ABSTRACT
Vanadium(II) sulphate is a versatile reductant; it can change its oxidation number by one, two, or three units, depending on the oxidation potential of the half-cell with which it reacts and on the mode of titration. The end-point of many of the reactions can be detected by means of indicators. A back-titration method is usually needed for the three-electron oxidation of vanadium(II).
ABSTRACT
Methods for the standardisation of vanadium(II) sulphate are described. With weak oxidising agents, e.g., Fe(III)/Fe(II) in phosphoric acid, vanadium(II) undergoes a one-electron change, which is satisfactorily indicated by Neutral Red, Phenosafranine or Safranine T, or by potentiometry. Stronger oxidising agents oxidise vanadium(II) to vanadium(IV); Variamine Blue, Methylene Blue, o-dianisidine, cacotheline, ferroin, bis(alpha,alpha'-bipyridyl)iron(II) and ammonium molybdate are satisfactory indicators. Binary mixtures of oxidising agents can be determined.
