Determination of hexacyanoferrate based on its catalytic influence on the oxidation of p-phenylenediamines by iron(III)

Summary Both hexacyanoferrate(III) and hexacyanoferrate(II) catalyze the oxidation of p-phenylenediamines by iron(III)_aq. The rate of this reaction in the presence of a sample with an unknown amount of hexacyanoferrate is compared with the reaction rate of solutions containing well defined concentrations of this substance. In this way, hexacyanoferrate can be determined photometrically down to <10^–9 mol/l. Although this procedure is very sensitive, the analysis can be performed with a simple photometer. Absorbance changes >0.2 can easily be obtained in 1 cm cuvettes, even at extremely small concentrations of hexacyanoferrate, because it is not an absorbance proportional to the concentration of hexacyanoferrate but rather the formation rate of p-semiquinonediimine which enables the quantitative determination of hexacyanoferrate.