Electromotive force measurements of the cell Na ion-selective elec-trode(glass) |NaBr(m)

MeOH(w)

H2O(100-w)|Br ion-selective electrode (solid state)were made at molalities of NaBr ranging from 0.01 to 3.0 in the mixed solvent of methanol-water containing 0

10

20

40

60

80

90

and 100 weight% methanol

respectively

at 298. 15K. Calculations of standard cell potential were made by using the Pitzer equation

and then the mean activity coefficient values of NaBr were evaluated. The association constant of NaBr in methanol were evaluated from the experimental E. M. F. data by using the Pitzer-Simonson equation extended to the binary systems containing ion pairing. The weak association of NaBr in methanol was shown. At each fixed solvent composition

the mean activity coefficients of NaBr first decreased and then increased with rising NaBr molality. At each fixed NaBr molality

they decreased with an increase of methanol in solvent mixtures.