Promoting effect of iodide on the hexacyanomanganate(IV)-arsenic(III) redox reaction, for kinetic determination of iodide

The rate of the reaction between hexacyanomanganate(IV) and arsenic(III) in an acid medium is strongly accelerated by iodide. The reaction kinetics indicates that the iodide activity decreases throughout the reaction, probably because manganese(IV) oxidizes iodide to iodate (an inactive form). This behaviour is defined as promotion, rather than catalysis, and this rate-modifying effect has been used to determine iodide by a kinetic method. A linear calibration plot was obtained by a two-point fixed-time procedure. A detection limit of 0.2 ng/ml, a quantification limit of 0.6 ng/ml and relative standard deviations of 5.5 and 13% for the 6.7 and 0.6 ng/ml levels respectively have been found. Positive kinetic interferences from osmium(VIII) and iodate have been observed, and copper(II), silver(I) and mercury(II) inhibit the iodide activity by precipitaton. The method has been applied to determination of iodide in sodium arsenite (reagent grade) and table salt. The method has been validated by recovery experiments.