The Determination of the Chemical Diffusion Coefficient of n-Type AgBr by Means of a D.c. Polarization Cell

Abstract

Abstract The electronic conductivity of AgBr and the chemical diffusion coefficient of n-type AgBr were measured by means of a d.c. polarization cell, \ominus Ag/AgBr/inert electrode ⊕, in the temperature range from 325 °C to 400 °C. In order to suppress the electrolysis of AgBr, a platinum plate was used instead of carbon as the inert anode. The chemical diffusion coefficient, \ ildeD, was determined by two methods. One was the intermediate probe method (IP method) whereby the relaxation of the voltage between the Ag reversible electrode and the Pt intermediate probe, inserted into the middle of AgBr, was followed after an abrupt change in the polarization voltage. The other was the open-circuit method (OC method) whereby the terminal voltage of the cell was followed after the circuit of the cell had been opened. The electronic conductivity in the steady state was independent of the nature of the inert electrodes (Pt or C), provided that the inert electrode was completely sealed. However, it took more time for the cell with a carbon electrode to reach the steady state than that with a platinum one. The chemical diffusion coefficient, \ ildeD, determined by the IP method for the Ag/AgBr/Pt cell agreed with that for the Ag/AgBr/C cell. The chemical diffusion coefficient, \ ildeD, determined by the OC method for the Ag/AgBr/Pt cell agreed with that obtained by the IP method. The chemical diffusion coefficients determined by these methods were expressed by \ ildeD=3.0×103exp\left(\frac−89.9 kJ mol−1RT\ight)⁄cm2 s−1. No chemical diffusion coefficient can be obtained by the OC method for the Ag/AgBr/C cell, because of the slow diffusion of the Br2 gas produced by the electrolysis of AgBr from the carbon electrode.