Polarographic determination of sorbic acid in fruit juices and soft drinks

A simple differential-pulse polarographic method using a laboratory-built hanging mercury drop electrode as the working electrode was developed for the determination of sorbic acid in fruit juices and soft drinks. Sorbic acid was extracted from the samples with diethyl ether. After reduction of the ethereal solution to a small volume by direct evaporation, the residual ether was dissolved in the supporting electrolyte (25 ml of acetonitrile + 1 ml of 0.06 M acetic acid + 0.8 g of tetraethylammonium bromide). Peak current was measured at -1.7 V. The working range of the method, without dilution or pre-concentration of the samples, was from 4 to 229 p.p.m. for the original juice and drink samples. The validity of the method was confirmed by parallel determinations using the method of the Association of Official Analytical Chemists and by recovery tests on a large variety of juice samples. Satisfactory recoveries and agreement in results from the two methods were obtained. The recovery and precision (relative standard deviation) of the method were 97 +/- 4 and 100 +/- 3%, respectively, for blackcurrant juice for five determinations.