Determination of Iodide in Brines by Membrane Permeation Flow Injection Analysis†

Abstract

A flow injection method is described for the determination of iodide based on the oxidation of iodide to iodine, which after permeation through a PTFE membrane is detected spectrophotometrically. The sample is injected into a carrier stream of water and merged with acidic dichromate reagent, oxidising iodide to iodine. The iodine permeates through the membrane into a collector stream containing iodide. The iodine reacts with iodide forming triiodide, and is measured spectrophotometrically in a flow cell at 350 nm. The repeatability of the technique was better than a relative standard deviation of 2% at 5 mg l^–1 level, with a throughput of 50 samples h^–1. The detection limit was 0.2 mg l^–1 iodide. The method is suitable for analysing high saline waters. No significant matrix effect is found for sea-water up to 32‰ S, or sodium chloride concentrations up to 50 g l^–1. The method is tested for potential interfering substances (Br^–, COOH, CH₃COOH, HCO₃^–, Fe²⁺, Mn²⁺ and S^2–). The only interfering compound found was sulfide, which can be removed by preheating the samples after acidification. The method was tested by analysing marine pore water samples from Ocean Drilling Program Leg 164; the results compared well with data obtained by the manual method.