Potentiometric determination of sulphite by use of mercury(I) chloride-mercury(II) sulphide electrodes in flow injection analysis and in air-gap electrodes

Abstract

Flow injection analysis, using as the detector a solid-state ion-selective electrode with a mercury(II) sulphide-mercury(I) chloride membrane, can be used for determining sulphite or dissolved sulphur dioxide in water. At concentrations in the range 1.5–10 mg 1^–1 of sulphite, the method has a Nernstian response of 60 mV per decade, but at lower concentrations (down to 0.1 mg 1^–1) the e.m.f. is linearly related to the sulphite concentration. Although the flow injection method is less sensitive than direct use of the electrode, it avoids the problem of chloride interference and permits the determination of sulphur dioxide in the commonly used tetrachloromercurate absorbent. The only serious interference found was from sulphide, although a small effect was also obtained from thiosulphate. Measurements in the range 0.1–10 mg 1^–1 of sulphite had relative standard deviations for single results of no more than 2%. The method requires only two reagents (dilute nitric acid solutions) and is simple to operate. Each analysis is complete in less than 5 min.

Air-gap electrodes, using the same sensor, had sub-Nernstian responses of very poor reproducibility and were not considered to be a practical means of determining sulphite.