Elimination of double peaks in the iodimetric flow injection visible spectrophotometric determination of sulphite using a single-channel manifold

A flow injection method of determining sulphite with amperometric monitoring of iodine using a single-channel manifold in which iodine is formed in the reverse flow injection (rFl) manner and reacts with sulphite dispersing in the normal flow injection (nFl) manner has been adapted for use with visible spectrophotometry. The carrier stream consists of an alkaline solution containing iodate and an excess of iodide: injections of acid and then of acidified sulphite are made. The decrease in the iodine signal (measured at 352 nm) in the presence of sulphite is proportional to the sulphite concentration in the injectate. The alkalinity of the carrier stream was adjusted to reduce the signal widths and to prevent the appearance of double peaks. A rectilinear decrease in signal size (down to ca. 10% of the signal size in the absence of sulphite) was obtained in the range 1 × 10–5–7 × 10–4M sulphite using a single-channel manifold consisting of 3 m of 0.8 mm bore transmission tubing with a flow-rate of 5 ml min–1 and an injection volume of 15 µl, when the carrier stream was 6.7 × 10–6M in iodate, 6.7 × 10–2M in iodide and 3.5 × 10–3M in sodium hydroxide, and the sample solution was 0.1 M in hydrochloric acid.