This paper gives values for the osmotic coefficients and mean activity coefficients of uni-univalent electrolytes in aqueous solutions at 25 °C. The values are expressed on the molality or weight basis. The data available in the literature have been corrected to the presently accepted scales of atomic weights (1969) and temperature (IPST 1968) and, where necessary, to the absolute electrical units of 1969 and the fundamental constants of 1963. The selected values of osmotic coefficients and mean activity coefficients for individual electrolytes have been made internally consistent thermodynamically. In some cases estimated values are given; in other cases, references only are given when the data are sparse or unsuited to critical evaluation. Values of the osmotic coefficients and mean activity coefficients of 79 compounds are given together with the standard deviation, variance, and normalized standard deviation of their fit to equations which express these quantities as functions of electrolyte concentration. Finally, literature references are given to data on 51 additional uni-univalent electrolytes.

Sinclair and Robinson3 have developed the isotonic or isopiestic method4 so that it is more precise than any of the direct methods of measuring the chemical potential (or activity) of the solvent in which the temperature of measurement does not vary with the concentration. We have modified their method to give a still greater precision. We believe that our error is not greater than 0.1% of , the osmotic coefficient when the total molality, vm, is greater than one molal, and not greater than 0.001 in for smaller concentrations.In order to determine the chemical potential of the solvent, or any related quantity, from isotonic measurements, however, it is necessary to know it as a function of the composition for one solute. On the other hand, the method gives a way of comparing the measurements with different solutes. We have, therefore, made such measure-these substances.Cambridge, Mass.

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Vol. 57 gested with 3 M sulfuric acid as recommended by Vosburgh and Eppley,4 and the final preparation was kept under 0.02 M sulfuric acid in the absence of light until used. The electrodes were