Automatic continuous-flow determination of sulphite in the presence of a high concentration of thiosulphate

A spectrophotometric method for the determination of sulphite in the presence of up to 200-fold of thiosulphate is proposed. The derivatizing reaction, which is based on the decoloration of p-rosaniline in neutral or slightly basic medium, is developed in an unsegmented flow system. A linear calibration range between 1 and 20 μg ml−1 was obtained, with a relative standard deviation of 1.5% and sampling frequency of 40 h−1. The method has been applied to the determination of the analyte in samples from an ammonium thiosulphate production plant, where the concentration of sulphite is the quality parameter. The concentration of sulphite and thiosulphate in these samples [0.2–1.0% and 50–60% (w/w), respectively] makes necessary a 1:1000 on-line dilution, and the use of peak-width measurements. In these working conditions the linear range was 2.7–20.0 g l−1 and the R.S.D. values ranged between 1.3 and 2.8%, depending on the constant absorbance value selected for measurement. The sampling frequency was 8 h−1.