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Determination of iron, nickel, chromium and vanadium by means of redox and ion-exchange columns
Tahnta, 1968.Vol. 15.pp. 735to 740. Pagamon Press. Printedin NorthernIreland
DETERMINATION OF IRON, NICKEL, CHROMIUM VANADIUM BY MEANS OF REDOX AND ION-EXCHANGE COLUMNS
AND
G. A. H. ROBERTS Esso Research Centre, Abingdon, Berkshire, U.K. (Received
11Junua*y 1968. Accepted9 March 1968)
Summary-Redox and anion-exchange columns have been used to separate and determine iron, nickel, chromium and vanadium in solution. The anion-exchange columns provide some of the separations, and the redox columns are used for the determination of the iron, chromium and vanadium. The chromium and vanadium may be determined in the presence of the iron by adjustment of the acidity in the redox column. By using a column “memory” technique, titration of the actual metal solution has been avoided. The method shows some advantages over conventional methods.
into the formation of corrosion deposits on probes inserted into oil-tied furnaces in power-stations resulted in a need for simple methods of analysis of mixtures of metals occurring in steels and fuel-oils. Typical metals occurring in such deposits are iron, chromium and nickel from the probe-steel, and vanadium and sodium from the fuel-oil. Previous studies1 on the use of redox reactions on columns for the analysis of metals occurring in more than one oxidation state suggested that, used in conjunction with ion-exchange columns, such columns could give simple and effective separations and determinations. The redox columns consist of tetrachlorohydroquinone deposited on a suitable inert substrate such as diatomaceous earth. Passage of an iron(II1) solution at a suitable pH results in reduction to iron plus the formation of an equivalent amount of the oxidized form, tetrachloroquinone, of the redox compound. This tetrachloroquinone acts as a “memory” of the iron(II1) that has passed through the column, and reduction with excess of ascorbic acid followed by a back-titration with iodine solution provides a measure of the amount of iron passed. Because the iron solution itself is not titrated, titration difficulties due to the presence of other metals are avoided. As the potentials of redox couples depend on pH and concentrations of complexing species, a certain degree of flexibility is provided; e.g., iron@) is reduced to iron(I1) in 0.03M hydrochloric acid but not in 2M hydrochloric acid (complex formation effect), whereas vanadium(V) and chromium(W) are reduced to vanadium(W) and chromium(II1) respectively in 244 hydrochloric acid (pH effect). The anion-exchange separations used in this paper have been described by Kraus and Nelson.2 In chloride media, the chloride complexes and oxy-anions formed by metals are adsorbed on to anion-exchange resins to varying degrees according to the concentration of chloride ions present. Thus these authors report that alkali metals, alkaline earths, alummium(II1) and nickel(I1) are not adsorbed even in the presence of 12M hydrochloric acid ; vanadium(IV), chromium(II1) and manganese(I1) are slightly adsorbed from a 12M hydrochloric acid medium; iron and vanadium(V) are adsorbed but eluted when the medium is diluted to 7M hydrochloric acid, and iron(III) is eluted by 1M hydrochloric acid. Chromium(W) in the form of the oxy-anion is very strongly adsorbed and difficult to remove in chloride media. INVESTIGATIONS
2
735
736
G. A. H.
ROBERTS
The two types of column (redox and ion-exchange) have therefore been used to determine the composition of a solution containing iron(III), vanadium(V), chromium(VI) and nickel(U). EXPERIMENTAL Materials
AmberliteResin CG 400, 100-200 mesh, B.D.H. Ltd. C&e A, 80-120 mesh, B.D.H. Ltd. Tetrachlorohydroquinone, Eastman Organic Chemicals,
(Kodak Ltd.).
Preparation of redox columns Tetrachlorohydroquinone (15-2 g) was deposited on 5 g of diatomaceous earth (C&e A) from ethereal solution.lra To the paste obtained after evaporation of most of the ether were added successive quantities of 0~03M hydrochloric acid until a smooth slurry was obtained. This slurry was poured into a l-cm diameter glass column fitted with a sintered disc (porosity 0) and tap, the resulting redox column being about 10 cm long. Preparation of anion-exchange columns Amberlite Resin CG 400, 100-200 mesh, was washed several times with distilled water and decanted to remove the fines. A column 5 cm in length was prepared by pouring the aqueous slurry into a column of 2-cm bore glass tubing into the bottom end of which was sealed a sintered glass disc of porosity 3. The column was conditioned by washing six times with a cycle of 2M sodium hydroxide, water. 2M hydrochloric acid and water. Neithe; fype of colk was allowed to run dry as this would allow air-bubbles to collect in the beds and slow the flow-rates. The columns were always conditioned before use by passage of 20-25 ml of the appropriate solvent (0.03M hydrochloric acid or water) used in the test solution. The columns were also washed through after the passage of the test solution. Preparation of solutions of corrosion deposits Corrosion deposits may be dissolved by several standard methods, e.g., 1 g of tiely ground deposit is mixed with 5 g of 100: 15 w/w sodium carbonate/borax mixture and fused in a platinum crucible. The product is then dissolved in sulphuric acid. Saltzmann’ has described a method for the oxidation of the metals to their highest oxidation states. After oxidation with potassium permanganate in dilute sulphuric acid, the excess of permanganate is destroyed with 2 % sodium azide solution. The principles of the method are outlined diagrammatically in Fig. 1. Vanadium and chromium were first determined together after passage of 20 ml of solution A (plus sufficient concentrated hydrochloric acid to give a 2M solution) down a redox column, and washing through with 2M hydrochloric acid followed by distilled water. A flow-rate of approximately 30 ml/hr was used, care being taken that the column capacity was sufficiently high. This could be checked visually by observing the extent of the yellow band of tetrachloroquinone in the column. This tetrachloroquinone, equivalent to the vanadium(V) and chromium(W), was then reduced by passing through the column an excess (20 ml) of 0-W ascorbic acid and washing it through with 20-30 ml of distilled water. The unreacted ascorbic acid was back-titrated with 0-W iodine solution, with starch as indicator. ChromiumWI) was then removed from 60 ml of solution A (made 0*5M in hvdrochloric acid) by passing it do& anion-exchange column I and washing it with 0.5M hydrochloric acid. The effluent was made up to 100 ml (solution B), 20 ml of which were made 2M in hydrochloric acid and passed down a redox column to determine the amount of vanadium(V) present. A The nickel was obtained free from interfering metals by passing 20 in1 of solution B (made 8M in hydrochloric acid) down anion-exchange column II. The nickel passed through unadsorbed, and a further 20-30 ml of 8M hydrochloric acid were passed through to ensure complete removal of the nickel. The nickel was then determined gravimetrically with dimethylglyoxime. It should be noted that anion-exchange columns that have been used to remove chromium should not be used for nickel removal because the 8M hydrochloric acid would reduce the adsorbed chromium(W) to chromium(II1) and then elute it, thereby contaminating the nickel. After the nickel removal, 20-30 ml of 4M hydrochloric acid were passed through column II to remove the vanadium, and this effluent was discarded because a certain amount had also emerged in the nickel solution.
Redox and ionexchange
737
columns
Redox Column 2MHCl SOLUTION A---------+ PI + [Crl Ni(II), CrO, V(v), Fe(III)
~
,
~%z?r”
Red~~~~~
SOL&ON B Ni(II), V(V), Fe(II1) Anion-exchange column II 8M HCl
Cr(VL) on column
l
WI
I
1 Part of V; Fe on column
SOL&ON Part of V, Ni -+ wi]
4M HCl J SOLUTION PartofV
F&II) on column Water Y
FIG. l.-Separation
and determination
Redox Column SOLUTION [Fe] 0.03 M HCL Fe(III) of Ni(II), Cr(VI), V(V) and Fe(II1).
The iron was obtained by passing 20-30 ml of water down column II. Collection was started soon as the yellow colour of the iron chloro-complex appeared through the column sinter. For the purpose of determining the iron concentration on a redox column it was essential that the acidity should not exceed 0*03M. Accordingly, the iron eluate was made slightly alkaline with dilute ammonia and then the brown gelatinous precipitate was just redissolved with dilute hydrochloric acid so that the final acidity did not exceed 0*03M. The iron content was then determined by passing the solution through a redox column (conditioned with 0*03M hydrochloric acid) followed by reduction with ascorbic acid and back-titration with iodine solution.
as
RESULTS
AND
DISCUSSION
Ion-exchange column behaviour Nickel. A solution of nickel chloride in concentrated hydrochloric acid was passed through the column; the eluate was shown to contain nickel. Elution of the column with O-H4 hydrochloric acid gave no further nickel in the eluate, i.e., nickel was not adsorbed on the column. Chromium. Passage of a solution of potassium dichromate in 0.5M hydrochloric acid resulted in the complete adsorption of chromium(X). This could only be eluted with concentrated hydrochloric acid, which slowly reduced it to chromium(III) which was then eluted. Vanadium. Kraus and Nelson2 reported that vanadium(V) is adsorbed from concentrated hydrochloric acid solutions and eluted by 7M hydrochloric acid. The hydrochloric acid concentration of the sample solution, which after passage down column I is 0*5M, must therefore be increased and this could be achieved by saturation with hydrochloric acid gas. A more convenient alternative was sought by adding
G. A. H. ROBERTS
738
concentrated hydrochloric acid (11 M) to the sample solution. A 1OM acid concentration requires the addition of 100 ml of concentrated acid to 10 ml of sample solution but this large total volume, besides being inconvenient to handle, appeared to cause the elution of the vanadium (see Table I)_ Therefore a lower con~ntration of 8M was chosen because this gave adsorption of all but a trace of the vanadium TABLEI.--SEPARATION OF NICKEL,VANADIUM ANDIRON BY ANION-EXCHANGE Sample volume, ml Cont. HCl added, ml ResuItant [HCI], M Metals eluted: a, immediately b, by 4MHCl c, by 03MHCl
10
5
100
22.5
20
10
9
8
Ni, V (trace) V Fe
Ni, V (trace) V Fe
Ni, V* None Fe
7.5
* Elution of vanadium probabIy due to large volume of hydr~~ori~ acid used. with the minimal
addition of hydrochloric acid. An interesting observation was made during the subsequent elution of the vanadium from the column. The use of 7M hydrochloric acid gave a blue eluate of vanadium(IV) whereas 4M acid gave a yellow eluate of vanadium(V). A similar observation had been made with chromium in the previous section where the passage of concentrated hydrochloric acid led to the reduction of chro~um~V1) to chromium(I~. Strelow and Bothma also noted the reduction of chromium(X) on a anion-exchange column and ascribed it to reduction by the quaternary ammonium resin. Iron. When solutions of iron(III) chloride in concentrated hydrochloric acid and in 4h4 hydrochloric acid were passed down a column there was no elution of iron(III) that could be detected by the addition of ammo~um thiocyanate. The addition of O*SM hydrochloric acid or water eluted all of the iron absorbed on the column.
Separation and determination of nickel(U), chromium( VI), vanadium(V) and iron(Ill) The information obtained was used to devise a scheme for separating the four metals. Solution A was prepared by mixing Xl-ml quantities of O*lM nickel chloride, 0*05M potassium dichromate, 0.1M ammonium vanadate and 0.1M iron(III) chloride and diluting to 250 ml with water. Then 25 ml of this solution were made 0*5M in hydrochloric acid and passed down a column. The first 10 ml of eluate were discarded as being the hold-up volume. The ~hromium(V~ was adsorbed as an orange layer. The column was washed with 0*5M hydrochloric acid until the effluent was colourless. This eluate (total volume 30 ml) contained nickel, vanadium and iron, and was divided into three parts for passage down a second column at varying hydrochloric acid concentrations. The details and results are given in Table I. Some vanadium passes through with the nickel, but does not interfere with the subsequent determination with dimethylglyoxime. These separation techniques were then combined with use of redox columns to determine the amounts of the four metals in Solution A. It has previously been reportedl*a that iron(II1) in 0.03M hydrochloric acid is completely reduced by a redox column but not at all in 2N hydrochloric acid, and that vanadium(V) and
Redox and ion-exchange
columns
739
chromium(VI) are completely reduced in 2M hydrochloric acid. However, noinformation was available for vanadium in 0*03M hydrochloric acid. A check experiment showed that ammonium vanadate is 40% reduced by passage down a redox column in 0.03M hydrochloric acid, and so, for the determination of iron, it is essential that vanadium is absent. When solution A (made 2M in hydrochloric acid) was passed down a redox column, the recovery of vanadium(V) plus chromium(VI) was 0*0790-0~0848 mequiv/ ml, average 0.0806, expected value 0*0800. Table II gives results obtained for the determination of vanadium(V) after the removal of chromium(V) by the anion-exchange column I. These showed that a TABLE II.-DETERMINATION
OF VANADIUM oFcHRohlmM
mu
Vanadium, HCl concentration used in chromium removal, M 0.03
o-2 0.5
_
REMOVAL
mequiv/ml
Found
Taken
OO236 0023, o-0225 0.024 O-021, O-021 0022 O-020 O-020 0020
O-020
o-020 0.020
concentration of at least 05M hydrochloric acid was necessary in the chromium removal step. Lower concentrations of chloride ion gave higher apparent vanadium values, i.e., not all of the chromium was being removed. Nickel recovery, after removal of chromium(V1) with an anion-exchange column and 05M hydrochloric acid and of vanadium(V) and iron(III) with the second anion-exchange column and 8M hydrochloric acid, ranged from 100 to 106%, average 103%. TASLE III.--ANALYSIS OF MIXTURES OF IRON, NICKEL, CHROMIUM
AND VANADIUM
Metal
Solution I Taken* Found *
Solution II Taken* Found*
Solution III Taken* Found*
Solution IV Taken* Found*
Iron Nickel Chromium Vanadium
O-0833 0.0344 00104 0.0611
0.0313 0.0917 0.0312 0.0204
0.1459 0.0344 0.0104 0.0204
0.0313 0.0573 OG416 0.0815
0.0838 0.0339 0.0107 0.0616
0.0321 0.0923 0.0318 0.0196
0.1483 0.0339 0.0109 0.0196
0.0332 0.0581 O&409 0.0818
* In g/100 ml of solution.
Iron(II1) recovery by final elution with water from the second anion-exchange column was 98 %. As a further check, four solutions containing various proportions of the four metals were analysed. The results obtained are given in Table III.
G. A. H. ROBERTS
740
ZusammenfasstmB-Zur Trennung und Bestimmung von Eisen, Nickel, Chrom und Vanadium in L.&sung wurden Redox- und Anionenaustauschsiiulen verwendet. Die Anionenaustauschs&den ermoglichen einige der Trermungen, die Redoxtiulen werden xur Bestimmung von Eisen, Chrom und Vanadium gebraucht. Chrom und Vanadium kbnnen neben Eisen durch Kontrolle der Acidit& in der RedoxsB;ule bestimmt werden. Durch Verwendung eines Speicher-(“memory”)verfahrens in der Saule wurde die Titration der MetallL%mg vermieden. Die Metbode xeigt Vorteile gegen ber bisher bekannten Verfahren. R&stun&On a utilid des colonnes redox et echangeuses d’anions pour s&parer et determiner le fer, le nickel, le chrome et le vanadium en solution. Les colonnes echangeuses d’anions four&sent quelques-unes des separations et l’on utilise les colonnes redox pour la determination du fer, du chrome et du vanadium. On peut determiner le chrome et le vanadium en presence du fer par ajustage de l’acidite dans la colonne redox. En utilisant une technique de colonne “a m&moire”, on a &it6 le titrage de la solution reelle de m&al. La m6thode montre quelques avantages par rapport aux methodes ordinaires. REFERENCES 1. 2. 3. 4. 5.
R. K. E. B. F.
Belcher, J. R. A. Kraus and Cerrai and C. E. Saltzmarm, W. E. Strelow
Majer and G. A. H. Roberts, TuCunfa, 1967, 14, 1245. F. Nelson, Am. Sot. Testing Mater. Spec. Tech. Publ. No. 195,1956, 27. Testa, Anal. Chim. Actu, 1963, 28,205. Anal. Chem., 1952,24,1016. and C. J. C. Bothma, ibid., 1967, 39, 595.
DETERMINATION OF IRON, NICKEL, CHROMIUM VANADIUM BY MEANS OF REDOX AND ION-EXCHANGE COLUMNS
AND
G. A. H. ROBERTS Esso Research Centre, Abingdon, Berkshire, U.K. (Received
11Junua*y 1968. Accepted9 March 1968)
Summary-Redox and anion-exchange columns have been used to separate and determine iron, nickel, chromium and vanadium in solution. The anion-exchange columns provide some of the separations, and the redox columns are used for the determination of the iron, chromium and vanadium. The chromium and vanadium may be determined in the presence of the iron by adjustment of the acidity in the redox column. By using a column “memory” technique, titration of the actual metal solution has been avoided. The method shows some advantages over conventional methods.
into the formation of corrosion deposits on probes inserted into oil-tied furnaces in power-stations resulted in a need for simple methods of analysis of mixtures of metals occurring in steels and fuel-oils. Typical metals occurring in such deposits are iron, chromium and nickel from the probe-steel, and vanadium and sodium from the fuel-oil. Previous studies1 on the use of redox reactions on columns for the analysis of metals occurring in more than one oxidation state suggested that, used in conjunction with ion-exchange columns, such columns could give simple and effective separations and determinations. The redox columns consist of tetrachlorohydroquinone deposited on a suitable inert substrate such as diatomaceous earth. Passage of an iron(II1) solution at a suitable pH results in reduction to iron plus the formation of an equivalent amount of the oxidized form, tetrachloroquinone, of the redox compound. This tetrachloroquinone acts as a “memory” of the iron(II1) that has passed through the column, and reduction with excess of ascorbic acid followed by a back-titration with iodine solution provides a measure of the amount of iron passed. Because the iron solution itself is not titrated, titration difficulties due to the presence of other metals are avoided. As the potentials of redox couples depend on pH and concentrations of complexing species, a certain degree of flexibility is provided; e.g., iron@) is reduced to iron(I1) in 0.03M hydrochloric acid but not in 2M hydrochloric acid (complex formation effect), whereas vanadium(V) and chromium(W) are reduced to vanadium(W) and chromium(II1) respectively in 244 hydrochloric acid (pH effect). The anion-exchange separations used in this paper have been described by Kraus and Nelson.2 In chloride media, the chloride complexes and oxy-anions formed by metals are adsorbed on to anion-exchange resins to varying degrees according to the concentration of chloride ions present. Thus these authors report that alkali metals, alkaline earths, alummium(II1) and nickel(I1) are not adsorbed even in the presence of 12M hydrochloric acid ; vanadium(IV), chromium(II1) and manganese(I1) are slightly adsorbed from a 12M hydrochloric acid medium; iron and vanadium(V) are adsorbed but eluted when the medium is diluted to 7M hydrochloric acid, and iron(III) is eluted by 1M hydrochloric acid. Chromium(W) in the form of the oxy-anion is very strongly adsorbed and difficult to remove in chloride media. INVESTIGATIONS
2
735
736
G. A. H.
ROBERTS
The two types of column (redox and ion-exchange) have therefore been used to determine the composition of a solution containing iron(III), vanadium(V), chromium(VI) and nickel(U). EXPERIMENTAL Materials
AmberliteResin CG 400, 100-200 mesh, B.D.H. Ltd. C&e A, 80-120 mesh, B.D.H. Ltd. Tetrachlorohydroquinone, Eastman Organic Chemicals,
(Kodak Ltd.).
Preparation of redox columns Tetrachlorohydroquinone (15-2 g) was deposited on 5 g of diatomaceous earth (C&e A) from ethereal solution.lra To the paste obtained after evaporation of most of the ether were added successive quantities of 0~03M hydrochloric acid until a smooth slurry was obtained. This slurry was poured into a l-cm diameter glass column fitted with a sintered disc (porosity 0) and tap, the resulting redox column being about 10 cm long. Preparation of anion-exchange columns Amberlite Resin CG 400, 100-200 mesh, was washed several times with distilled water and decanted to remove the fines. A column 5 cm in length was prepared by pouring the aqueous slurry into a column of 2-cm bore glass tubing into the bottom end of which was sealed a sintered glass disc of porosity 3. The column was conditioned by washing six times with a cycle of 2M sodium hydroxide, water. 2M hydrochloric acid and water. Neithe; fype of colk was allowed to run dry as this would allow air-bubbles to collect in the beds and slow the flow-rates. The columns were always conditioned before use by passage of 20-25 ml of the appropriate solvent (0.03M hydrochloric acid or water) used in the test solution. The columns were also washed through after the passage of the test solution. Preparation of solutions of corrosion deposits Corrosion deposits may be dissolved by several standard methods, e.g., 1 g of tiely ground deposit is mixed with 5 g of 100: 15 w/w sodium carbonate/borax mixture and fused in a platinum crucible. The product is then dissolved in sulphuric acid. Saltzmann’ has described a method for the oxidation of the metals to their highest oxidation states. After oxidation with potassium permanganate in dilute sulphuric acid, the excess of permanganate is destroyed with 2 % sodium azide solution. The principles of the method are outlined diagrammatically in Fig. 1. Vanadium and chromium were first determined together after passage of 20 ml of solution A (plus sufficient concentrated hydrochloric acid to give a 2M solution) down a redox column, and washing through with 2M hydrochloric acid followed by distilled water. A flow-rate of approximately 30 ml/hr was used, care being taken that the column capacity was sufficiently high. This could be checked visually by observing the extent of the yellow band of tetrachloroquinone in the column. This tetrachloroquinone, equivalent to the vanadium(V) and chromium(W), was then reduced by passing through the column an excess (20 ml) of 0-W ascorbic acid and washing it through with 20-30 ml of distilled water. The unreacted ascorbic acid was back-titrated with 0-W iodine solution, with starch as indicator. ChromiumWI) was then removed from 60 ml of solution A (made 0*5M in hvdrochloric acid) by passing it do& anion-exchange column I and washing it with 0.5M hydrochloric acid. The effluent was made up to 100 ml (solution B), 20 ml of which were made 2M in hydrochloric acid and passed down a redox column to determine the amount of vanadium(V) present. A The nickel was obtained free from interfering metals by passing 20 in1 of solution B (made 8M in hydrochloric acid) down anion-exchange column II. The nickel passed through unadsorbed, and a further 20-30 ml of 8M hydrochloric acid were passed through to ensure complete removal of the nickel. The nickel was then determined gravimetrically with dimethylglyoxime. It should be noted that anion-exchange columns that have been used to remove chromium should not be used for nickel removal because the 8M hydrochloric acid would reduce the adsorbed chromium(W) to chromium(II1) and then elute it, thereby contaminating the nickel. After the nickel removal, 20-30 ml of 4M hydrochloric acid were passed through column II to remove the vanadium, and this effluent was discarded because a certain amount had also emerged in the nickel solution.
Redox and ionexchange
737
columns
Redox Column 2MHCl SOLUTION A---------+ PI + [Crl Ni(II), CrO, V(v), Fe(III)
~
,
~%z?r”
Red~~~~~
SOL&ON B Ni(II), V(V), Fe(II1) Anion-exchange column II 8M HCl
Cr(VL) on column
l
WI
I
1 Part of V; Fe on column
SOL&ON Part of V, Ni -+ wi]
4M HCl J SOLUTION PartofV
F&II) on column Water Y
FIG. l.-Separation
and determination
Redox Column SOLUTION [Fe] 0.03 M HCL Fe(III) of Ni(II), Cr(VI), V(V) and Fe(II1).
The iron was obtained by passing 20-30 ml of water down column II. Collection was started soon as the yellow colour of the iron chloro-complex appeared through the column sinter. For the purpose of determining the iron concentration on a redox column it was essential that the acidity should not exceed 0*03M. Accordingly, the iron eluate was made slightly alkaline with dilute ammonia and then the brown gelatinous precipitate was just redissolved with dilute hydrochloric acid so that the final acidity did not exceed 0*03M. The iron content was then determined by passing the solution through a redox column (conditioned with 0*03M hydrochloric acid) followed by reduction with ascorbic acid and back-titration with iodine solution.
as
RESULTS
AND
DISCUSSION
Ion-exchange column behaviour Nickel. A solution of nickel chloride in concentrated hydrochloric acid was passed through the column; the eluate was shown to contain nickel. Elution of the column with O-H4 hydrochloric acid gave no further nickel in the eluate, i.e., nickel was not adsorbed on the column. Chromium. Passage of a solution of potassium dichromate in 0.5M hydrochloric acid resulted in the complete adsorption of chromium(X). This could only be eluted with concentrated hydrochloric acid, which slowly reduced it to chromium(III) which was then eluted. Vanadium. Kraus and Nelson2 reported that vanadium(V) is adsorbed from concentrated hydrochloric acid solutions and eluted by 7M hydrochloric acid. The hydrochloric acid concentration of the sample solution, which after passage down column I is 0*5M, must therefore be increased and this could be achieved by saturation with hydrochloric acid gas. A more convenient alternative was sought by adding
G. A. H. ROBERTS
738
concentrated hydrochloric acid (11 M) to the sample solution. A 1OM acid concentration requires the addition of 100 ml of concentrated acid to 10 ml of sample solution but this large total volume, besides being inconvenient to handle, appeared to cause the elution of the vanadium (see Table I)_ Therefore a lower con~ntration of 8M was chosen because this gave adsorption of all but a trace of the vanadium TABLEI.--SEPARATION OF NICKEL,VANADIUM ANDIRON BY ANION-EXCHANGE Sample volume, ml Cont. HCl added, ml ResuItant [HCI], M Metals eluted: a, immediately b, by 4MHCl c, by 03MHCl
10
5
100
22.5
20
10
9
8
Ni, V (trace) V Fe
Ni, V (trace) V Fe
Ni, V* None Fe
7.5
* Elution of vanadium probabIy due to large volume of hydr~~ori~ acid used. with the minimal
addition of hydrochloric acid. An interesting observation was made during the subsequent elution of the vanadium from the column. The use of 7M hydrochloric acid gave a blue eluate of vanadium(IV) whereas 4M acid gave a yellow eluate of vanadium(V). A similar observation had been made with chromium in the previous section where the passage of concentrated hydrochloric acid led to the reduction of chro~um~V1) to chromium(I~. Strelow and Bothma also noted the reduction of chromium(X) on a anion-exchange column and ascribed it to reduction by the quaternary ammonium resin. Iron. When solutions of iron(III) chloride in concentrated hydrochloric acid and in 4h4 hydrochloric acid were passed down a column there was no elution of iron(III) that could be detected by the addition of ammo~um thiocyanate. The addition of O*SM hydrochloric acid or water eluted all of the iron absorbed on the column.
Separation and determination of nickel(U), chromium( VI), vanadium(V) and iron(Ill) The information obtained was used to devise a scheme for separating the four metals. Solution A was prepared by mixing Xl-ml quantities of O*lM nickel chloride, 0*05M potassium dichromate, 0.1M ammonium vanadate and 0.1M iron(III) chloride and diluting to 250 ml with water. Then 25 ml of this solution were made 0*5M in hydrochloric acid and passed down a column. The first 10 ml of eluate were discarded as being the hold-up volume. The ~hromium(V~ was adsorbed as an orange layer. The column was washed with 0*5M hydrochloric acid until the effluent was colourless. This eluate (total volume 30 ml) contained nickel, vanadium and iron, and was divided into three parts for passage down a second column at varying hydrochloric acid concentrations. The details and results are given in Table I. Some vanadium passes through with the nickel, but does not interfere with the subsequent determination with dimethylglyoxime. These separation techniques were then combined with use of redox columns to determine the amounts of the four metals in Solution A. It has previously been reportedl*a that iron(II1) in 0.03M hydrochloric acid is completely reduced by a redox column but not at all in 2N hydrochloric acid, and that vanadium(V) and
Redox and ion-exchange
columns
739
chromium(VI) are completely reduced in 2M hydrochloric acid. However, noinformation was available for vanadium in 0*03M hydrochloric acid. A check experiment showed that ammonium vanadate is 40% reduced by passage down a redox column in 0.03M hydrochloric acid, and so, for the determination of iron, it is essential that vanadium is absent. When solution A (made 2M in hydrochloric acid) was passed down a redox column, the recovery of vanadium(V) plus chromium(VI) was 0*0790-0~0848 mequiv/ ml, average 0.0806, expected value 0*0800. Table II gives results obtained for the determination of vanadium(V) after the removal of chromium(V) by the anion-exchange column I. These showed that a TABLE II.-DETERMINATION
OF VANADIUM oFcHRohlmM
mu
Vanadium, HCl concentration used in chromium removal, M 0.03
o-2 0.5
_
REMOVAL
mequiv/ml
Found
Taken
OO236 0023, o-0225 0.024 O-021, O-021 0022 O-020 O-020 0020
O-020
o-020 0.020
concentration of at least 05M hydrochloric acid was necessary in the chromium removal step. Lower concentrations of chloride ion gave higher apparent vanadium values, i.e., not all of the chromium was being removed. Nickel recovery, after removal of chromium(V1) with an anion-exchange column and 05M hydrochloric acid and of vanadium(V) and iron(III) with the second anion-exchange column and 8M hydrochloric acid, ranged from 100 to 106%, average 103%. TASLE III.--ANALYSIS OF MIXTURES OF IRON, NICKEL, CHROMIUM
AND VANADIUM
Metal
Solution I Taken* Found *
Solution II Taken* Found*
Solution III Taken* Found*
Solution IV Taken* Found*
Iron Nickel Chromium Vanadium
O-0833 0.0344 00104 0.0611
0.0313 0.0917 0.0312 0.0204
0.1459 0.0344 0.0104 0.0204
0.0313 0.0573 OG416 0.0815
0.0838 0.0339 0.0107 0.0616
0.0321 0.0923 0.0318 0.0196
0.1483 0.0339 0.0109 0.0196
0.0332 0.0581 O&409 0.0818
* In g/100 ml of solution.
Iron(II1) recovery by final elution with water from the second anion-exchange column was 98 %. As a further check, four solutions containing various proportions of the four metals were analysed. The results obtained are given in Table III.
G. A. H. ROBERTS
740
ZusammenfasstmB-Zur Trennung und Bestimmung von Eisen, Nickel, Chrom und Vanadium in L.&sung wurden Redox- und Anionenaustauschsiiulen verwendet. Die Anionenaustauschs&den ermoglichen einige der Trermungen, die Redoxtiulen werden xur Bestimmung von Eisen, Chrom und Vanadium gebraucht. Chrom und Vanadium kbnnen neben Eisen durch Kontrolle der Acidit& in der RedoxsB;ule bestimmt werden. Durch Verwendung eines Speicher-(“memory”)verfahrens in der Saule wurde die Titration der MetallL%mg vermieden. Die Metbode xeigt Vorteile gegen ber bisher bekannten Verfahren. R&stun&On a utilid des colonnes redox et echangeuses d’anions pour s&parer et determiner le fer, le nickel, le chrome et le vanadium en solution. Les colonnes echangeuses d’anions four&sent quelques-unes des separations et l’on utilise les colonnes redox pour la determination du fer, du chrome et du vanadium. On peut determiner le chrome et le vanadium en presence du fer par ajustage de l’acidite dans la colonne redox. En utilisant une technique de colonne “a m&moire”, on a &it6 le titrage de la solution reelle de m&al. La m6thode montre quelques avantages par rapport aux methodes ordinaires. REFERENCES 1. 2. 3. 4. 5.
R. K. E. B. F.
Belcher, J. R. A. Kraus and Cerrai and C. E. Saltzmarm, W. E. Strelow
Majer and G. A. H. Roberts, TuCunfa, 1967, 14, 1245. F. Nelson, Am. Sot. Testing Mater. Spec. Tech. Publ. No. 195,1956, 27. Testa, Anal. Chim. Actu, 1963, 28,205. Anal. Chem., 1952,24,1016. and C. J. C. Bothma, ibid., 1967, 39, 595.