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Determination of vanadium(V), chromium(VI) and cerium(IV) in binary or ternary mixture
Tchnta,
Vol 22, pp. 10>106.
Pergamon
Press, 1975. Printed in Great Br~tam.
LETTER TO THE EDITOR
DETERMINATION OF VANADIUM(V), CHROMIUM(V1) AND CERIUM(IV) IN BINARY OR TERNARY MIXTURE SIR,
Potentiometric titration with iron differentiates vanadium(V) and cerium(IV) but not vanadium(V) and chromium(VI), or chromium(W) and cerium(IV) when present together. The methods’ available fqr the determination of these species in binary or ternary mixture are generally cumbersome and less accurate. I now propose a different scheme for this determmation, based on the following consideratrons. Vanadium(IV) can be titrated2 with cerium(IV) sulphate at room temperature, using ferroin indicator, even in the presence of chromium(II1). Chromium(V1) is reduced completely3 by vanadium(IV) in a short time if the latter is used in excess. Although it seems that arsenic(II1) selectively reduces chromium(VI) when added in excess to a solution containing chromium(V1) and cerium(IV) in dilute sulphuric acid, it is not possible to titrate the latter oxidant with iron(I1) without interference from arsenic(III), when ferroin is used as indicator. At the concentrations used, arsenic(II1) and cerium(IV) are not without reaction as is generally assumedi and the reaction is catalysed by chromium(II1) formed from the reaction of chromium(V1) and arsenic(II1). It is even possible to determine arsenic(II1) through a reaction between arsenic(II1) and cerium(IV) catalysed by chromium(II1) at room temperature and the results are correct to within +0.2%. The procedure for this is as follows. To 510 ml of O.OSMcerium(IV) sulphate add 2-6 ml of _ O.OSNarsenic(II1) (free from chloride) and 1 ml of O.lM chromium(II1) [prepared from potassium chromium(II1) sulphate], stir the mixture well and allow it to stand for 5 min. Add 5 ml of 20N sulphuric acid and dilute to 45 ml, and titrate the excess of cerium(IV) with 0.05M iron( using a drop of O.OlM ferroin as indicator. It is possible that the reaction occurs through the following steps: Cr (III) + Ce (IV) &Cr (IV) + Ce (III) Cr (IV) + Ce (IV)* Cr (V) + Ce (III) Cr (V) + As (III) fast Cr (III) + As (V)
(1) (2) (3)
Evidently step (2) is faster than the rate-determining step of the uncatalysed arsenic(III)+zerium(IV) reaction. It is also not possible to determine titrimetrically the total of vanadium(V) and cerium(IV) when chromium(V1) is present in the same solution, by using the selective reduction of the latter with arsenic(IIIk and for the same reason. It has been observed that chromium(VI)4 and/or vanadium(V) do not interfere in the potentiometric titration of cerium(IV) in 05-l .OM nitric acid with oxalate. It seems that this is possible owing to the slow kinetics of the reactions of chromium(W) and vanadium(V) with oxalate. RECOMMENDED
PROCEDURES
~a/anadiutn(V)+ chromium(V1) or oanadium(V) + cerium(lV) (i) Titrate with 0.05M iron(I1) a solution containing 2-5 ml of _ 0.05M vanadium(V) and 2-5 ml of -@05N chromium(W) [or 2-5 ml of +-005M cerium(IV) sulphate in 1N sulphuric acid] in 56M sulphuric acid medium, using a drop of O.OlM ferroin as indicator. Alternatively, treat the solution containing vanadium(V) and chromium(V1) [or cerium(IV)] with 20 ml of 0.05M iron(I1) solution and titrate the unreacted iron(I1) with 0.05N dichromate or cerium(IV) sulphate in 56M sulphuric acid medium, using ferroin as indicator. This gives the sum of vanadium(V) and chromium(V1) [or cerium(IV)]. (ii) Treat a similar solution of vanadium(V) and chromium(V1) [or cerium(IV)] as in (i) with 20 ml of 0.05M iron(I1) or vanadium(IV) and titrate the unreacted reductant (after a lapse of 5 min in the latter case) with cerium(IV) sulphate in 7-8.5M acetic acid medium, using ferroin as indicator. Alternatively, for cerium(IV) use a procedure similar to (iii) below. The value of chromium(V1) or cerium(IV) is calculated and the value of vanadium(V) obtained by difference. Chromium( VI) + cerium(lV)
A procedure similar to (i) or (ii) above is used but the titrations in the former can be done in 2N sulphuric acid medium [cerium(IV) sulphate is used for the titration of excess of iron(I The sum of chromium(V1) and cerium(IV) is then calculated. A procedure similar to (iii) below is used to determine the value of cerium(IV). The value of chromium(V1) is then obtained by difference. 105
Iu6
LETTER
l-0 THE EDITOR
Vanadium(V) + chromium(V1) + cerium(lV)
A procedure similar to (i) is used to determine the sum of vanadium(V), chromium(V1) and cerium(IV), then a procedure similar to (ii) is used to determine the sum of chromium(V1) and cerium(IV). (iii) Titrate a solution containing 2-5 ml bfeach of005M vanadium(V), O*fbSNchromium(W) and 0.05M cerium(IV) in 50 ml of 0.5-I.OM nitric acid, with 0.05N oxalic acid to a potentiometric end-point. The potential drop at the equivalence point is 150-200 mV per 0.04 ml of titrant as against 300-400 mV in the titration of cerium(IV) alone. The oxalic acid used corresponds to the cerium(IV) in the mixture. The values of the chromium(V1) and vanadium(V) are obtained by difference. The relative errors’ of the visual procedures and potentiometric methods used are within +02 and *@5x respectively. Department of Chemistry Andhra Uniwrsity Post-Graduate Centre Gun&-$22005. India
K. SRIRAMAM
25 July 1974 REFERENCES
1. 2. 3. 4.
I. M. Kolthoff and R. Relcher, Vo’olumefricAnalysis, Vol. III, pp. 605 and 131. Interscience, New York, 1957. K. Sriramam and G. G. Rao, Talanta, 1966, 13, 1468. K. Sriramam, ibid., 1971,18, 361. G. G. Rao, K. S. Murty and M. Gandikota, ibid., 1972 19, 65.
Vol 22, pp. 10>106.
Pergamon
Press, 1975. Printed in Great Br~tam.
LETTER TO THE EDITOR
DETERMINATION OF VANADIUM(V), CHROMIUM(V1) AND CERIUM(IV) IN BINARY OR TERNARY MIXTURE SIR,
Potentiometric titration with iron differentiates vanadium(V) and cerium(IV) but not vanadium(V) and chromium(VI), or chromium(W) and cerium(IV) when present together. The methods’ available fqr the determination of these species in binary or ternary mixture are generally cumbersome and less accurate. I now propose a different scheme for this determmation, based on the following consideratrons. Vanadium(IV) can be titrated2 with cerium(IV) sulphate at room temperature, using ferroin indicator, even in the presence of chromium(II1). Chromium(V1) is reduced completely3 by vanadium(IV) in a short time if the latter is used in excess. Although it seems that arsenic(II1) selectively reduces chromium(VI) when added in excess to a solution containing chromium(V1) and cerium(IV) in dilute sulphuric acid, it is not possible to titrate the latter oxidant with iron(I1) without interference from arsenic(III), when ferroin is used as indicator. At the concentrations used, arsenic(II1) and cerium(IV) are not without reaction as is generally assumedi and the reaction is catalysed by chromium(II1) formed from the reaction of chromium(V1) and arsenic(II1). It is even possible to determine arsenic(II1) through a reaction between arsenic(II1) and cerium(IV) catalysed by chromium(II1) at room temperature and the results are correct to within +0.2%. The procedure for this is as follows. To 510 ml of O.OSMcerium(IV) sulphate add 2-6 ml of _ O.OSNarsenic(II1) (free from chloride) and 1 ml of O.lM chromium(II1) [prepared from potassium chromium(II1) sulphate], stir the mixture well and allow it to stand for 5 min. Add 5 ml of 20N sulphuric acid and dilute to 45 ml, and titrate the excess of cerium(IV) with 0.05M iron( using a drop of O.OlM ferroin as indicator. It is possible that the reaction occurs through the following steps: Cr (III) + Ce (IV) &Cr (IV) + Ce (III) Cr (IV) + Ce (IV)* Cr (V) + Ce (III) Cr (V) + As (III) fast Cr (III) + As (V)
(1) (2) (3)
Evidently step (2) is faster than the rate-determining step of the uncatalysed arsenic(III)+zerium(IV) reaction. It is also not possible to determine titrimetrically the total of vanadium(V) and cerium(IV) when chromium(V1) is present in the same solution, by using the selective reduction of the latter with arsenic(IIIk and for the same reason. It has been observed that chromium(VI)4 and/or vanadium(V) do not interfere in the potentiometric titration of cerium(IV) in 05-l .OM nitric acid with oxalate. It seems that this is possible owing to the slow kinetics of the reactions of chromium(W) and vanadium(V) with oxalate. RECOMMENDED
PROCEDURES
~a/anadiutn(V)+ chromium(V1) or oanadium(V) + cerium(lV) (i) Titrate with 0.05M iron(I1) a solution containing 2-5 ml of _ 0.05M vanadium(V) and 2-5 ml of -@05N chromium(W) [or 2-5 ml of +-005M cerium(IV) sulphate in 1N sulphuric acid] in 56M sulphuric acid medium, using a drop of O.OlM ferroin as indicator. Alternatively, treat the solution containing vanadium(V) and chromium(V1) [or cerium(IV)] with 20 ml of 0.05M iron(I1) solution and titrate the unreacted iron(I1) with 0.05N dichromate or cerium(IV) sulphate in 56M sulphuric acid medium, using ferroin as indicator. This gives the sum of vanadium(V) and chromium(V1) [or cerium(IV)]. (ii) Treat a similar solution of vanadium(V) and chromium(V1) [or cerium(IV)] as in (i) with 20 ml of 0.05M iron(I1) or vanadium(IV) and titrate the unreacted reductant (after a lapse of 5 min in the latter case) with cerium(IV) sulphate in 7-8.5M acetic acid medium, using ferroin as indicator. Alternatively, for cerium(IV) use a procedure similar to (iii) below. The value of chromium(V1) or cerium(IV) is calculated and the value of vanadium(V) obtained by difference. Chromium( VI) + cerium(lV)
A procedure similar to (i) or (ii) above is used but the titrations in the former can be done in 2N sulphuric acid medium [cerium(IV) sulphate is used for the titration of excess of iron(I The sum of chromium(V1) and cerium(IV) is then calculated. A procedure similar to (iii) below is used to determine the value of cerium(IV). The value of chromium(V1) is then obtained by difference. 105
Iu6
LETTER
l-0 THE EDITOR
Vanadium(V) + chromium(V1) + cerium(lV)
A procedure similar to (i) is used to determine the sum of vanadium(V), chromium(V1) and cerium(IV), then a procedure similar to (ii) is used to determine the sum of chromium(V1) and cerium(IV). (iii) Titrate a solution containing 2-5 ml bfeach of005M vanadium(V), O*fbSNchromium(W) and 0.05M cerium(IV) in 50 ml of 0.5-I.OM nitric acid, with 0.05N oxalic acid to a potentiometric end-point. The potential drop at the equivalence point is 150-200 mV per 0.04 ml of titrant as against 300-400 mV in the titration of cerium(IV) alone. The oxalic acid used corresponds to the cerium(IV) in the mixture. The values of the chromium(V1) and vanadium(V) are obtained by difference. The relative errors’ of the visual procedures and potentiometric methods used are within +02 and *@5x respectively. Department of Chemistry Andhra Uniwrsity Post-Graduate Centre Gun&-$22005. India
K. SRIRAMAM
25 July 1974 REFERENCES
1. 2. 3. 4.
I. M. Kolthoff and R. Relcher, Vo’olumefricAnalysis, Vol. III, pp. 605 and 131. Interscience, New York, 1957. K. Sriramam and G. G. Rao, Talanta, 1966, 13, 1468. K. Sriramam, ibid., 1971,18, 361. G. G. Rao, K. S. Murty and M. Gandikota, ibid., 1972 19, 65.