Differential coulometry with controlled potential. Determination of vanadium (5) and (4)

The application of the method of differential coulometry for precision determination of vanadium (5) and (4) in an orthophosphoric acid medium is considered. The specific feature of the method is the necessity of consistent control of the main electrochemical parameter, i.e. the potential of the working electrode, until the electrolysis is completed in the cell with the standard specimen and then in the cell with the test specimen. The diagram of the set-up and the operating procedure are given. The analysis procedure is established. 4.0 M H3PO4 is selected as the optimum background electrolyte. The electrolysis was carried out at potentials of 0.2 V (cathode process) and 1.25 V (anode process). The procedure developed enables one to determine vanadium (5) and (4) in the presence of a 1-to-50-fold excess of the conjugate form with a relative error of 0.1 %