An automated flow-injection titrator for spectrophotometric determinations of total acidity in wines, using a single standard solution and gradient calibration

Abstract

An automated flow-injection titrator to perform spectrophotometric determinations of acidity in wine samples is described. The proposed titrator exploits concentration gradients generated in the flow-injection system and, in contrast to the automated flow titrators previously proposed, it does not require analytical curves based on several standard solutions of tartaric acid. Only one single standard solution, the proper titrant, is directly used for gradient calibration and calculating of the analyte concentrations. A simple laboratory-made photometer that uses an LED (Light-Emitting Diode) as light source and a phototransistor as detector was built in to this titrator. Yellow LED was usually used for spectrophotometric measurements because m-cresol purple (7.4 < pH < 9.0 yellow–purple transition) was selected as acid–base indicator. The system is fully controlled by microcomputer and the software for control and for acquisition and treatment data was written in C language. A good agreement between results yielded by the proposed titrator and those produced by a reference method of the Association of Official Analytical Chemistry (AOAC) was obtained and no statistical difference between results at the 95% probability level was confirmed by applying the paired t-test. An overall relative accuracy of about 0.9% and a mean relative standard deviation lower than 1.5% were obtained with nine replicate titrations. The proposed titrator allows 72 determinations per hour to be carried out, consuming just 200 µl of the sample and 1.5 ml of the titrant.

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