150°C for 2 hr. Weighing form: Mn(C 9 H 6 ON) 2 Note. The method is fairly rapid and in spite of the low precision it yields satisfactory results (see Table 8.16., measurements of F. Szabadvâry). (c) Copper can be deposited electrolytically from a chloride-free, sulphuric acid solution b y electrolysis (see Chapter 8.1.1.), a n d manganese can be precipitated from t h e electrolyte b y one of t h e simple gravimetric methods (see Chapter 25.). Note. F. Szabadvâry obtained the following results for copper in the presence of approximately equal amounts of manganese: 20-9, 20-4, 20-2 mg. True value: 20-1 mg.

114

COPPER

Other methods for the Cu-Mn separation: (d) CuSCN-Mn(H) (Chapter 8.2.), (e) Cu2S with Na 2 S 2 0 3 , similar to the Cu-Zn separation (see Chapter 8.2.I.D.).

TABLE 8.16. Cu-Mn separation according to Chapter 8.22.b DeviaDeviaWeight of Weight of Cu(aHeON)a T r u e tion from Mn(CeHeON)8 True tion from Mn Cu true value value true precipiprecipi[Cu(C,H,ON)t] ΓΜη(0,ΗβΟΝ),] value mg tate mg value tate mg Δ% mg 275-2 278-1 279-9 mean:

277-7

277-9

469-3 474-4 478-5

— 0-06

mean:

465-4

+ 1-7

474-0

8.23. Cu-Te Neutralize the solution with ammonia, add a small excess of ammonium sulphide, dilute to 200 ml and dissolve enough solid potassium cyanide in the mixture to dissolve the precipitate completely. Heat the solution to boiling in a fume-cupboard, add 10-15 ml of freshly saturated sodium sulphite solution and boil for 20 min. Collect the precipitate in a filter crucible, wash with hot water, dry at 110°C and weigh. Weighing form: Te. Determination of copper in the filtrate. Acidify the solution with sulphuric acid in a well-ventilated fume-cupboard, and boil until the copper(I) sulphide precipitates completely. Saturate the solution with hydrogen sulphide, collect the precipitate on a filter paper, and wash with 2% acetic acid saturated with hydrogen sulphide until sulphate can no longer be detected in the filtrate. Roast the precipitate to the oxide in a porcelain crucible, and ignite at 850 to 950°C for 1-3 hr. Weighing form: CuO.

REFERENCES to Table 8.1. 1. W. GIBBS, Z. anal. Chem., 3, 334 (1864); C. LUCKOW, Z. anal. Chem., 8, 23 (1869); A. CLASSEN, Quant. Anal, durch Electrolyse, 3rd ed. Springer, Berlin, 1892, p. 78; F. FÖRSTER, Ber., 39, 3029 (1906); Z. Electrochem., 14, 90, 208 (1908); 15, 232 (1909); 32, 525 (1926); F. FÖRSTER, Electrochemie wässriger

REFERENCES

115

Lösungen, Barth, Leipzig 1923. H . E L P H E , Z, anal, Chem., 126, 241 (1934); F . H E C H T and J . DONAU, Anorg, Mikrogewichtsanal, Springer, Wien, 1940, p. 161; W. D. TREAD WELL, Elektroanalytische Methoden, Borntraeger, Berlin, 1915, p . 65. 2. M. L. RIVOT, Compt, rend,, 38, 205 (1918); I. M. KOLTHOFF and G. H . P .

M E E N E , Z, anal, Chem,, 72, 337 (1927); J . BODNAR and V. TOLNAY, Z, anal,

Chem., 120, 336 (1940); R. BELCHER and T. S. WEST, Anal Chim. Ada, 6, 337 (1952).. 3. H . ROSE, Ann. Physik, 110, 138 (1860); G. VORTMANN, Z. anal, Chem,, 20, 416 (1881); A. ORLOVSZKY, Z. anal. Chem., 21, 215 (1882); C. HOLTHOFF, Z. anal. Chem,, 28, 680 (1889); F . L. HAHN, Ber, 63, 1616 (1930); E . BRENNECKE, Schwefelwasserstoff als Reagens in der quant. Analyse, Enke, Stuttgart, 1939. p . 19. 4. W. GIBBS, Z. anal. Chem., 7, 258 (1868); W. WAUBEL, Z. angew. Chem., 22, 1716 (1910); F . P . TREAD WELL, Lehrbuch der anal. Chem., H . l l t h ed. Deuticke, Wien, 1949, p . 151. 5. F . PISANI, Compt. rend., 47, 294 (1858); L. W. WINKLER, Z. anal. Chem., 63, 352 (1923). 6. E . RIEGLER, Z. anal. Chem., 43, 214 (1904). 7, 8. A. RECOURA, Bull. soc. chim. France, [4] 7, 832 (1910); I. MARIN and C. DUVAL, Anal. Chim. Acta, 6, 57 (1952). 9. G. REICHINSTEIN, Zavodskaya Lab., 15, 1407 (1949).

10. H . F U N K and M. D I T T , Z. anal. Chem., 93, 241 (1933); P . W E N G E R and

Z. BESSO, Mikrochemie, 29, 240 (1941); D. GUNEV, Khim. i Ind., 20, 170 (1941); C. A., 37, 5331 (1943).

H . H . BILTZ and E. HÖDTKE, Z. anorg. Chem., 66, 426 (1910); J . H A N U S and

A. SOUKUP, Z, anorg. Chem., 68, 52 (1910); H . WEBER, Z. anal. Chem., 50, 50 (1911); R. FRESENIUS, Z. anal. Chem., 50, 35 (1911); S. A. BRALEY, J. Ind. Eng. Chem., 11, 1144 (1919); I. BELLUCCI and A. CHIUOINI, Gazz. chim. ital., 49, 187 (1919). 12. O. BAUDISCH and S. HOLMES, Z. anal. Chem., 119, 241 (1940). 13. P . RAY and M. K. BÖSE, Z. anal. Chem., 95, 400 (1933); T. K I B A and S. SATO,

J. Chem. Soc. Japan,

61, 133 (1940); C. A,, 34, 4687 (1940).

14. A. K. MALLIK and A. K. MAJUMDAR, Sei, and Cult,, 14, 477 (1949); C,

A,,

43, 6935 (1949). 15. F . EPHRAIM, Ber,, 63, 1928 (1930); 64, 1210 (1931); W. R E I F , Mikrochemie N. S. 3, 424 (1931); F . HECHT and R. REISSNER, Mikrochemie, 17,127 (1935); M. JEAN, Bull, soc, chim, France, 10, 201 (1943); L. DUCRET, Bull. soc. chim. France, 12, 880 (1945). 16. F . FEIGL, Ber., 56, 2083 (1923); R. STREBINGER, Mikrochemie, 1, 72 (1923); L. SILVERMAN, Ind. Eng. Chem. Anal. Ed., 12, 343 (1940); I. R. SHIK, Zavodskaya Lab., 9, 542 (1940); C, A„ 34, 7204 (1940). 17. R. BERG, Z. Anal. Chem., 70, 341 (1927); J . CALVET, Compt. rend., 195, 148 (1932); H . R. FLECK and A. M. WARD, Analyst, 58, 388 (1933); I. R. SHIK, Zavodskaya Lab., 4, 1161 (1935); C. A., 21, 1778 (1927). 18. R. BERG and W. ROEBLING, Z. angew. Chem., 48, 597 (1935); T. UNEMURA,

J. Chem. Soc. Japan,

61, 25 (1940); <7. A., 34, 3616 (1940).

19. G. SPACU and M. K U R A S , Z. Anal. Chem., 102, 24 (1935); M. K U R A S , Coll.

Czech. Chem, Commun., 11, 367 (1939).

116

COPPER

20. P . SPACU, Z. anal Chern., 115, 423 (1939); S. H . C. BRIGGS, Z. Anorg. Ohem., 56, 260 (1908). 21. G. SPACU, BulL Soc. Stiinte. Cluj., 1, 284 (1922); C. A , 17, 1772 (1923); G. S P A C U a n d J .

DICK,

Z. anal.

Chem.,

7 8 , 241

(1929); J . T. D O B B I N S

and E . S . GILREATH, J. Chem. Educ., 2 2 , 119 (1945); C. ^ . . , 3 9 , 1819 (1945).