IODIDE-SELECTIVE ELECTRODES IN REACTING AND IN EQUILIBRIUM SYSTEMS

When H2O2 is added to iodate in acid solution, iodine and reactive intermediate compounds— e.g., HIO, HIO2, H2I2O3—thereof are formed, a complex reaction system results, and hydrogen peroxide is catalytically decomposed (1). Earlier investigations of this system (2) are being continued with major emphasis at present on measurements of the iodide activity, [I~], long recognized as a crucial variable and thought at times to be below 10 W. The usual Ag/AgI electrode could not be trusted in this reactive system. Of much greater promise are the iodide-selective electrodes (5) now available, in which crystalline silver iodide is interposed as a membrane between the unknown [I-] solution (here, the reaction mixture) and presumably an Ag/AgI electrode in contact with aqueous silver nitrate. When such an electrode is used in a reaction mixture, the membrane is a possible reactant. For example, if the mixture contains no silver ion, the reaction [formula omitted] The iodide produced in Reaction 1 of course influences the electrode reading. When this iodide is at least comparable with the iodide to be measured, the electrode reading will be too large. It will approach [formula omitted] fo as a limit (the solubility limit of the electrode) as the initial [I-] approaches zero. © 1969, American Chemical Society. All rights reserved.