Direct spectrophotometric determination of aluminium oxide in Portland cement and cement clinker. An insight into the solution equilibria and analytical aspects of the aluminium–quinizarin system

Abstract

A spectrophotometric study of the complexation reaction between Al³⁺ and quinizarin (QUIN) was carried out to ascertain the suitability of the complex formed for direct spectrophotometric determination of aluminium. The absorbance at 550 nm, due to the aluminium–QUIN complex, formed at pH 3.8, is recommended for the determination of the alumina content of Portland cement and cement clinker. The proposed method is simple and rapid and possesses reasonable selectivity. Interference of iron(III), generally present in Portland cement, is eliminated by addition of ascorbic acid. The results obtained for several SRM Portland cement samples and for a variety of cement materials demonstrate that the proposed method allows the precise and accurate determination of Al₂O₃ content over the concentration range 1.4–5.75 μg mL⁻¹ of aluminium. The determination of Al can be carried out successfully in the absence of a masking agent by using first-derivative spectrophotometry. Verification and the use of control charts in the spectrophotometric determination of Al is achieved. The record of the verifier response during routine operation establishes that the method is being maintained in statistical control.