Potentiometric study of the solid state ion-exchange reaction between α-AgI and β″-alumina

Abstract

The ion-exchange reaction between β″-alumina and solid α-AgI was investigated by measuring the EMF of a galvanic cell with a β″-alumina solid electrolyte. The following cell was assembled and EMF measurements were taken in a temperature range between 342 to 816 K: Mo, Na(s, 1)| β″-alumina|Ag+AgI+ β″-alumina, Mo (I). Ag, AgI and β″- alumina powders were mixed in a molar ratio of Ag : AgI : Na (in β″-alumina powder) of 1.00 : 1.00 : 1.00. The cell was heated and maintained at 630 K for 30 d. The ion-exchange between Ag + ion in α-AgI and Na + ion in β″-alumina powder gradually proceeded and formed two solid solutions of (Ag + −Na + )I − and (Na + −Ag + )-β″-alumina. The resulting EMF was higher than that of cell ( II) : Mo, Na(s, 1)| β″-alumina|NaI+Ag+AgI, Mo(II). The 30 days retention at 630 K gave a cell EMF of 2.1370 V which was higher than the EMF of cell (II) by 9.6 mV. When the cell temperature was decreased below 600 K, a NaI saturated with AgI, (Na + −Ag + )I − sat. phase appeared and the EMFs of cell (I) agreed with the EMFs of cell (II). By heating the cell to a particular temperature on the liquidus line on the AgI side of the AgINaI system, a liquid phase appeared and the ion-exchange between α-AgI and β″-alumina smoothly proceeded via the liquid. After that, two kinds of three-phase-equilibria of (Ag −Na−I) sat.+(Ag + −Na + )I − sat.+(Na + −Ag + )I − sat. and (Ag −Na−I) sat.+ (Ag + −Na + )I − sat.+liquid (Ag + −Na + )I − were attained below and above 657 K, respectively. Then, the EMFs of cell (I) agreed with the value of cell (II) below 657 K.