Study of heterogeneous equilibria in saturated solutions of some sparingly soluble 8-hydroxyquinolines

Heterogeneous equilibria in saturated aqueous solutions of 8-hydroxyquinoline (A), 2-methyl-8-hydroxyquinoline (B) and 5-chloro-8-hydroxyquinoline (C) were investigated at constant ionic strength (1M sodium chloride), at 25.0 +/- 0.1 degrees , by the application of the following methods: the formation function method, the method of bound protons, the method of free proton sites and the solubility method. The following equilibrium constants were determined: K(s1)=H(2)A(+)]/H(3)O(+)], K(s2)=HA] and K(s3)=H(3)O(+)] A(-)]. Their mean pK values are: -2.88, 2.42 and 12.09 for A, -3.27, 2.71 and 12.73 for B, and -0.34, 3.88 and 12.75 for C. In addition, the acidity constants of investigated 8-hydroxyquinolines were determined by pH-metric titrations. The values obtained for these constants were consistent with those calculated on the basis of corresponding equilibrium constants determined in heterogeneous systems.