Determination of sulphite and acetic acid in foods by gas permeation flow injection analysis

Abstract Two simple, rapid and sensitive gas-diffusion conductimetric methods were used for the determination of acetic acid and sulphite, separately. For sulphite, the method was based on formation of sulphur dioxide in acid medium (1.5 mol L −1 HCl). The sulphur dioxide formed diffuses through an hydrophobic permeable membrane into an acceptor stream (de-ionised water). The change in the conductivity of the de-ionised water was measured and referred to sulphite content in the samples. The method is applicable in the range of 1.0–50.0 mg L −1 sulphite and the limit of detection is 0.03 μg mL −1 . The relative standard deviation for 10 replicate determinations of 10.0 μg mL −1 of sulphite is 0.2%. Sulphite was determined by the method of standard addition. Recovery of sulphite from wine samples ranged from 97.3% to 99.3%. The method was successfully applied to the determination of sulphite in wines and compares well with the standard iodometric method. For acetic acid determination, the method was based on the change of the conductivity of acceptor solution (de-ionised water) when acetic acid from the matrix, diffuses via a permeable membrane into the receptor solution. The increase in conductivity of the de-ionised water was proportional to the acid content. A linear calibration graph in the range of 0.010–0.100 mol L −1 of acetic acid with a relative standard deviation of 0.8% (0.010 mol L −1 acetic acid, N  − 10) was obtained. Sample throughputs of 120 h −1 for sulphite and 80 h −1 for acetic acid were achieved. The systems were successfully applied for the assays of sulphite in wines and fruit juices and acetic acid content in vinegar, respectively.