Reagent-injection spectrophotometric determination of citric acid in beverages and pharmaceutical formulations based on its inhibitory effect on the iron (III) catalytic oxidation of 2,4-diaminophenol by hydrogen peroxide

A new simple and rapid reagent-injection method is reported for the spectrophotometric determination of citric acid in beverages and pharmaceutical formulations. The method is based on the inhibitory effect of citric acid upon the Fe(III) catalytic oxidation of 2,4diaminophenol (DAP) by H2O2. Citric acid forms stable complexes with Fe(III), thus, reducing its catalytic effect. The decrease in absorbance of the oxidation product is monitored at 500nm. The various chemical and physical variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 0 to 1000mgl-1 citric acid. The precision was very good (sr=1.9%) and the detection limit was satisfactory (cL=0.96mgl-1). The sampling rate was 90 injections per hour. The average accuracy was evaluated by comparison of the results with those obtained by the BoehringerMannheim UV-method test combination kit for citric acid and it was also very good (er= 0.96%). The method was found to be adequately selective considering the ions the samples contain, and has been successfully applied to the determination of citric acid in commercial beverages and pharmaceutical formulations.