SPECTROPHOTOMETRIC STUDY OF THE COMPLEXATION EQUILIBRIA BETWEEN COPPER(II) AND 2-HYDROXYNICOTINIC, 2-MERCAPTONICOTINIC AND HYDROXYPICOLINICACIDS

The complexation reaction of copper(II) with 2-hydroxynicotinic acid ( hyna), 2-mercaptonicotinic acid (mena) and 3-hydroxypicolinic acid (hy pa) have been studied spectrophotometrically as a function of pH in 50 % (v/v) ethanol-water medium at 25 degrees C and an ionic strength of 0.1 M (NaClO4). The existence of CuLH(+), CuL, CuL(2-) and CuL(OH)(2)( 2-) complex species has been established for hyna (LH(2) symbolising t he non-ionised ionised form of reagents). The formation of CuLH(+) com plex with hypa is not proved, while the noncharged monoligand complex prevails in solutions of pH 5.2-7.5 and starts at similar to 1.5 pH un its lower than that with hyna. Coordination of two ligand molecules wi th Cu2+ takes place at pH 6.5 and 7.8 in solutions with a preponderanc e of hyna and hypa, respectively. In the Cu-mena system there is only formation of 1:1 complex in protonated or deprotonated forms according to the pH. A simple, selective and sensitive method for UV-spectropho tometric determination of trace levels of copper was proposed based on the formation of the Cu(hypa) complex at pH 5.5 (lambda = 335 nm, eps ilon = 1.42 x 10(4)L mol(-1) cm(-1)). Interference caused by a number of ions was masked by the addition of fluoride ions as masking agent.