A polarographic and coulometric study of the iron(III)-Solochrome Violet RS system

Experimental results are presented which clearly illustrate the kinetic character of the polarographic waves/peaks of the iron(III)-Solochrome Violet RS (SVRS) complex formed at pH 4.7. Even in the presence of an excess of iron(III), at the 10(-3)M level of SVRS only half of the azo-dye wave is displaced to more negative potentials. At lower concentrations (e.g., 10(-6)M), however, a larger fraction is displaced. The undisplaced wave/peak corresponds to reduction of free dye dissociated from the complex, whereas the displaced wave/peak corresponds to the reduction of the complexed dye directly. The temperature coefficient for the undisplaced peak is 12%/deg at the 10(-5)M level, indicating that the height of this peak is controlled, at least in part, by the rate of dissociation of the iron(III) complex. The rate of increase of the total limiting current produced by reduction of the complex and of the free dye dissociated from it, becomes less at higher concentrations and is an indication of adsorption occurring at the dropping mercury electrode. In the controlled-potential coulometric reduction of the complex at a mercury-pool cathode at pH 4.7, the SVRS is reduced completely in a four-electron reaction at a potential only slightly more negative than that for the reduction of the free dye, indicating that the dye is dissociated from the complex and then reduced at this potential; the iron(III) is reduced simultaneously to iron(II). In view of the kinetic nature of the polarographic waves of the SVRS-iron(III) complex, care must be taken in using SVRS as a reagent for the polarographic determination of iron(III).