The determination of cobalt in the water and waste water

Water Research, Pergamon Press 1967. Vol. 1, pp. 695-715. Printed in Great Britain.

THE DETERMINATION OF COBALT IN WATER A N D WASTE WATER K. BENFORD, MARIANNE GILBERT a n d S. H. JENKINS Chemists Department, Upper Tame Main Drainage Authority 156 Newhall Street, Birmingham 3, England (Received 6 September 1967) Abstract--Two suitable alternative methods are described for the determination of cobalt in water or waste water. In the first method the cobalt is complexed with 3-methoxynitrosophenol and the absorbance of its chloroform extract measured at 410 mg. Zinc, cadmium and lead do not interfere. Interference by nickel and copper is prevented using the procedure described. Using a slight modification, interference by iron is greatly reduced. Up to a certain limit Cr 6+ can be tolerated. The tolerance limit for Cr 3+ is higher than that of Cr 6+. With the second method the cobalt is complexed with ammonium thiocyanate, the complex extracted with methyl isobutyl ketone and the absorbance of the extract measured at 620 m/t. Zinc, lead, cadmium, nickel, chromium as Cr a + and Cr 6 + can be tolerated at high concentrations without causing interference. Interference caused by copper is prevented by reducing it to the cuprous state and then precipitating it as the iodide, while iron interference is prevented by the addition of chromate to precipitate the ferrous or ferric iron. The first method is suitable for cobalt concentrations of up to 3 mg/l. With the second method it is possible to use up to 10 mg of cobalt/1. COBALT r a r e l y occurs in n a t u r a l waters in c o n c e n t r a t i o n s sufficient to affect p l a n t s o r animals. It m a y occur infrequently in effluents f r o m the t r e a t m e n t o f m e t a l s a n d its presence in waters is likely to result f r o m c o n t a m i n a t i o n with such effluents. C o n sequently the e l e m e n t is to be expected in a s s o c i a t i o n with o t h e r metals whenever it is f o u n d in water. F o r this reason, when a m e t h o d suitable for the d e t e r m i n a t i o n o f c o b a l t in w a t e r was required, a t t e n t i o n h a d to be given to the possibility o f interference b y those metals likely to occur with cobalt. METHOD T h e m o s t p r o m i s i n g m e t h o d a p p e a r e d to be t h a t described b y JOHNSON (1964) b a s e d on the o b s e r v a t i o n b y TORn (1955) t h a t the c o m p l e x f o r m e d b y c o b a l t a n d 3-methoxyn i t r o s o p h e n o l m a y be used for the q u a n t i t a t i v e d e t e r m i n a t i o n o f cobalt. T h e following p r o c e d u r e was carried o u t in o r d e r to s t u d y the effect o f possible interference b y o t h e r metals. A k n o w n v o l u m e o f solution c o n t a i n i n g n o t m o r e t h a n 30 pg o f c o b a l t was placed in a 100 ml s e p a r a t i n g funnel a n d distilled w a t e r a d d e d to b r i n g the v o l u m e u p to 10ml. One d r o p o f 5 N h y d r o c h l o r i c acid was a d d e d , followed b y 5 m l o f 3 - m e t h o x y n i t r o s o p h e n o l (0.05 g dissolved in h o t water, which was then cooled, filtered a n d m a d e u p to 100 ml) a n d 5 ml o f a m m o n i u m citrate solution (50 g t r i - a m m o n i u m citrate dissolved in I00 ml distilled water). T h e m i x t u r e was then allowed to stand for a b o u t 5 min in o r d e r to ensure c o m p l e t e f o r m a t i o n o f the water-insoluble c o b a l t - 3 - m e t h o x y n i t r o s o p h e n o l complex. T h e n 5 ml o f c h l o r o f o r m were a d d e d a n d the mixture was s h a k e n E W

695

696

K. B~h'FORD,MARIANNEGILBERTand S. H. JENKINS

until the aqueous upper layer was green in colour. So long as the aqueous upper layer remained orange in colour the extraction was incomplete. The chloroform layer was then run into a second separating funnel. The aqueous layer was again extracted by shaking briefly with another 5 ml of chloroform. The chloroform was then added to the first extract and the aqueous layer discarded. The combined chloroform extracts were washed for about 5 min with 10 ml sodium carbonate (10 g anhydrous sodium carbonate dissolved in 100 ml distilled water) in order to remove the excess reagent. After separation of the two layers the lower, chloroform layer was run into a 25 ml volumetric flask. The aqueous sodium carbonate

3C Cobalt present /~g In

25

2S~of 2C

15

IC S

C

I0

20 30 Reod~ on Eel a b s o q ~ t ~ -

40

50

FIo. 1. Calibration graph for cobalt using 3-metboxynitrosophenol and 1 cm cell. Absorbance at 410 m/z using an EEL absorptiometer. was washed with 5 ml of chloroform, which, after separation of the layers, was run into the volumetric flask. The volume was then made up to the 25 ml m a r k with chloroform. The O.D. of the solution was measured in a 1 cm cell at a wavelength of 410 m/z, with chloroform in the compensating cell. Although the optimum absorbance is stated to be 375 m# the authors think there is some advantage in selecting a wavelength attainable on ordinary absorptiometers. Using amounts of cobalt in solutions containing from 0 to 30 #g it was found that the relationship between O.D. and cobalt concentration was a straght line which passed very close to the point of origin, as shown by FIG. 1. It was also found that at laboratory temperatures the cobalt complex in chloroform remained stable for about 2 hr. The 3-methoxynitrosophenol solution remained stable for about 3 weeks, after which the blank value on the reagents appeared to increase. The optical density of a given solution of the cobalt complex in chloroform was measured at different wavelengths between 410 and 685 m/~, which was the range of the E E L instrument used. Under these conditions m a x i m u m absorbance was exhibited at 410 m#. It should be pointed out that in a reference to this method (JOHNSON, 1964) the m a x i m u m absorbance is given as 375 m/~. EFFECT OF O T H E R METALS ON THE D E T E R M I N A T I O N The effects of copper, nickel, iron as Fe 2 + and Fe 3 +, chromium as Cr 3 + and Cr 6 +,

The Determination o f Cobalt in Water and Waste Water

697

lead and cadmium were investigated. The amount of cobalt taken was 20 pg and this was mixed with 20,000/~g of each metal, unless the metal interfered with the analysis: when interference occurred the amount of metal taken was reduced in order to determine the maxium concentration that had no effect on the determination of the cobalt. Where interference by a particular metal was found methods of reducing the interference were investigated.

Effect of copper Copper added in solution was found to interfere seriously with the method. TABLE 1 shows that interference occurred when the ratio of copper to cobalt exceeded 1 : 1. TABLE | . EFFECT OF COPPER ON THE DETERMINATION OF COBALT USING 3-METHOXYNITROSOPHENOL

Cobalt present

Copper present

Cobalt found

in sample analysed

in sample analysed

by analysis

(/./g)

(/2g)

(//g)

20 20 20 20

0 20 100 200

20 20 27.5 33'5

To reduce the interference by copper the following six methods were tried. 1. Two calibration curves were prepared, one from solutions which all contained 5 mg Cu/l but with increasing concentrations of cobalt; the other, a similar curve but with 10 mg Cu/l. Although the relationship between absorbance and cobalt concentration was approximately linear the lines were n o t parallel with each other or with the line obtained in the absence of copper. Consequently, it would have been necessary in any analysis of cobalt by this method to have known the exact copper concentration and to have a calibration curve for every copper concentration. 2. An attempt was made to mask the effect of copper by addition of 10 ml of a 10 per cent w/v aqueous solution of thiourea to the a m m o n i u m citrate used in the determination. It was found necessary to allow the aqueous reagents to stand for 15 min before adding the chloroform. By this method up to 10 times as much copper as cobalt could be tolerated. However, erratic results were obtained so that this method was abandoned. 3. An attempt to remove copper interference by forming the cuprammonium ion Cu(NH3), 2+ was made by adding an excess of 0.880 ammonia solution, dropwise, until a stable blue colour was obtained. The basic procedure was then followed, omitting the addition of HCI so as not to destroy the cuprammonium ion. However, on shaking with chloroform, no colour was formed in the solventlayer. It was evident that the p H must be below 7 for a stable complex of cobalt and methoxynitrosophenol to be formed. 4. An effort was also made to complex the copper with glycine but it was found that

698

K . BENFORD,

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on addition to the HC1 and a m m o n i u m citrate the copper-glycine complex was broken up and the characteristic bluish-green colour of copper reappeared in the solution. 5. The copper was also complexed with sodium diethyldithiocarbamate (dithizone), a reagent that forms complexes both in acid and alkaline conditions. After acidifying the mixture of cobalt and copper with HC1, a few drops of a 1 per cent aqueous solution of dithizone were added. The complex of copper formed with the dithizone was completely removed from the aqueous solution by extraction with chloroform. However, on completing the determination of cobalt in the aqueous solution it was found that cobalt was absent, probably by co-precipitation as cobaltous diethyldithiocarbamate.

TABLE2. EFFECT OF CITRIC ACID IN REDUCING THE INTERFERENCE CAUSED BY COPPER IN THE DETERMINATION OF COBALT USING 3-METHOXYNITROSOPHENOL

Cobalt present in sample

Copper present in sample

(~g)

~g)

Cobalt found by analysis (m)

20 20 20 20 20

0 200 1000 2000 5000

20 20 20 19 6

6. A second extraction with citric acid was also tried in order to remove copper. After the cobalt-3-methoxynitrosophenol complex had been extracted with chloroform and the two chloroform extracts combined in a 100 ml separating funnel, 5 ml of a solution of 5 g citric acid in 100 ml distilled water was added to the chloroform extract and the mixture was shaken for about 5 min. The chloroform layer was run into a third separating funnel, whilst the citric acid layer was washed with another 5 ml of chloroform and shaken briefly. The chloroform layer was run into the combined extracts in the third separating funnel and washed with sodium carbonate solution as described in the basic method. The remainder of the procedure was the same as that described previously. The results are given in TABLE 2. It will be seen from TABLE 2 that by washing the chloroform extracts with citric acid it is possible for cobalt to be determined in the presence of between 50 and 100 times as much copper as cobalt.

Interference by nickel When the basic procedure was used in the presence of nickel salts it was found that up to 200/tg of nickel could be tolerated in the presence of 20/~g of cobalt. With a 50fold excess the interference was severe.

The Determination of Cobalt in Water and Waste Water

699

The method adopted for reducing interference by nickel was to give a second extraction with citric acid, as previously described in paragraph (6) above. The results obtained are given in TABLE 3, which also includes the results when this procedure was omitted. TABLE 3. EFFECT OF CITRIC ACID IN REDUCING THE INTERFERENCE CAUSED BY NICKEL IN THE DTERMINATION OF COBALT USING 3-METHOXYNITROSOPHENOL

Cobalt present in sample

Nickel present in sample

Cobalt found by analysis

(gg)

(,ug)

(Pg)

Omiting the extraction with citric acid 20 0 20 20 20 200 20 1000

20 20 21 25

Including the extraction with citric acid 20 0 20 1000 20 2500 20 5000 20 I0,000 20 20,000

20 20 20 20 19 16

TABLE 3 shows that the citric acid extraction procedure makes it possible for 250-500 times as much nickel as cobalt to be tolerated. Two alternative methods were considered to prevent nickel interference (1) by precipitating it with dimethylglyoxime or (2) with 8-hydroxyquinoline. However, dimethylglyoxime necessitates the use of an ammoniacal solution, which it was previously shown prevented the subsequent determination of cobalt. The disadvantage of using 8-hydroxyquinoline was that it would co-precipitate cobalt as well as nickel as the oxinate. Consequently no further work along these lines was attempted. Interference by iron, F e 3 +, and Fe 2 +

It was found that at low concentrations iron a s Fe 3 + interfered with the determination of cobalt by giving an enhancement of the colour of the cobalt complex. A second extraction with citric acid had the effect of reducing the interference but not sufficiently for the authors' purpose. The interference by Fe 2 + was caused by the formation of a green complex with 3-methoxynitrosophenol. To reduce interference from this source 1 ml of 20 vol. hydrogen peroxide was added to the mixture of cobalt and iron solutions and after

700

K . BENFORD,

MARIANNEGILBERTand S. H. JENKINS

mixing, in order to ensure oxidation of Fe 2 + to Fe 3 +, the reagents were added as described in the basic method. The interference caused by ferrous iron was thereby reduced but this, of course, did not affect the interference caused by the ferric iron. Complexing the iron with triethanolamine was also tried: hydrogen peroxide was added to the cobalt and iron solution as before and this was followed by the addition of 5 ml of 50 per cent triethanolamine (a) before and (b) after the addition of the 3-methoxynitrosophenol. The interference due to iron was satisfactorily reduced but TABLE4.

EFFECT OF IRON ON THE DETERMINATION OF COBALT USING 3 -METHOXYNITROSOPHENOL

Cobalt present in sample (m)

Iron present in sample (m)

Cobalt found in sample (ag)

(a) Using the basic method, without precautions to prevent inferrefence by iron 20 0 20 20 20 22 20 200 40 Co)Oxidizing the iron with H202 and adding the reagents in the order described in the text 20 0 20 20 1000 20 20 5000 21 20 10,000 22 20 20,000 22'5

as the colour of the cobalt complex was also reduced, this method of preventing interference was not pursued. The formation of complexes of iron as Fe 3 ÷ with 8-hydroxyquinoline, thioglycollic acid and o-phenanthroline was also tried in order to prevent interference by iron. Investigation showed that these reagents also formed stable complexes with cobalt, so that no cobalt-methoxynitrosophenol complex, extractable by chloroform, was formed. With iron the most successful method for overcoming interference was found to be to oxidize any ferrous iron in the acidified sample to the ferric state and to follow this by adding a m m o n i u m citrate and 3-methoxynitrosophenol in the following order: (1) Cobalt and iron in aqueous solution; (2) 1 drop of 5 N HC1; (3) 1 ml of 20 vol. H 2 0 2; (4) 5 ml of a m m o n i u m citrate; and (5) 5 ml of 3-methoxynitrosophenol. This mixture was allowed to stand for 5 min and the extraction procedure already described was then followed. The results obtained in the presence of iron when no precautions were taken to prevent interference, and when the procedure described in the previous paragraph was used, are shown in TABLE 4.

701

The Determination of Cobalt in Water and Waste Water

The results in TABLE 4 show that by this modification the determination of cobalt using 3-methoxynitrosophenol is satisfactory in the presence of 250 times as much iron as cobalt. Interference by zinc, lead and cadmium To separate solutions containing 20 #g of cobalt increasing amounts of zinc or lead or cadmium were added and the concentration of cobalt was determined using the basic procedure. The results are given in TABLE5. TABLE 5. EFFECT OF ZINC~ LEAD AND CADMIUMON THE DETERMINATIONOF COBALT USING 3-METHOXYNITROSOPHENOL IN THE BASIC PROCEDURE

Interfering metal

Cobalt present in sample (fig)

Interfering metal present in sample (fig)

Cobalt found by analysis (/.tg)

Zinc

20 20 20 20 20 20 20

0 20 200 1000 2000 10,000 20,000

20 20 20 20 20 20 20

Lead

20 20

0 20,000

20 20

Cadmium

20 20 20

0 10,000 20,000

20 20 19

The conclusion drawn from TABLE 5 is that cobalt can be determined satisfactorily in the presence of a 1000-fold excess of zinc, lead and cadmium. Effect of chromium as Cr 3 ÷ and Cr 6 ÷ This was found by adding increasing amounts of chromium in the trivalent form and as hexavalent chromium to separate solutions containing 20 pg of cobalt. The results are given in TABLE6. These show that whereas up to a 1000-fold excess of Cr 3+ could be tolerated, Cr 6 + already showed interference when a 25-fold excess of chromate was present. This suggested that interference by chromate would be avoided by reducing the chromate to trivalent chromium before analysis. To test the effect of reducing agent on the determination a solution of 15 per cent of titanium chloride in water was added in increasing amounts to solutions containing 20 #g of cobalt, and the basic method of analysis was carried out. The results showed that the titanium interfered, probably by formation of a complex with the methoxynitrosophenol, giving high figures for cobalt on analysis.

K. BENFORD,MARIANNE GILBERT and S. H. JENKINS

702

To consider whether it would be possible to separate the cobalt complex from the titanium Complex (if one were formed), the basic method of analysis was carried out on a solution containing cobalt to the point of forming the complex of cobalt and TABLE 6,

EFFECT OF CHROMIUM AS C r 3+ AND C r 6+ ON THE DETERMINATION OF COBALT USING 3-METHOXYNITROSOPHENOL IN THE BASIC PROCEDURE

Chromium as Cr 6 ÷

Chromium as Cr 3 + Cobalt present in sample (gg) 20 20 20 20 20 20 20

C& ÷ present in sample (gg) 0 20 200 1000 5000 10,000 20,000

Cobalt found by analysis (gg)

Cobalt present in sample (gg)

20 20 20 20 20 20 22

20 20 20 20 20

Cr 6 ÷ present in sample (gg) 0 250 500 1000 10,000

Cobalt found by analysis (fig) 20 20 19 18 17

3-methoxynitrosophenol and then extracting it with chloroform. The titanium salt in solution was then added to the aqueous residue and this was re-extracted with chloroform. This modification had hardly any effect in reducing the interference caused by the titanium. It was concluded that titanous chloride is not a suitable reductant for chromate in this method of determining cobalt. A second method of reducing chromate by sodium sulphite was tried. To begin with, the effect of sodium sulphite in increasing concentrations on the determination of cobalt was examined. The results are given in TABLE7. TABLE 7.

EFFECT OF SULPHITE ON THE DETERMINATION OF COBALT USING 3-METHOXYNITROSOPHENOL

Cobalt present in sample (pg) 20 20 20 20 20 20 20 20 20

Sodium sulphite added to sample (pg SOa) 0 1000 2000 5000 10,000 15,000 20,000 40,000 60,000

Cobalt found by analysis (/.tg) 20 20 20 20 20 18 17 15 13

The Determination of Cobalt in Water and Waste Water

703

It will be seen from these results that up to 10,000 #g of S O 3 2 - in the presence of 20 #g of cobalt could be tolerated without affecting the accuracy of the determination. Therefore, starting with a solution containing 20 #g of cobalt and 6000 #g Cr 6 +, amounts of sodium sulphite solution were added in order to find how much sulphite it was necessary to add to reduce the C r 6+ to Cr 3+ and still leave less than 10,000 #g of SO32- in solution (the maximum that could be tolerated). When 10,000 #g of SO32- were present the yellow colour of chromate was still present. With 20,000 l~g SO32- there was slight interference in the determination and marked interference with larger amounts. T A B L E 8. C O B A L T DETERMINATION IN THE PRESENCE OF CHROMATE

Cobalt present in sample

Sulphite present in sample

Chromate present in sample

Cobalt found by analysis

SO3 2 -

Cr 6 +

(fig)

(fig)

(fig)

(fig)

20 20 20 20 20 20 20

0 30,000 30,000 30,000 30,000 30,000 30,000

0 300 3000 4500 600O 9000 15,000

20.0 20.0 20.0 20.0 19.5 19"5 17.5

In the analyses recorded in this table hydrogen peroxide was added so as to destroy any sulphite that might have prevented cobalt from being determined in the presence of chromate. It appeared that if a large excess of sulphite could be added to reduce the chromate and the excess then destroyed it might be possible to determine cobalt in the presence of 6000 #g of Cr 6 +. This was tried by adding increasing amounts of chromate to a solution containing 20 #g of cobalt and 30,000 # g SO32- as sodium sulphite, followed by 1 ml of 20 vol. hydrogen peroxide to oxidize the sulphite. A selection of the results is shown in TABLE 8. The results in TABLE 8 show that with the modification used, chromium could be tolerated in amounts of up to about 9000 #g C r 6 ÷ per 20 #g Co. It was observed that when the amount of chromate in the previous test exceeded 9000 #g C r 6 + a transient blue colouration appeared in the solution on addition of the hydrogen peroxide. This transient blue colour is rapidly followed by changes that eventually result in the formation of Cr a +. The chemistry of this reaction is uncertain although it is believed that CrO5 is one intermediary product in the decomposition. In order to test whether the chromate could be oxidized directly to the pentavalent state and thus make it unnecessary to add sulphite, solutions containing 20 #g cobalt and increasing amounts of chromate were treated with 1 ml of 20 vol. H202 and the cobalt determined. The results obtained by this method are included in the appropriate column in TABLE 9 and show that up to 9000 #g of C r 6 ÷ can be tolerated by omitting sulphite reduction.

704

K . BENFORD,

MARIANNEGILBERTand S. H. JENKINS

In a test using mixtures of cobalt and chromate, omitting the reduction procedure with sulphite but adding hydrogen peroxide, it was found that up to 21,000 pg Cr 6 ÷ could be tolerated in the presence of 20/tg Co. However, it is advisable, in the light of the evidence given in TABLE9, to avoid the presence of more than 9000 pg of Cr 6 + in the sample undergoing analysis. TABLE 9. RESULTS OBTAINED BY DETERMINING COBALT IN THE PRESENCE OF VARIOUS METALS ADDED IN AMOUNTS PREVIOUSLY FOUND TO BE WITHIN OR AT THE LIMIT OF INTERFERENCE, USING 3METHOXYNITROSOPHENOL, AND TAKING PRECAUTIONS TO PREVENT INTERFERENCE BY IRON, COPPER AND CHROMIUM

Cobalt present in 10 ml Zinc sample (pg)

20 20 20 20 20 20 20 20 20 20 20 20 20

(pg)

20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000

Amount of metal present in samples as

Lead (pg)

20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000

Cobalt found Cadmium Nickel Iron as Copper Chromium Chromium by Fe3+ Cr6÷ Cr 3+ analysis (pg)

20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000 20,000

(pg)

5000 5000 5000 5000 5000 5000 5000 5000

(/~g)

5000 5000 5000 5000 5000 5000 5000

(pg)

1000 1000 1500 I000 1500 1000 1000 1000 1000

as (ug)

as (pg)

9000 18,000 9000

15,000 30,000 15,000

(pg)

20"0 20-0 19-5 20.0 19"5 15'0 19"5 16"5 19"5 11"0 20"0 13"0 12-0

It should be pointed out that the results with Cr 3 ÷ and Cr 6 + are anomalous; if Cr 6 + breaks down to Cr a+ in the analytical procedure given, then the tolerance limits should be the same for Cr 3 ÷ and Cr 6 +. TABLE9 shows that these limits are different. The reasons for this apparent anomaly are not known, but it is possible that Cr 6 + does not decompose completely to Cr a ÷ by the method used.

Recommended method for cobalt in the presence of zinc, lead, cadmium, nickel, iron, copper, chromium as Cr 6+ and Cr 3+, using 3-methoxynitrosophenol The method used is to transfer to a 100 ml separating funnel a known volume of up to 20 ml of sample containing the metals in almost neutral solution. The volume is made up to 20 ml with distilled water. One drop of 5 N HCI is added together with 1 ml 20 vol. H 2 0 2. The funnel is briefly shaken and then 5 ml of 50 per cent ammonium citrate solution is added, followed by 5 ml of 0.05 per cent 3-methoxynitrosophenol. The mixture is allowed to stand for 5 min to ensure completion of the cobalt complex. Then 5 ml of chloroform is added and the mixture shaken for 1 min until the aqueous layer is green in colour. The lower, organic layer is run into a second separating

The Determination of Cobalt in Water and Waste Water

705

funnel. The residual, aqueous layer is re-extracted with 3 ml of chloroform and, after separation of the two layers, the second chloroform extract is added to the first. To this mixture of the chloroform extracts 5 ml of 5 per cent citric acid is added, the funnel shaken for 30 sec, and after separation of the layers the chloroform layer is run into a third separating funnel. The aqueous layer is again extracted with 3 ml of chloroform and after separation of the layers the choloform is added to that in the third separating funnel. The combined chloroform extracts are then shaken for 30 sec with 10 ml of 10 per cent sodium carbonate solution in order to remove excess solvent. The washed chloroform is run into a 25 ml volumetric flask and the remaining sodium carbonate solution is washed with 2-3 ml of chloroform. This chlorform is run into the volumetric flask and the volume made to the mark with 3-methoxynitrosophenol. The optical density of the solution is then determined at 410 m/z. (The tolerance limits of several metals are given in TABLE9.) It was confirmed that by using a second extraction with citric acid the interference by copper is eliminated if the amount taken is kept below 1000/~g and that even in the presence of other metals this method satisfactorily removes copper interference. The method as described is suitable for sample volumes of 20 ml. Larger volumes may be used but it must be realized that greater volumes of the reagents, especially the methoxynitrosophenol and ammonium citrate must also be used, otherwise, as has been proved by experiment, formation of the cobalt-methoxynitrosophenol complex will be incomplete. The modified method is therefore suitable for the determination of up to 20 #g of cobalt in 20 ml of sample in the presence of the following metals, alone or in combination with one another.

Metal

Amount of metal tolerated in the presence of 20/gg Co (~g)

Copper 1000 Nickel 5000 Iron, Fe3÷ 5000 Zinc 20,000 Lead 20,000 Cadmium 20,000 Chromium, Cr 3+ 18,000 Chromium, CP ÷ 9000

D E T E R M I N A T I O N OF COBALT AS THE THIOCYANATE In order to develop a simpler method than the one just described, which involves the use of 3-methoxynitrosophenol, and increase the ratio of interfering metal to cobalt a study was made of the conditions required for the extraction of cobalt thiocyanate using an organic extraction in the presence of other metals. Salts of Co z+ form a blue coloured complex with ammonium thiocyanate, which

706

K. BENFORD, MARIANNE GILBERT and S. H. JENKINS

is soluble in methyl isobutyl ketone. This solvent has a very low miscibility with water. This has formed the essence of a method of determining cobalt based on a study of the extraction equilibrium beween the aqueous phase and the organic phase, since the blue c010ured complex formed, (NH4)2Co(SCN)4, is in equilibrium in the solvent with the complex Co(SCN)2 formed in the aqueous phase (TRIBALAT,1964). Thus, in slightly acid solution the following reaction occurs in the aqueous phase: COC12 + 2 NH4SCN = Co(SCN)2 + 2 NH4C1.

(1)

In the presence of a solvent of low miscibility the equilbrium conditions are shown by the following equation in which the blue complex (NH4)Co(SCN)2 in the organic phase is in equilibrium with the Co(SCN)4 in the aqueous phase: Co(SCN)2 + 2 NH4SCN~(NH4)2Co(SCN)4 Aqueous phase Aqueous Organic phase phase

(2)

Any Co g+ present in the aqueous phase can be determined after it has been transferred in the form of the complex (NH4)2Co(SCN)4 into the organic phase, for the reason that the constant removal of the complex from the aqueous to the organic phase allows the reaction in equation 2 to proceed to completion. By making use of these reactions and the theoretical data concerning the extraction of the solvent a method has been devised to determine microgram quantities of cobalt. Basic Method

The basic method is as follows: A volume of the sample, which should not be more than 25 ml or contain more than 50 #g of cobalt is placed in a 100 m separating funnel. To this are added 5 ml of a solution made by dissolving 50 g ammonium thiocyanate in distilled water made up to I00 ml (this solution should have a pH of 5.5-6.0) and 5 ml of methyl isobutyl ketone. Distilled water sufficient to bring the volume in the separating funnel to approximately 25 ml is added. The contents are then shaken for 30 sec and the two phases are allowed to separate. The aqueous, lower layer is run into a second 100 ml separating funnel and the upper layer, which will be turquoise blue if cobalt is present, is run into a 10 ml volumetric flask. The aqueous layer is shaken after addition of 3 ml methyl isobutyl ketone. The two layers are allowed to separate and after running off the lower layer to waste the solvent layer is added to the first extract in the 10 ml volumetric flask. The volume is then made up to 10 ml with methyl isobutyl ketone. The absorbance of this solution is then measured in an absorptiometer at 620 m# in a 1 cm cell, using methyl isobutyl ketone in the compensating cell. The absorbance obtained with an unknown concentration of cobalt is compared with that previously obtained using standards of known concentration. Maximum absorbance

In the reference cited (TRmALAT, 1964) the cobalt complex (NHa)2Co(SCN)4 in methyl isobutyl ketone is stated to have a maximum absorbance at 620 m#. In order to establish this point solutions containing 100 #g cobalt were treated with ammonium

The Determination of Cobalt in Water and Waste Water

707

thiocyanate and extracted with methyl isobutyl ketone in the manner already described. The absorbance was then measured at different wavelengths ranging from Absorbol c e 24

20

/

t6

t2

/I

./

/

x x x

8

xx % \x \

4

xx Oi __ SO0

I 600

I 610

I 620

I 630

~

I ......

I 640

',4

6SO

70O

mr

FIG. 2. Determination of Co II with N H , CNS. Absorbance at different wavelength in mythyl isobutyl ketone, using a 1 cm cell.

500 to 700 m/,. The results obtained, shown in FIG. 2, indicate that the maximum absorbance occurs within a broad peak between 620 and 625 m/~. Standard graph. In order to determine the relationship between absorbance and concentration, solutions containing from 10 to 100/tg of cobalt were extracted by the Absorbanc¢

20

mtpressed aS 16 a reading on on Eel

12

Absorptiome~r 8

0

o

,b

2b

3b

4b

Cobalt

token /t~

I

60

I

70

I

I

90

,do

FIG. 3. Relationship between the amount of cobalt taken for analysis and the absorbance, measured in a 1 cm cell with an EEL absorptiometer at 620 ma.

method described and the absorbance of the extract plotted against weight in micrograms of the cobalt taken. The results appear in FiG. 3 and show that the relationship was linear from 0 to 50 yg of cobalt but not beyond 50/~g. The shape of the curve did

708

K. BENFORD, MARIANNE GILBERT and S. H. JENKINS

not suggest that this was because Beer's law was not obeyed but rather that the cobalt complex was not being extracted completely at the higher concentrations of cobalt. The extraction procedure was therefore slightly modified by using two extractions, each with 10 ml methyl isobutyl ketone, and making up the final volume to 25 ml. Using this procedure with solutions containing from 60 to 100/~g of cobalt the results shown in FIG. 4 were obtained. It is clear that the linear relationship applies to all amounts of cobalt up to 100/~g, provided the total volume of methyl isobutyl ketone used in the two extractions is increased from 8 ml to 20 ml when the amount of cobalt exceeds 50 #g. In the authors' opinion, since the lower range of concentrations of cobalt are likely to be of greater interest in the field of water pollution control, it is preferable to limit the cobalt present in the sample to an amount which makes it unnecessary to use more than 8 ml of methyl isobutyl ketone. Absorbance. ~

on

II

IO

Eel Al~tlon~tt, r 9 8

7

6

f

60

I

70

c_.~

80

I

90

I

IOO

tt~X. ~

FIG. 4. Relationship between the amount of cobalt taken for analysis and the absorbance, measured in a 1 cm cell with an EEL absorptiometer at 620 m/~.

Effect of other metals on the method Since, as previously stated, cobalt would probably occur in waters in association with other metals, the effect on the determination of those metals likely to be present was found by adding to the volume of solution taken for analysis, which contained 50 pg cobalt, increasing amounts of solutions of the metals zinc, lead, cadmium, nickel, chromium 3 + and chromium 6÷. The effect of one interfering metal at a time was determined. In the case of cadmium, a suspension of cadmium hydroxide was added in order to avoid having to add a very large volume of the sparingly soluble cadmium hydroxide. This necessitated filtering the methyl isobutyl ketone extract through cotton wool before it was transferred to the 10 ml volumetric flask. The absorbance was determined in an EEL absorptiometer at 620 my and the cobalt concentration read off a calibration graph prepared from solutions of known cobalt concentrations. The results are given in TABLE 10. It will be seen that interference does not occur until the following amounts of metal in the sample taken are exceeded: Zinc 100,000/~g; lead 50,000 #g; cadmium 150,000 /zg; chromium 3 + 20,000 #g; chromium 6 + 250,000 #g.

TABLE 10. AMOUNT OF COBALT FOUND IN SOLUTIONS CONTAINING 50 pg COBALT AND INCREASING AMOUNTS OF INTERFERING METAL

Metal

Amount of metal present

Ratio of metal added to cobalt

(ug) Zinc

Lead

Cadmium

Nickel

C h r o m i u m Cr 3 +

C h r o m i u m Cr 6 +

0 10,000 20,000 30,000 50,000 100,000 150,000 200,000 0 I0,000 20,000 30,000 50,000 100,000 150,000 0 10,000 20,000 30,000 50,000 100,000 150,000 200,000 250,000 300,000 0 10,000 20,000 30,000 50,000 100,000 150,000 0 10,000 20,000 30,000 50,000 100,000 150,000 0 10,000 20,000 30,000 50,000 100,000 150,000 200,000 250,000 300,000

Amount of cobalt f o u n d

(ug) 200 400 600 1000 2000 3000 4000

-:1 : ~1 :1 :1 :1 :1 :1

50-0 50.0 49.5 40.0 50-0 50-0 33.0 0

200 400 600 1000

-:1 :1 :1 :1

50-0 50"0 50-0 49-0 49.5 33.2 0

200 400 600 1000 2000 3000 4000 5000 6000

-:1 :1 :1 :1 :I :1 :1 ." 1 :1

50.0 50-0 50-0 50.0 49.5 49.0 50.0 46"5 40"0 33.2

-:1 :1 :1 :1 :1 :1 -:1 :1 :1 :1 :1 :1 -:1 :1 :1 :1 :1 :1 :1 :1 :1

50.0 50'0 49.0 50-0 50.0 53.2* 57-0* 50.0 50.0 50.0 33.2 26.6 20.0 6.7 50"0 50"0 49'0 50"0 50'0 50.0 49.5 50-0 50.0 46-5

200 400 600 1000 2000 3000 200 400 600 1000 2000 3000 200 400 600 1000 2000 3000 4000 5000 6000

Cobalt added, 50/~g. * T h e colour is e n h a n c e d owing to t h e n o r m a l t u r q u o i s e blue colour becoming purple with high c o n c e n t r a t i o n s o f nickel.

710

K. BENrORD,MARIANNEGILBERTand S. H. JENKINS

Interference by copper and iron Copper, as would be expected, was found to interfere seriously with method on account of the formation of insoluble cuprous thiocyanate; this gave a pale green colour to the solvent layer, which masked the turquoise blue colour of the cobalt thiocyanate complex. It was also thought that the formation of the ammonium cobalt complex with thiocyanate might be suppressed in the presence of copper or that there might be co-precipitation of copper and cobalt thiocyantes, this thought being based on the assumption that the cuprammonium ion [Cu(NH3)4] 2+ would be formed with copper salts at a sufficiently high pH, and not the complex [Co(NH3)6] C12. The advantage of forming [Cu(NH3)42+] is that it has a negligible solubility in methyl isobutyl ketone. To test the worthiness of this idea the determination was carried out using 50/~g cobalt and 5000/~g copper, with thiocyanate buffered to different pH values with ammonia. At pH 9 an optical density was obtained indicating complete recovery of the cobalt. At this pH, determinations of cobalt were carried out in the presence of increasing amounts of copper. The results showed that, at pH 9, 50/~g cobalt could be determined accurately in the presence of up to 12,500/~g of copper. No further work was carried out along these lines because it meant that the method would have to be modified in the presence of copper. Iron in the ferric state also interferes in the original method, owing to the formation of the red thiocyanate Fe(CNS)3. The possibility was considered of (a) reducing ferric iron to the ferrous condition which does not form a coloured thiocyanate or (b) using thiocyanate buffered at pH 9 to precipitate iron as a hydroxide or (c) removal of the iron by precipitation. With method (a) reduction did not appear to be a practicable procedure, owing to the difficulty in keeping the iron in a reduced state. With (b) using thiocyanate at pH 9 there was serious interference, mainly due to the colour of the cobalt complex being masked by ferric thiocyanate in the organic layer. With (c) precipitation of ferrous and ferric iron as the insoluble chromates appeared to offer possibilities, especially in view of the lack of interference in the method by high concentrations of chromate, and the insolubility of most chromates other than those of the alkali metals, in addition to the instability of ammonium chromate. This last possibility was explored by determining the cobalt in a sample containing 50/~g of cobalt and increasing amounts of iron as ferrous or ferric iron. The sample was placed in 100 ml separating funnel and then 1 ml of a 5 per cent solution of potassium chromate was added. The ammonium thiocyante was added as already described and the cobalt complex extracted with methyl isobutyl ketone. The precipitated iron was removed by filtration through cotton wool. The results are given in TABLE 1 1. The results show that precipitation of ferrous and ferric iron, in situ, by addition of chromate is effective in preventing interference in the determination of 50 pg of cobalt in the presence of up to 20,000 pg of iron. When this modification was used to determine cobalt in the presence of iron and one other metal it was found to be successful except in the case of copper, which was added in the proportion of 10,000/~g to 50/~g of cobalt. Copper also interfered when it was added to the various combinations of metals which themselves did not affect the accuracy of the determination of cobalt. The conclusion was therefore reached that the modification involving the use of ammonia to bring the pH to 9 was satisfactory in the presence of copper if iron was

The Determination of Cobalt in Water and Waste Water

711

absent, and that the modification using chromate to precipitate iron was successful if copper was absent. TABLE 11. DETERMINATIONOF COBALT IN A SAMPLE CONTAINING 50 /.tg CO a n d INCREASING AMOUNTS OF FERROUS OR FERRIC IRON~ USING POTASSIUMCHROMATE TO PRECIPATE THE IRON IN ORDER TO PREVENT INTERFERENCE FROM IRON

Ferrous iron present (/tg)

Cobalt found

Ferric iron

Cobalt found

(/tg)

(/2g)

(/tg)

0 5000 I 0,000 15,000 20,000 30,000 50,000

50.0 50.0 50.0 49.0 50-0 48'2 23.5

0 5000 10,000 15,000 20-000 30,000 50,000

50.0 50.0 49.0 50.0 49-0 30-0 15.0

Removal of copper and iron to prevent their interference Since copper and iron are likely to be present in water or industrial effluents containing cobalt, methods for the removal of copper and iron were considered. The most promising seemed to be to remove these metals from the sphere of action either by complexing or precipitating them. The addition of cupferron (ammonium nitrosophenyl hydroxylamine) in acid solution was tried. This was unsuccessful because the turquoise blue cobalt complex was not formed with ammonium thiocyanate, its place being taken by cobaltous chloride. In neutral solution the reaction was also unsuccessful. Electrolysis of solutions containing copper and cobalt in order to remove copper was also tried. Although an appreciable proportion of metallic copper was desposited on the cathode, the colour of the ammonium cobalt thiocyanate complex was masked by a green colour, possibly the complex of copper, cobalt and thiocyanate Cu 2 [Co ( C N S ) J produced by the reduction of divalent copper to the monovalent state during electrolysis. Since divalent salts of copper had been used in the work so far carried out to test the interference due to copper it was thought that the interfering effect might be removed by forming a complex or a salt of the copper reduced to the monovalent state. A cupric salt was therefore reduced to the cuprous state in neutral solution by adding potassium iodide solution according to the reaction: Cu 2 ÷ + 2I- = Cu ÷ + 12. Iodine is soluble in methyl isobutyl ketone. Therefore it would have been necessary to remove iodine liberated. This could be effected by having sodium sulphite present in the solution so that any iodine liberated reacted with the sulphite by the action I 2 + S O 3 2 - - - * S O 4 2 - + 2 I - the nett reaction being 2Cu2++2I~Cu212, which is removed from further action by precipitation. The copper was therefore precipitated from solutions containing 50/tg of cobalt and up to 300,000 pg of copper by adding, dropwise, an excess of 10 per cent aqueous F W

712

K. BENFORD,MARIANNEGILBERTand S. H. JENKINS

s o d i u m sulphite. This was s h o w n b y the absence o f a n y further p r e c i p i t a t i o n w h e n m o r e sulphite was a d d e d . Excess o f a 10 p e r cent s o l u t i o n o f p o t a s s i u m i o d i d e was t h e n a d d e d , this being i n d i c a t e d when further a d d i t i o n o f i o d i d e ceased to give a n y m o r e p r e c i p i t a t i o n o f the c r e a m - c o l o u r e d c u p r o u s iodide. T h e liquid was filtered into a 100 ml s e p a r a t i n g funnel a n d the analysis c o n t i n u e d as described previously. T h e results given in TABLE 12 were o b t a i n e d . TABLE 12. DETERMINATION OF COBALT I N A SAMPLE CONTAINING 5 0 f i g OF COBALT I N THE PRESENCE OF INCREASING AMOUNTS OF COPPER,

PRECIPITATING THE COPPER AS

CUPROUS IODIDE

Copper present

Ratio of copper to cobalt

Cobalt found

(jug)

(jug)

0 10,000 20,000 40,000 50,000 100,000 200,000 300,000

TABLE 13.

-200 : 1 400 : 1 800 : 1 1000 : 1 2000 : 1 4000 : 1 6000 : 1

50"0 50"0 49"5 50-0 49-0 50"0 51"5 60+

DETERMINATION OF COBALT I N THE PRESENCE OF MIXTURES OF INTERFERING METALS,

PRECIPITATING COPPER AS CUPROUS IODIDE AND IRON AS CHROMATE TO PREVENT INFERFERENCE

Content of metal in the solutions analysed Cobalt

Copper

Zinc

Lead

Cadmium

(/lg) Co

(/~g) Cu

(/zg) Zn

~ug) Pb

(/~g) Cd

50 50 50 50 50 50 50 50

50,000 50,000 50,000 50,000 50,000 50,000 50,000 50,000

50,000 50,000 50,000 50,000 50,000 50,000 50,000

50,000 50,000 50,000 50,000 50,000 50,000

50,000 50,000 50,000 50,000 50,000

Ratio Cobalt of total found Nickel Iron as Crom- Chrommetal equal ium ium (other weights trival- hexavalthan of ferrous ent ent cobalt to and ferric cobalt) (/~g) (/lg) (/lg) (fig) Ni Fe Cr Cr Co

50,000 50,000 50,000 50,000

20,000 20,000 20,000

20,000 20,000

30,000

1000 2000 3000 4000 5000 5400 5800 6800

:I -l :1 •I •I :I :I •I

50.0 50-0 49.0 50'0 50"0 49"0 49"5 50'0

The Determination of Cobalt in Water and Waste Water

713

The results show that by precipitating the copper as cuprous iodide the method is satisfactory when the copper to cobalt ratio is 2000 • 1, i.e. when up to 100,000 pg of copper is present. In order to test the combined effect on the determination of cobalt of copper and iron, using precipitation of the copper as cuprous iodide and precipitation of iron as the chromate to prevent interference, a series of solutions containing 50/zg of cobalt and 50,000 #g of copper were taken. To these were added various combinations of other metals and the cobalt determined. The composition of the mixtures analysed and the results obtained are given in TABLE 13. The results show that microgram quantities of cobalt can be determined accurately in the presence of a large excess of the metals likely to be present in association with cobalt in industrial waste water using the method described below. METHOD Reagents, made with analytical grade chemicals: 1. l g sodium sulphite, Na2SO35H20, is dissolved in l0 ml distilled water, made daffy or as required. This is kept in a dropping bottle. 2. 1 g potassium iodide, KI, is dissolved in l0 ml distilled water. This is kept in a dropping bottle, out of sunlight. 3. 5 g potassium chromate, K2CrO4, is dissolved in 100 ml distilled water. The solution keeps indefinitely. 4. 50 g ammonium thiocyanate, (NH4)CNS, is dissolved in 100 ml distilled water. The solution keeps indefinitely. 5. Methyl isobutyl ketone. An aliquot of the sample is placed in a 100 ml beaker. The volume of the sample should not be more than 25 ml and should not contain more than 10 mg cobalt/l. Sodium sulphite solution is then added dropwise. If no green precipitation occurs the addition of sulphite should cease. If a green precipitate is formed, indicating that copper is present, the addition of sulphite is continued until no further precipitation occurs. Potassium iodide solution is added dropwise until the colour of the precipitate has been changed to a white or off-white colour, but this addition is unnecessary if no green precipitate is obtained. The sample as treated above is filtered through Whatman 542 filter paper, or a similar grade of paper, and the filtrate together with the washings used to transfer the sample to the filter paper are collected in a 100 ml separating funnel. To the contents of the separating funnel 1 ml potassium chromate solution is added. This is followed by 5 ml ammonium thiocyanate solution and 5 ml isobutyl ketone. The mixture is shaken by hand for 30 sec and the two phases are allowed to separate. The lower, aqueous layer is transferred to a second 100 ml separating funnel and the upper layer, which will be turquoise blue if cobalt is present, is run through a cotton-wool plug in the stem of a glass funnel, into a 10 ml volumetric flask. The aqueous layer in the separating funnel is re-extracted with 3 ml of methyl isobutyl ketone. After separation the lower layer is run to waste. The residual extract is added to the first extract in the 10 ml volumetric flask through the cotton wool plug. The volume is made up to 10 ml with methyl isobutyl ketone. The absorbance of the extract is measured at 620 m/~ in a 1 cm cell, using the solvent in the reference cell.

714

K. BENFORD,MARIANNEGILBERTand S. H. JENKINS

A standard graph is obtained by carrying out the procedure on standard solutions of cobalt, omitting the addition of sulphite, iodide and chromate. The following volumes of standard cobalt, containing 10 #g Co/ml, may be u s e d - - l , 2, 3, 4, 5 ml. The above procedure may be modified suitably if iron or copper are absent. In the absence of copper the addition of sulphite and iodide is unnecessary. In the absence of iron the chromate need not be added. The following table summarizes ratio of the maximum amount of interfering metal to cobalt that may be used in each method. In the 3-methoxynitrosophenol method 20 pg of cobalt was made to 25 ml in the solution used for absorbance measurement. In the thiocyanate method 50 #g of cobalt was finally made up to l0 ml for absorbance measurement.

Zinc Lead Cadmium Nickel Copper Iron Chromium Cr6+ Chromium Cr3+

Method 3-methoxynitrosophenol

Method ammonium thiocyanate

1000 : 1 1000 : 1 1000 : 1 250 : 1 50 : 1 250 : 1 450 : 1 750 : 1

1000 : 1 1000 : 1 1000 : 1 1000 : 1 1000 : 1 400 : 1 600 : 1 400 : 1

SUMMARY 1. Two methods have been worked out for the determination of cobalt in waters and industrial waste water. 2. In the first method the complex formed by cobalt and 3-methoxynitrosophenol is extracted with chloroform and the absorbance measured at 410 m/~. 3. Zinc, cadmium and lead can be tolerated in high concentrations relative to the amount of cobalt in the sample. Nickel interferes at low concentrations but an extraction with citric acid prevents this interference. Copper seriously interferes with the determination but this also is removed by critric acid extraction; the limiting amount of copper permissible in a sample containing up to 20 ~lg cobalt is 1000/~g Cu. Iron must be present as Fe 3 + but even then interference is caused in the determination; by mixing the reagents in strict order the interference is greatly reduced. Although large amounts of chromium as Cr 3 + could be tolerated, in the form of Cr 6 + interference resulted. Considerable attention was given to a study of this interference. a. The second method devised for determining cobalt is to complex it with ammonium thiocyanate, extract the complex with methyl isobutyl ketone and measure the absorbance of the complex at 620 my. 5. Zinc, lead and cadmium could be tolerated in concentrations of at least 1000 times that of the cobalt; nickel only began to interfere when it was present in 1000 times the amount of cobalt. Interference by chromium as Cr 6 + was insignificant, even at very high concentrations. There was some interference by Cr 3+ but only when about 4000 times as much chromium as cobalt was present

The Determination of Cobalt in Water and Waste Water

715

6. The m o s t serious interferences were caused by copper a n d iron. Both these difficulties were overcome by m e t h o d s which are described in detail in the paper. The effect of copper was prevented by reducing it to the c u p r o u s state with sulphite a n d then precipitating it as the c u p r o u s iodide. I n this f o r m it did n o t interfere with the f o r m a t i o n o f the a m m o n i u m cobalt thiocyanate complex. P r e v e n t i o n of interference by iron was effected b y a d d i n g a n excess of c h r o m a t e in order to precipitate ferrous a n d ferric chromate, which did n o t affect the f o r m a t i o n of the a m m o n i u m cobalt thiocyanate complex n o r its s u b s e q u e n t extraction.

REFERENCES JOHNSON W. C. (Editor) (1964) 3-Methoxynitrosophenol. In Organic Reagents for Metals, Vol. 2, pp. 128-133. Hopkins & Williams, Essex. Torul T. (1955) A new colorimetric determination of small amounts of cobalt with m-methoxy-onitrosophenol. J. Chem. Soc. Japan 76, 328. TRraALAT S. (1944) l~luilibre D'l~xtraction et Reactions Chimiques en Solvants Peu Ionisants. In Theory and Structures of Complex Compounds, Papers presented at the Symposium held in Wroclaw, Poland, 15-19 June 1962, pp. 447-455, Pergamon Press.

ADDENDUM (13 November 1967) The authors are indebted to Mr C. H. Hammerton for pointing out that the extraction of the cobalt complex (NH4)2Co(CNS)4 into methyl isobutyl ketone is hindered under certain conditions of acidity or alkalinity. When the cobalt in solutions containing 50/~g Co at different pH values was determined, 100K recovery was obtained at pH values of 4.5 to 6.5. The recovery figures were too high when the pH was low and too low at high pH values. It was proved that a value within the pH range 4.5-6.5 was satisfactory by tests in which a volume of cobalt solution containing 50/tg Co was present. This cobalt solution was prepared by adding 0.1 N HCI to 10 mg Col1 until the pH was 1"0. To the solution containing 50 ~g Co ammonia solution was added dropwise until the pH was 6.0. The cobalt was then determined by the recommended method. The cobalt recovered was 49.0/~g Co. Thus, the procedure described in the paper requires the following addition on page 713, para. 5, immediately before the sentence beginningwith sodium sulphite solution • "Add either dilute HC1 or NH4OH dropwise to the sample in order to bring the pH within the range 4.5-6.0. Filter off any precipitate". It should be pointed out that if salts such as sodium sulphite or potassium chromate are added in amounts that raise the pH above 6-5 the recovery of cobalt may be incomplete.