AUTOMATIC CONTINUOUS-FLOW DETERMINATION OF SULFITE IN THE PRESENCE OFA HIGH-CONCENTRATION OF THIOSULFATE

A spectrophotometric method for the determination of sulphite in the p resence of up to 200-fold of thiosulphate is proposed. The derivatizin g reaction, which is based on the decoloration of p-rosaniline in neut ral or slightly basic medium, is developed in an unsegmented flow syst em. A linear calibration range between 1 and 20 mu g ml(-1) was obtain ed, 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 a nalyte in samples from an ammonium thiosulphate production plant, wher e the concentration of sulphite is the quality parameter. The concentr ation of sulphite and thiosulphate in these samples [0.2-1.0% and 50-6 0% (w/w), respectively] makes necessary a 1:1000 on-line dilution, and the use of peak-width measurements. In these working conditions the l inear 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).