Chelate formation between lead(II) and 1,2,5,8-tetrahydroxyanthraquinone (quinalizarin): Spectrophotometric studies on the composition and stability

Abstract

The formation of a bluish violet chelate between bivalent lead and 1,2,5,8-tetrahydroxyanthraquinone (quinalizarin) with maximum absorption at 520mμ has been studied. The chelate is soluble in aqueous ethanol. The composition of the chelate has been determined by three methods. The chelate has a composition of 1 : 1 and is stable at pH 6.1-8.5. The values of log K determined in 50% ethanolic medium by the method of Dey et al., the continuous variation method, and the mole ratio method, using absorbance data, are 4.1 ± 0.1, 4.0 ± 0.0, and 4.1  ± 0.1 respectively at 30°, pH 6.3. The free energy change of formation, calculated from the three values of log K, is -5.7 ± 0.1, -5.6 ± 0.0, and -5.7 ± 0.1 kcal respectively.