72 



THEORY OF COLLOIDAL BEHAVIOR 



salts are alike if plotted over the pH, as Fig. 16 shows. The 

 maximal osmotic pressure lies at pH of about 3.0. 



There is then no doubt that the curves for the osmotic pressures 

 of the three proteins, gelatin, crystalline egg albumin, and 

 casein obey the valency rule, and show no appreciable influence 

 of the nature of the ion except that of the sign of charge and 

 valency. 



FIG. 16. Osmotic pressure of 1 per cent solutions of casein chloride and casein 

 phosphate as function of pH. The two curves are almost identical. 



In the older experiments in which the hydrogen ion concentra- 

 tions were not measured, the action of weak acids led the investi- 

 gators into error. In the Hofmeister series it is generally 

 contended that acetic acid acts like sulphuric acid and not like 

 hydrochloric or nitric acids. This is due to the fact that the 

 investigators compared the effects of different acids at equal 

 molecular concentrations instead of comparing the effects of 

 different acids at the same pH. If this is done it is found that 

 acetic acid .acts like HC1 and not like H2SO4. Figure 17 gives 

 the curves for the osmotic pressure of about 0.8 per cent solutions 



