82 HYDROGEN ION CONCENTRATION 



especially since the presence of NaCl depresses enormously the solu- 

 bility of the sodium urate. Thus we have the remarkable case in 

 which the presence of the alkali-salt, at least in NaCl-containing 

 solutions, diminishes, instead of increasing, the solubility of a weak 

 acid. Recently Kanitz calculated from available data the second 

 dissociation constant of uric acid, and he came to the conclusion 

 that the primary sodium urate in pure solution is markedly hydro- 

 lyzed into free uric acid and the secondary sodium salt. Such a 

 solution contains, therefore, free uric acid which must exercise a 

 great influence upon the solubility of the urate. On the other hand, 

 the sodium urate tends to an abnormally great extent to form very 

 stable supersaturated solutions, even in the presence of the urate 

 as the solid phase. Schade considered these supersaturated solu- 

 tions as colloidal in nature, but Kohler and Gudzent showed this 

 assumption to be improbable by various means (ultrafiltration, 

 compensation dialysis, conductivity). 



Schade and Boden showed that uric acid solutions may be easily 

 made to form gels, which however slowly crystallized out. On the 

 other hand, Gudzent demonstrated by means of compensation 

 dialysis that in the blood uric acid is present in true solution. At 

 any rate, Bechold and Ziegler found that they could dissolve large 

 amounts of free uric acid in blood serum, and they designated 

 this condition as the "overfilling" ("Uberfiillung") of the blood with 

 uric acid. Evidently in this case the uric acid is present 

 in the blood in the form of supersaturated sodium urate in solution. 

 The whole problem demands extensive reinvestigation, for we have 

 not yet reached an adequate solution. 



22. The relation of hydrion concentration to the solubility of 



difficultly soluble salts 



The solubility of true neutral salts (NaCl, KNO3, etc.) does not 

 depend upon the hydrion concentration which is the case for the 

 salts of strong bases and weak acids, and reversely, for the salts 

 of strong acids and weak bases. A biologically important instance 

 is furnished by the solubility of calcium carbonate, which was in- 

 vestigated by Rona and Takahashi."*^ 



We shall discuss this problem in a somewhat different way from 

 these authors, and more in accord with our previous considerations. 



^^ P. Rona and D. Takahashi, Biochem. Zeitschr. 49, 370 (1913). 



