HETEROGENEOUS EQUILIBRIA 263 



It will be observed that, according to these calculations, the concen- 

 tration of the un-ionized fraction decreases markedly as the concentra- 

 tion of the added electrolyte increases. In a 0.3 normal solution of 

 potassium sulphate, the calculated concentration is less than one half 

 that in pure water. The ion product increases due to the addition of 

 the second electrolyte, this increase depending upon the nature of the 

 added electrolyte. On the addition of 0.3 N equivalents of potassium 

 sulphate, the ion product increases from 204.9 to 258.6. . On the addition 

 of 0.2 N equivalents of potassium chloride, the ion product increases from 

 204.9 to 290.0. The increase in the case of potassium chloride, there- 

 fore, is approximately twice that for potassium sulphate. If the assump- 

 tions underlying these calculations are correct, the concentration of the 

 un-ionized fraction is greatly reduced on the addition of a relatively small 

 amount of a second electrolyte. Since it has commonly been assumed 

 that the isohydric principle holds for strong electrolytes, many writers 

 have accepted as correct the result that the concentration of the un- 

 ionized fraction of the salt is greatly depressed on the addition of an 

 electrolyte. As was pointed out in a preceding section, the applicability 

 of the iso-ionic principle to mixtures of strong electrolytes is doubtful. 

 It is doubtful, therefore, that the above values represent correctly the 

 state of the solutions in question. 



The solubility depression of the un-ionized fraction is much greater 

 than might be expected from the effect of electrolytes upon the solubility 

 of non-electrolytes. The solubility depression of hydrogen in water at 

 15 for different salts at normal concentration is in the neighborhood of 

 20 per cent, that of oxygen in the neighborhood of 30 per cent, and that 

 of nitrous oxide in the neighborhood of 20 per cent. The solubility de- 

 pression of phenylthiourea at normal concentration of the added salt is 

 24 per cent for potassium chloride, 10 per cent for sodium nitrate, and 

 for ammonium nitrate there is a solubility increase of 7 per cent. The 

 solubility curves, moreover, while not quite linear, are only slightly 

 convex toward the axis of concentrations. Furthermore, on the addition 

 of hydrochloric acid, the solubility depression of non-electrolytes is rela- 

 tively very small. At normal concentration and 25, it is 7 per cent for 

 hydrogen, 6.8 per cent for oxygen, and 4.4 per cent for nitrous oxide. 

 From the effect of electrolytes on the solubility of non-electrolytes, it 

 must be concluded, not only that the effect varies greatly with the nature 

 of the added electrolyte, but, also, that the magnitude of the effect is 

 much lower than that derived from the values calculated on the basis of 

 the isohydric principle. Furthermore, it follows from the work of Ken- 



