New approach to evaluating dissolved iodine speciation in natural waters using cathodic stripping voltammetry and a storage study for preserving iodine species

A simple and fast new method to measure dissolved total reducible iodine in natural waters using cathodic stripping voltammetry is presented here. Iodate is promptly reduced to iodide by ascorbic acid (0.5 mM) at pH ≤ 2.7, and total iodine is measured as iodide. This new method minimises possible interference and contamination from the relatively high concentration of added chemicals used in other electrochemical methods and lowers the limit of detection for iodide and total iodine to ˜ 0.1 and ˜ 0.2 nM, respectively, using 1 min of adsorption. This new method, as well as an adaptation of the existing electrochemical method for iodide (Luther et al., 1988), was used to determine the dissolved iodine speciation in a variety of seawater samples stored in diverse ways. The iodine speciation was preserved in samples stored for at least 2 months at −16 °C, 4 °C, and room temperature. However, after longer periods of storage (up to 1 year) at 4 °C and room temperature, iodide concentration increased up to 3 times the original value, while total iodine concentrations remained unchanged. The reduction of iodate to iodide is postulated to be bacterially mediated. Although random losses of iodide and iodate were observed in samples preserved frozen, this method, amongst those studied here, appeared to be the most suitable way of preserving iodine speciation when long-term storage is required.