Complexation equilibria and spectrophotometric determination of iron(III) with resorcylic acid.

The complexation equilibria of Fe(III) with resorcylic acid (2,4-dihydroxybenzoic acid, DHB) were studied spectrophotometrically in ethanol-water (4 + 6, v/v) at an ionic strength of 0.1 M NaClO4. The complexation reactions were demonstrated and characterized. A simple, rapid, and sensitive method based on the formation of the Fe(III)-DHB complex at pH 2.5 (lambdamax = 520 nm, epsilon = 0.8 x 10(4) L/mol x cm) was developed for the spectrophotometric determination of Fe(III). The effect of diverse ions on the sensitivity of the proposed method was studied. The Fe-DHB complex was isolated and characterized by both elemental analysis and infrared spectroscopy. The thermal behavior of the complex in dynamic nitrogen gas was studied by thermogravimetric and differential thermogravimetric analysis. Thermal events encountered throughout the course of decomposition were monitored. A computer program was used for regression analysis and for determination of kinetic and thermodynamic parameters from experimental nonisothermal thermogravimetric data. The proposed method was tested by determinations of iron in various synthetic samples and Portland cement materials.