Studies on flow injection analysis with sulphide ion-selective electrodes

Abstract

Three sulphide ion-selective electrodes (ISEs), namely, the Orion Model 94–16A, EDT Research EES and EDT Research EES flow-through types have been calibrated for between 10^–2 and 10^–6M concentrations of sulphide and shown to have near-Nernstian slopes. The different E° values between the Orion and EDT electrodes are attributed to differences between membranes or electrical contacts.

Standard antioxidant buffer (SAOB) containing sufficient ascorbic acid antioxidant (40 g dm^–3 in 2 M sodium hydroxide solution) can control any deleterious effects by hydrogen peroxide on sulphide ISEs, as illustrated by studies on the Orion Model 94–16A electrode. Affected electrodes can be reconditioned by soaking in 0.1 M sodium sulphide solution for 2 d followed by polishing of the membrane surface.

Flow injection analysis (FIA) indicated that an appropriate carrier stream contains a 5 × 10^–6M concentration of sodium sulphide in normal SAOB (20 g dm^–3 ascorbic acid in 2 M sodium hydroxide solution). Flow-rate and sample volume parameters were also studied. For subsequent work on the FIA of sewage effluent samples, a flow-rate of 2.23 cm³ min^–1 was selected to go with tubing of diameter 1.3 mm and length 12 cm from injection head to ISE. FIA is superior to direct potentiometry for the determination of sulphide in sewage effluents, and both the Orion Model 94–16 “cascade” and EDT Research EES “flow-through” modes of sulphide ISEs yielded similar sulphide data to those obtained by a colorimetric method. The sulphide contents of samples analysed ranged from 0.1 to 17.8 p.p.m. of sulphide. The lower sulphide contents were for samples pre-treated with hydrogen peroxide to reflect a recognised method of controlling deleterious sulphide types in effluents.