Spectrophotometric study of the complexation equilibria of zinc(II) with lawsone, juglone and naphthazarin

The complexation equilibria of zinc(II) with 2-hydroxy-1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone and 5,8-dihydroxy- 1,4-naphthoquinone have bee n studied spectrophotometrically as a function of pH in 50% (v/v) ethanol-w ater medium at 25 degreesC and an ionic strength of 0.1M NaClO4. The spectr al study of the complexation reactions has been carried out in solutions co ntaining equimolar concentrations or an excess of one component in the pH r ange of study. The chelation capabilities of these hydroxynaphthoquinones w ith zinc(II) have been examined. The complexation equilibria have been demo nstrated and characterized by analysis of the absorbance versus pH graphs. The composition, molar absorptivities, equilibrium constants and stability constants of zinc(II) complexes with these reagents have been determined. T he fundamental conditions favouring direct spectrophotometric determination of trace amounts of zinc(II) using naphthazarin have been investigated.