80 HYDROGEN ION CONCENTRATION 



in water both the solid phase and the liquid phase consist of undis- 

 sociated acid, H-ions and acid anions. According to the Henr}^- 

 Nernst law of distribution each of these three molecular species has 

 its own distribution coefficient in respect to each phase, and the 

 saturated solution should consequently contain these three com- 

 ponents distributed between its two phases in concentrations lim- 

 ited by the appropriate coefficients. But this would lead to an 

 unequal concentration of the positive H-ions and of the negative 

 acid ions in solution, which would be impossible because of the 

 electro-static forces arising from such a condition. Instead of this, 

 a condition of equilibrium is established in the system entailing 

 equal concentrations of positive and negative ions in solution and 

 the formation of an electric potential difference at the boundary of 

 the liquid and solid phases (this point to be discussed later). This 

 conception is based upon the distribution coefficient of only one of 

 the molecular species, the undissociated acid, which determines the 

 "partial solubility" of the benzoic acid, and it is also based upon the 

 mass action law which regulates the concentrations of the other 

 molecular species in solution so as to maintain equilibrium. 



A biologically important example of the relation of the hydrion 

 concentration to the solubility of an acid is furnished by the case 

 of uric acid^^ Unfortunately this problem is not yet completely 

 solved. The investigation of this question has hitherto taken a 

 tortuous path because of a lack of a clear conception of the inter- 

 dependence of solubility and hydrion concentration. We shall 

 attempt to interpret the available data in accord with the above 

 theory. First the partial solubility, X, of uric acid must be deter- 

 mined. For this purpose its ionization must be completely sup- 

 pressed by means of the addition of HCl. His and Paul found a 

 solubility in 1.0 A' HCl at 18° of one mol in 7137 Hters, or, X = 

 1.401 X 10~^. According to the same authors the constant of dis- 

 sociation in the first step, ki = 1.5 X 10"" (according to Gudzent, 



^3 Literature : His and Paul, Pharmazeut. Zeit. 1900. — Paul, Zeitschr. f. 

 physiol. Chem. 31, 1 and 64 (1900).— Gudzent, Zeitschr. f. physiol. Chem. 

 56, 150 (1908);! 60, 25, 38; 63, 253, 455 (1909).— Bechhold and Ziegler, Biochem. 

 Zeitschr. 20, 189 (1909); 24, 146 (1910).— Ringer, Zeitschr. f. physiol. Chem. 

 67, 332 (1910).— Kohler, Zeitschr. f. physiol. Chem. 70, 360; 72, 169; Zeitschr. 

 f. klin. Med. 78, 1.— Lichtwitz, Zeitschr. f. physiol. Chem. 84, 416 (1913).— 

 Schade andBoden, Zeitschr. f. physiol. Chem. 83, 347; 86, 416 (1913).— A. Kan- 

 itz, Zeitschr. f. physiol. Chem. 116, 96 (1921). 



