KINETICS OF OXIDATION OF THIOCYANATE ION BY MANGANESE(III) IN PYROPHOSPHATE MEDIUM

Mn(III) is stabilized by pyrophosphate in weakly acidic solutions. The nature of the complex formed was elucidated spectrophotometrically. The kinetics of Mn(III) oxidation of thiocyanate in pyrophosphate medium was investigated over the pH range 2-3. The oxidation followed first order kinetics with [Mn(III)]. The effects of varying [Mn(III)], [NCS-], added Mn(II) and metal ions, pH, total [P2O74-] and added ClO4-, Cl - and SO42- were studied. The order in [NCS-] was unity, and increasing [H+] increased the rate. Retardations with added P2O74- and Mn(II) were observed. Complexation of NCS- as K2Zn(NCS)4 decreased the reactivity without any change in overall mechanism. The dependence of the reaction rate on temperature was examined, and activation parameters were computed from Arrhenius and Eyring plots. A mechanism consistent with the results is proposed.