Determination of iodide by detection of iodine using gas-diffusion flow injection and chemiluminescence

This work describes development of a flow injection (FI) system for determination of iodide, based on the chemiluminescence (CL) reaction between iodine and luminol. Iodide in the sample zone is oxidized to iodine. Employment of a gas-diffusion (GD) unit allows for selective detection of the generated CL (425 nm). Preliminary results showed for concentrations of less than 2 mg L⁻¹, that signals were irreproducible and that the calibration was not linear.In order to solve these problems, a method of ‘membrane conditioning’ was investigated, in which iodide stream was continuously merged with oxidant to generate I₂ that conditioned the GD membrane and tubing. This minimized surface interaction between the active surface and the I₂ generated from the samples, thus improving both precision and sensitivity. By employing membrane conditioning, it has been possible to reliably detect concentrations down to 0.1 mg L⁻¹.At the optimized condition, an excellent linear calibration (r² = 0.999) was obtained from 0.1 to 1.0 mg L⁻¹. The method was successfully applied to determine iodide in some pharmaceutical products such as potassium iodide tablets and a liquid patent medicine. However, for vitamin tablets, ascorbic acid was found to interfere seriously by causing a negative signal.