Dec. 5, 1932
OSMOTIC
PROPERTIES OF
[CONTRIBUTION FROM
THE
AQUEOUSSODIUM CHLORIDE-CESIUM CHLORIDE
6035
CHEMISTRY DEPARTMENT, UNIVERSITY OF MALAYA]
The Osmotic Properties of Aqueous Sodium Chloride-Cesium Chloride Mixtures at 25' BY R.A. ROBINSON RECEIVED JUNE 16, 1952 Isopiestic vapor pressure measurements have been made of mixtures of sodium and cesium chloride in aqueous solution and from the results the variation of the activity coefficient of each salt with change in its molality at constant total molality of the solution has been studied. activity coefficient of this salt has already been measured up to 3 m by the e.m.f. method3 and up t o 5 m by the isopiestic method4; the latter measurements not being carried to higher concentrations although cesium chloride is a very soluble salt because a 5 m solution is in equilibrium with saturated potassium chloride and, a t that time, no other reference salts were available. Measurements have now been made up to 11.63 m (Table I ) and the osmotic and activity coefficients calculated (Table 11)t o supplement those given earlier.' For work on the mixtures six stock solutions containing sodium and cesium chloride in different ratios were prepared and with these isopiestic measurements were made over the range 0.5 to 6 m, using sodium chloride as reference salt. TABLE I Isopiestic measurements were made as close as possible to ISOPIESTICSOLUTIONS OF CESIUM CHLORIDE AND SODIUM round values of m and interpolated within a very small concentration range. Table I11 records these isopiestic ratios CHLORIDE OR CALCIUM CHLORIDE R = mR/?n where m is the total molality of the mixed salt mCoC1 7nNaCI mCaCI mNaCl mCaCl mNaCl mCsCl mNaCl solution and mR that of the sodium chloride solution of the 1.996 1.773 3.671 3.117 5.248 4.291 6.081 4.871 same vapor pressure. The molality of cesium chloride in 2.439 2.139 4.215 3.526 5.358 4.363 6.436 5.120 the mixture will be denoted by ml,that of sodium chloride 3.158 2.718 4.999 4.108 5.509 4.476 7.475 5.803 by w , where ml = xm and ml = (1 - x ) m .
Owen and Cooke' applied the isopiestic vapor pressure method to determine the thermodynamic properties of potassium and lithium chloride when mixed in aqueous solution a t constant total molality. Similar measurements have been reported recently2 on potassium chloride-sodium chloride mixtures. Nearly all other mixtures which have been studied have included hydrochloric acid as one component and the present study is intended to extend our knowledge of two salt mixtures.
3,319
2.840
mcsci
mcsciz
mcsci
m c m
7.856 7.879 8.455
2.947 2.949 3.089
9.261 9.354
3.279 3.302
mcaci
mcaciZ
9.620 3.361 10.364 3.525
mclci
mcsci2
10.985 3.652 11.630 3.779
TABLE I1 OSMOTIC AND AcTrvrrY COEFFICIENTS OF CESIUM CHLORIDE AT m
P
Y
2.0 2.5
0.864 .871
0.496 .485
3.0 3.5 4.0 4.5 5.0
.880 ,891 .901 .913
.479 .475 .474 .474
.923
.475
Discussion IfHarned's rule holds for this pair of salts log 71 = log YlCO, - a 1 m 2 log 7 2 = log 72(0) - orzlllzl and 55.51/xm2 X log Q ~ ( O ) / U ~ (=~ )(a12
25" m
P
Y
6.0
0.480
7.0 8.0 9.0 10.0 11.0
.486 *496 .503 .508 .512
'.0°4 1.013 1.018
+
a21)x
- 2a21
(1) (2) (3)
where uw(x)is the water activity of a mixed salt solution of total molality m, and uw(o)that of a sodium chloride solution of the same molality. The latter is represented very faithfully over the range 2 6 m 6 by the empirical equation
