DETERMINATION OF CITRIC-ACID BY MEANS OF COMPETITIVE COMPLEX-FORMATION IN A FLOW-INJECTION SYSTEM

A photometric method for the determination of citrate and other organic acids based on their ability to complex Fe3+-ions is presented. The red colored complex of [Fe(SCN)2]+, used as reagent, is destroyed upon contact with the sample because the organic acid complexes the Fe3+-ion. The decrease in absorption is monitored at 460 nm. The reaction is carried out in a simple flow injection system either in single or preferably double channel configuration. The influence of pH was investigated. Best results were obtained by adjusting the carrier stream to pH 2.0-2.5 with a KCl/HCl-buffer. With an increasing concentration of reagent the linear range is shifted to higher citrate concentrations. The slope of the calibration graph and the linear range are influenced by the sample volume. Other variations of parameters include flow rate, reactor volume and diameter of tubing. Generally speaking, optimum conditions for the flow system are not specified because they vary with the application. The typical conditions for a calibration graph from 1 to 8 mmol/l citrate were a reagent concentration of 2.6 mmol/l [Fe(SCN)2]+, a flow rate of 2.4 ml/min, a reactor length of 50 cm with tubing of 0.97 mm inner diameter and a sample volume of 100 mul. At these system settings the coefficients of variation were 2.5% and 1.6% for eight replicate measurements of samples containing 4 mmol/l and 8 mmol/l citrate, respectively. Up to 180 samples can be analyzed per hour. Naturally the method is disturbed by all other ions that form complexes or precipitates with Fe3+ -ions. Therefore its application is limited to samples with a known matrix, which was given in the analysis of citrate in lemon flavored soft drinks, where the citric acid usually accounts for 95 to 99% of the total acidity and other interfering ions are absent.