86 THEORY OF COLLOIDAL BEHAVIOR 



hydrogen ion concentration until they reach a maximum at pH 

 about 3.0 or slightly above. The curves then drop again. The 

 curves for the three salts, gelatin chloride, oxalate, and phosphate 

 are practically identical while the curve for gelatin sulphate is 

 considerably lower. 



Figure 23 gives the curves for the viscosity of gelatin citrate, 

 tartrate, and succinate. The three curves are practically 

 identical and also identical with the curves for gelatin chloride 

 and gelatin phosphate in Fig. 22. 



Figure 24 gives the curves for the viscosity of 0.8 per cent solu- 

 tions of originally isoelectric gelatin to which acetic and mono-, 

 di-, and trichloracetic acids have been added. The curves are 

 again identical with those for gelatin chloride, phosphate, etc. 



The titration curves with alkalies have shown that Ca and Ba 

 combine with proteins in equivalent proportions and we should 

 hence expect that the viscosity curves for Ba and Ca proteinates 

 would be lower than those for Li, Na, K, and NH 4 proteinates. 

 This was found to be correct. 



In experiments on the viscosity of casein solutions the limited 

 degree of solubility of the salts of casein has to be considered. In 

 the region from 4.7 to 3.0 or even a trifle below neither casein 

 chloride nor casein phosphate is sufficiently soluble to permit 

 the preparation of a 1 per cent solution, and in this region the in- 

 fluence of casein on the viscosity of water is, therefore, negligible. 

 The curve representing the relative viscosity of 1 per cent casein 

 chloride and phosphate solutions (as compared with that of pure 

 water) rises sharply at pH 3.0. With a further increase of the 

 hydrogen ion concentration the curve falls steadily as it did in 

 the case of the curve for gelatin. This indicates that the maxi- 

 mum for the influence of casein chloride on viscosity lies at pH 

 equal to or greater than 3.0. The curve for the influence of 

 casein phosphate on viscosity coincides with the curve for 

 casein chloride. 



The difference between the viscosity curve of Na caseinate 

 and Ba caseinate (Fig. 25) is also similar to that of the 

 corresponding gelatin salts. 



In the influence of monovalent or bivalent ions on those physi- 

 cal properties of proteins which are characteristic for colloidal be- 

 havior only the valency and the sign of charge of the ion play a role, 



