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

A spectrophotometric study of the complexation reaction between Al3+ and qu inizarin (QUIN) was carried out to ascertain the suitability of the complex formed for direct spectrophotometric determination of aluminium. The absor bance at 550 nm, due to the aluminium-QUIN complex, formed at pH 3.8, is re commended for the determination of the alumina content of Portland cement a nd cement clinker. The proposed method is simple and rapid and possesses re asonable selectivity. Interference of iron(iii), generally present in Portl and cement, is eliminated by addition of ascorbic acid. The results obtaine d for several SRM Portland cement samples and for a variety of cement mater ials demonstrate that the proposed method allows the precise and accurate d etermination of Al2O3 content over the concentration range 1.4-5.75 mu g mL (-1) of aluminium. The determination of Al can be carried out successfully in the absence of a masking agent by using first-derivative spectrophotomet ry. Verification and the use of control charts in the spectrophotometric de termination of Al is achieved. The record of the verifier response during r outine operation establishes that the method is being maintained in statist ical control.