Development of a sequential injection analysis method for the automatic simultaneous determination of sulfite and...

Existing analysis methods for sulfite and sulfate are mostly focused on the determination of one of the two analytes, and even when both analytes are targeted, the corresponding methods are not fully automatic. Herein, the development, optimisation, and validation of an automatic sequential injection analysis (SIA) method for the simultaneous determination of sulfite spectrophotometrically and sulfate turbidimetrically in water samples is reported for the first time. The analysis is based on the rapid reaction of sulfite with 5,5'-dithiobis (2-nitrobenzoic acid) (DTNB), a non-hazardous and inexpensive reagent, and the precipitation of sulfate with BaCl₂. The SIA method is characterised by linear calibration ranges of 0 - 40 mg L^-1 and 0 - 300 mg L^-1, LODs of 0.1 mg L^-1 and 4 mg L^-1, and RSDs (n = 10) of < 2.51 % (10 mg L^-1 SO₃^2-) and < 2.05 % (100 mg L^-1 SO₄^2-) for sulfite and sulfate, respectively, and a sampling rate of 16 h^-1. The SIA method was successfully applied to synthetic acid rainwater, tap water, and bottled waters spiked with sulfite and sulfate species and good recovery in the range 94 - 115 % was obtained.