obtained. Using this in equation 2 a value of GO for K 1 was obtaiiied from the solubility of 0.107 m with iio added halide ion. This value is high compared to the c.m.f. value of Alvaree-lhnes, Uraunstein, arid Blander (20 on molality basis).8 Actually, using the value of 20 for K1 leads to = 2.7 x 10-3, which is not unreasonable considering tlic 220' extrapolation and change from SaNOJ-Kn'Os to KNOo solveiit. Kz was cstiiiiated by solving eq. 1 aiid 3 siniultaiicously for [CI-] and K z . The values for rt2 ranged t)ci\\cwi 4 aiid I). However, using t h e coristarits it (:ai1 l x sec'ii that [AgClz- 1 caiiriot be iieglccted cvcii wlic~ii110 excess chloride is added. This would incari that Ka,,was actually smallci~ than 2.7 x 1 0 - 3 . For calculatiiig the chloride ion concentrations the following approsirnate values w r c used : lcSp= 2 x 10-3, rc, = 20, 1~ = o. The total cliloridc coiiccntration (found by analysis) \\-as
0.2-
X ul
0.1 t J
m 3 J
0
0 0
0*2 KX1KN03 ,
Fig. 3.-Solubiiitj
0,4
0.6
moles/ kg,
of AgCl and XgBr in KSOS.
obtaiiiing bromide ioii concciitratio~isfrom the total bromide found by aiialysis was
T h e final equation was used in calculating the chloride ion concentratioiis in Tablr I. Equation 3 iicglects complexation with lead, but the lead concentration in KXO3 never exceeded 0.013 rc, 111 addition, when the halide concciitration was so loic that 0.013 would be appreciable, the fraction of lead complcxcd with halide was low. In the case of hgI3r, K , lias t)cen measured at various tempcratures,8 and a t 430' the d u e is about 7 3 . Irom the solubility reported here ICsl, = 0 X 10-3 aiid K I = 30. Alvarcz-l;unes, 13rauiistt~in,a i d I3lander report Kz = 27 at 452' which shows good agreement for this constant. Their published 17aluc was used for calculatiiig bromide concentrations. The cquatioii used for (8) A. Alvnre+runes, -1. Rraunstcln, rrnd AI. Rlandcr, J . Am. Cliem. 1538 (lDR2).
S o r . , 84,
'Llius, the correction for bromidc was much less thaii for the chloride system, and sliglit errors in the coilstants would have less effect on the calculatioii of the lead halide dissociation constants. In fact, the constants for the line drawn through tlir uncorrected points in I'ig. 2 (curve A) \\-ere K = 0.37, IC, = 0.15, and ICZ = 0.038. Only liz was changed appreciably by this calculation. Obviously, it would be more satisfying to measure tlir halide ion activity directly i n the molten KKO3 phase, and hopefully, measurements of this type will be made in the future. Acknowledgment.--Some of the expclrimciits reported were performed at the du Polit II. I
