EQUILIBRIUM STUDY OF THE MIXED COMPLEX OF COPPER(II) WITH 2-HYDROXYNICOTINIC AND SALICYLIC ACIDS

The complexation equilibria between copper(II), 2-hydroxynicotinic acid (hyna) and salicylic acid (sa) have been studied using spectrophotometric and potentiometric pH-titration methods in ethanol-water medium (50%, v/v; I=0.1 M NaClO4, 25-degrees). Optimum conditions for the predominance of the complex forming equilibrium : Cu(hyna) + sa half arrow right over half arrow left Cu(hyna) (sa) were established. The stability constant of the Cu(hyna) (sa) mixed-ligand complex was determined and the ternary complex enhancement over binary complex formation was evaluated. For the equilibrium : Cu(hyna)2+Cu(sa)2 half arrow right over half arrow left 2Cu (hyna) (sa), the corresponding constant was log X = +0.7 (+ 0.08). The constant given in parenthesis is due to DELTAlog K(Cu) = log K(Cu(hyna)sa)Cu(hyna)-log K(Cu(sa))Cu. The mixed complex absorbs strongly at 355 nm in the pH range 6.2-6.8 and the ternary system obey's Beer's law upto 6.5 mug ml-1 of copper with molar absorptivity of 0.84x19(4) dm-3 mol-1 cm-1.