SPECTROPHOTOMETRIC DETERMINATION OF A MIXTURE OF WEAK ACIDS USING MULTIVARIATE CALIBRATION AND FLOW-INJECTION ANALYSIS TITRATION

A multivariate calibration technique has been developed for the determination of individual concentrations of acids present in mixtures of weak acids. Spectrophotometric data are generated by following the color change of a combination of acid-base indicators caused by an alkaline gradient. This gradient is generated by introducing a NaOH solution into a flow injection analysis (FIA) titration system. Absorption spectra of the indicator-sample-alkaline gradient mixture are acquired by a fast scanning, microcomputer controlled Hadamard spectrophotometer. The surfaces (absorbance x time x wavelength) generated by the FIA titration procedure reflect the acid mixture composition. Partial least squares (PLS) modelling is used twice to reduce and treat the titration data. First, the absorption spectra of the indicator mixture at different pH levels are employed to establish a multivariate calibration procedure to convert the absorption spectra to pH values. Thereafter, surfaces obtained by the titration procedure are reduced to a time x pH curve. These time x pH curves and the known acid compositions are used to establish another PLS model. This model predicts individual acid concentrations for mixtures of citric and tartaric acids in the 1.0-4.0 mM range, acetic and benzoic acid in the 1.0-4.0 mM range and hydrochloric and acetic acid in the 1.25-5.0 mM range with an average relative error from 2.8% to 6.5%. The overall sample processing time is about 5 min.