An insight into the solution equilibria of magnesium(II) with purpurin and spectrophotometric determination of magnesium

The complexation equilibria of magnesium(II) with purpurin (PURP) are studied spectrophotometrically as a function of pH in 50% (v/v) ethanol–water medium at 20°C. The uncharged complex formed at pH 9.5 allows precise and accurate determination of magnesium over the concentration range 0.8–4.3 μg ml−1. The molar absorptivity of the Mg–PURP complex at 540 nm and detection limit for Mg are 9.2×103 l mol−1 cm−1 and 75 ng ml−1, respectively. The proposed method is rapid and possesses reasonable selectivity. Under the optimum conditions, the use of first-derivative spectrophotometry has the advantage of high sensitivity than normal spectrophotometry and allows the determination of 0.2 μg ml−1 of magnesium. The validity of the method is examined by analysing several SRM Portland cement samples and a variety of cement materials of variable magnesia content.