Application of the reductive flow injection amperometric determination of iodine at a glassy carbon electrode to the iodimetric determination of hypochlorite and hydrogen peroxide

Abstract

Dissolved molecular oxygen is insignificantly reduced at a glassy carbon electrode that is being held at –0.2 V vs. SCE for the determination of iodine. The tendency for molecular oxygen to oxidise iodide to iodine in acidic solutions, however, is a serious problem in the development of flow injection amperometric methods based on the use of iodine reactions in which iodine is monitored at –0.2 V. In the work described here, iodine (l₂), has been determined at concentrations above 6 × 10^–6M in neutral, unsparged potassium iodide solution by injection through PTFE transmission tubing into a nitrogen-sparged 0.4% potassium iodide eluent in a flow injection system. By sparging the sample solution with nitrogen for 30 s before injection and shielding the PTFE transmission tubing with nitrogen, it was possible to determine iodine at concentrations above 1 × 10^–7M, with no significant loss of iodine being caused by the sparging.

A nitrogen-sparged eluent, consisting of 0.04% potassium iodide in 0.3% acetic acid solution, remains free of iodine for several hours when stored under nitrogen. Hypochlorite was determined directly on-line in the range 0.1 × 10^–4–2.5 × 10^–4M by injecting nitrogen-sparged sample solutions into this eluent. The reaction is fast and complete on-line. Hydrogen peroxide was determined on-line by injecting nitrogen-sparged sample solutions, 2.5 M in sulphuric acid, into a nitrogen-sparged 5% potassium iodide eluent. The reaction of hydrogen peroxide with iodide was incomplete on-line. Iodine formed in the molybdate-catalysed and the uncatalysed reactions of hydrogen peroxide with iodide off-line was determined by injection into a 1 M sulphuric acid eluent.