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Physio-chemical studies of the oxidation of solid potassium iodide by halogen gases Baijal, Mahendra Das

Abstract

The mechanism of oxidation of Potassium iodide by Chlorine and Fluorine gas has been studied using powdered KI, pressed pellets and single crystals (pressed pellets only in the case of Fluorine). The electrical conductivity of a KI pellet has also been determined as a function of temperature. The major part of the experimental work concerns changes in D.C. electrical conductivity during reaction. The main conclusions are as follows: (a) The room temperature conductivity of KI is always higher than that obtained by extrapolating the impurity-range, and the activation energy below about 92° C. is very small; this is tentatively attributed to a space-charge effect. (b) In the KI/CI₂ reaction, the products are invariably KCl and I₂ in various forms (including V-Centres), although poly-halides are thermodynamically more stable. Thus the reduction of Chlorine occurs in a manner which does not allow reorganization to ICI₂-. (c) In pressed pellets, the conductivity always increases at the start of oxidation, but the enhanced conductivity may be ionic or electronic in different circumstances, and the initial increase may be followed by several alternative processes which are structure-sensitive. There is an inverse correlation between initial conductance and reactivity which is interpreted in terms of competition between (i) trapping of positive holes at isolated cation vacancies and (ii) nucleation of solid Iodine at grain boundaries. (d) Unpressed powders show no increase in conductance during reaction; this is attributed to preferential oxidation of bound surface states.

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