MECHANISM OF A REDOX INDICATOR REACTION - KINETICS OF OXIDATION OF FERROCYPHEN BY NITROUS-ACID

The oxidation of [Fe(phen)2(CN)2] and [Fe(bipy)2(CN)2] by nitrous acid in sulphuric acid follows the kinetic equation rate = k[H+] [HNO2] [complex] at low acidities. The mechanism is a diffusion controlled reaction between NO+ and the complex. Reaction is too slow for satisfactory use as a redox indicator for nitrite titrations at low acidities (0.1 M) [H+]. The variation of rate with acidity in more concentrated sulphuric acid (up to 6 M) is interpreted in terms of protonation of the complex to form [Fe(phen)2(CNH)2]2+. © 1979 Springer-Verlag.