Hydrolysis of the Manganic Ion

Abstract

RECENTLY the kinetics of the reactions of manganic perchlorate in aqueous solution have become the subject of investigation, but so far the species of Mn(III) present have not been identified. From a spectrophotometric investigation of manganic perchlorate solutions prepared electrolytically, we find maxima in the region of 203–220 mµ, and at 470 mµ and a minimum of 350 mµ The results of a detailed examination of such solutions at 25° C in the region 300–550 mµ, with acidity varying between 0.10 M and 3.70 M HClO₄ at an ionic strength µ = 4.0 maintained by the addition of manganous perchlorate (which also stabilizes the manganic ion) can be interpreted in terms of only one equilibrium: where Mn³⁺H₂O is the hexa-aquo ion and MnOH²⁺ the hydroxopenta-aquo ion having extinction coefficients ε₁ and ε₂, respectively. The observed extinction coefficient ε should follow: and rearranging: When ε₁/ε is sufficiently < 1, a plot of [H⁺₃O] against 1/ε at a fixed wave-length should be approximately linear. Such plots are found to be linear up to about 1.0–1.5 NHClO₄: examples are shown in Fig. 1. From this plot at each wave-length an approximate value for K is found from the intercept, and then from the slope an approximate value for ε₂; by substitution of these values back in equation (1) an approximate value for ε₁ is obtained. This is then used in the full equation (2), and (1 − ε₁/ε) × [H⁺₃O] plotted against 1/ε for each wave-length for the whole acidity range 0.10–3.70 N HClO₄: fine adjustment is made to ε₁ to give the best straight line in each case over the whole acidity range, thus giving an accurate value for ε₁ and accurate values for K and ε₂ from the intercept and slope respectively.