JOHN C. SYNNOTT AND JAMES N. BUTLER
2474 tetrachloride did not remain constant after initial calibration but decreased rapidly toward zero. For example using a 0.1 M sample of DMF in carbon tetrachloride, the AT reading 2 min after the sample drop was placed on the thermistor was 5.20,l min later it was 3.20, and after 15 min AT was 0.03. The rate of AT decrease for DRlA solutions was slightly lower. This behavior would be expected with solutes whose vapor
pressures were too high for study by this method and would lead to spuriously high molecular weights. Acknowledgment. Support by the n'ational Institutes of Health (General Medical Sciences, GM 13342) is gratefully acknowledged. The authors wish to thank Dr. H. Fred Henneike for helpful discussions and for permission to use a portion of his molecular weight data prior to publication.
The Mean Activity Coefficient of Sodium Sulfate in Aqueous Sodium Sulfate-Sodium Chloride Electrolytes by John C. Synnott and James N. Butler Tyco Laboratories, Inc., Waltham, Massachusetts
OS154 (Received December 18, 1967)
The activity coefficient of NazSOc in aqueous NazS04-h'aC1electrolytes at 25" and total ionic strength 1.0 has been measured using the cell Pb(Hg)IPbSOl(s)/Na+,S04*-, C1-, HzO/glasselectrode where the glass electrode is reversible to Na+. If care is taken to exclude oxygen from the cell during preparation and measurements, reproducibility of the order of &0.05 mV is obtained. Harned's rule was found to be obeyed within experimental error for Na2S04 (component 2) in these mixtures, and the coefficient aZlat I = 1 was calculated by a least-squares method to be -0.035 i 0.005. This agrees with the value of azl calculated from published osmotic coefficients and activity coefficients of NaCl (component 1) in the corresponding mixtures.
Introduction We have published' measurements of the activity coefficients of NaCl in NaCl (component l)-NazS04 (component 2) electrolytes and have compared these with the measurements made using cation-sensitive glass electrodes.2 From these measurements and from the known osmotic coefficients of the pure aqueous solutions of the pure components, we calculated the Harned rule coefficient for Na2SO4in these mixtures (az1). Since there was some discrepancy between the a12 values of various workers at ionic strength 1, it is of interest to measure aZlby an independent method. We have made some measurements using a sulfatereversible cell Pb(Hg)IPbS04(s)INa+,Sod2-,C1-, HzOl sodium ion-sensitive glass electrode whose potential is given by
E
=
E"
+ RT - In 2P
The Jeurnal of Phgsicnl Chemistry
( ~ N ~ ~ ~ S O , Y Z I ~ )
(1)
where yzl is the mean activity coefficient of Na2S04in the mixed electrolytes, and the other symbols have their usual meanings. The lead amalgam-lead sulfate electrode should not respond to chloride ion since the solubility of PbClz is much larger than that of PbS04. The standard potential E" includes the asymmetry potential of the glass electrode, and is expected to vary slowly with time. Therefore, the measurements were made by transferring the same glass electrode between two cells, one of which contained a reference solution of NazS04without any added chloride.a This measurement is, in principle, the same as one using two sodium amalgam electrode cells, but is easier to apply because of the simplicity of working with the glass electrode. According to the work of Lanier,2aresults of comparable precision should be obtained. (1) J. N. Butler, P. T. Hsu, and J. C. Synnott, J . Phys. Chem., 72, 910 (1967). (2) (a) R. D. Lanier, ibid., 69, 3992 (1965); (b) J. M,Gieskes, 2. Physik. Chem. (Frankfurt), 5 0 , 78 (1966). (3) -4.J. Zielen, J . Phys. Chem., 67, 1474 (1963).
ACTIVITY COEFFICIENT OF NazS04 IN AQUEOUS Na2SOe-NaC1 ELECTROLYTES
Experimental Section The glass electrode used was a Corning sodium ion electrode, Type NAS 11-18. It was connected to an Instrumentation Laboratories Model 135-A electrometer, which operated a Sargent I\hdel SR recorder. Calibration was made with bias boxes differing in potential by a few millivolts. These were connected in place of the cell, and the change in deflection of the recorder was noted. The true potential difference was read from the bias boxes to AO.005 mV on a Leeds and Northrup Model I
