BIOELECTROCHEMICAL DETERMINATION OF CITRIC-ACID IN REAL SAMPLES USINGA FULLY AUTOMATED FLOW-INJECTION MANIFOLD

An enzymic method for the determination of citric acid in fruits, juic es and sport drinks is proposed, The method is based on the action of the enzymes citrate lyase, oxaloacetate decarboxylase and pyruvate oxi dase, which convert citric acid into H2O2 with the latter being monito red amperometrically with a H2O2 probe. The enzymes pyruvate oxidase a nd oxaloacetate decarboxylase mere immobilized, A multi-membrane syste m, consisting of a cellulose acetate membrane for the elimination of i nterferants, an enzymic membrane and a protective polycarbonate membra ne were placed on a Pt electrode and used with a fully automated flow injection manifold. Several parameters were optimized, resulting in a readily constructed and reproducible biosensor, Interference from vari ous compounds present in real samples was minimized, Calibration graph s mere linear over the range 0.01-0.9 mM pyruvate, 0.015-0.6 mM oxaloa cetate and 0.015-0.5 mM citrate, The throughput was 30 samples h(-1) w ith an RSD of 1.0% (n = 8); the mean relative error was 2.4% compared with a standard method, The recovery was 96-104%, A 8-10% loss of the initial activity of the sensor was observed after 100-120 injections.