ELECTROCHEMICAL DETERMINATION OF OXYGEN STOICHIOMETRY AND ENTROPY IN OXIDES

The oxygen stoichiometry of CeO2 and Ce0.82Gd0.18O1.91 is determined b y a combination of cyclic voltammetry and coulometric titration. The e lectrochemical cell employed is an oxygen pumping cell, in which the o xide is subjected to different oxygen pressures corresponding to the p otential imposed in the temperature range 800-1000 degrees C. With sca n rates of 2 mu V/s potential sweeps on CeO2 are reversible. The chang e in entropy is determined by either subtraction of e.m.f. curves obta ined by potential sweeps of different temperatures or by measuring the e.m.f. during a temperature scan. The latter method yields the most r eliable results. Potential sweeps on Ce0.8Ca0.2O1.8 suggest that calci um is expelled from the lattice.