Iodimetric and bromimetric flow injection amperometric methods of determining sulphite in which iodine or bromine is monitored at a glassy carbon electrode

Abstract

A flow injection amperometric method is recommended for the iodimetric determination of sulphite. Both the formation of iodine and its reaction with sulphite are carried out on-line in a single channel manifold consisting typically of 3 m of 0.71 mm bore transmission tubing. Iodine is formed reproducibly in the reverse FIA manner in an injected bolus of dilute acid using a slightly alkaline potassium iodate eluent containing an excess of potassium iodide, and is monitored at a glassy carbon electrode held at –0.2 V vs. SCE, without interference from dissolved molecular oxygen. When acidified sulphite standards or samples are injected into the same eluent a decreased iodine signal is obtained owing to the reaction of the sulphite injected with an equivalent amount of iodine. The decrease in signal size is rectilinear with the sulphite concentration of the injected solution in the range 0.1 × 10^–4–4 × 10^–4M when injection is made into an eluent 3 × 10^–5M in potassium iodate and 0.1 M in potassium iodide.

Methods of determining sulphite in which the reaction between iodine or bromine and sulphite is carried out off-line and the excess of iodine or bromine is determined amperometrically by injecting the reacted solution into an inert iodide or bromide eluent, or in which a slightly alkaline sample solution containing iodate-iodide is injected into an acidic eluent, have also been developed.