A UV-spectrophotometric approach for quantifying iodine in environmental samples for AMS studies

Full text: We report on a UV-spectrophotometric approach to measure total iodine extracted from a variety of environmental sample types. For seawater samples, the iodine extraction method we use follows several basic steps: 1) dissolved iodate is reduced to iodide; 2) iodide is oxidized to dissolved iodine; and 3) dissolved iodine is extracted in a separatory funnel using an organic solvent (Burr et al. 2020). Organic solvents like carbon tetrachloride and chloroform are often used for iodine extraction because they readily dissolve iodine and are immiscible with water. However they are also known carcinogens, so we have begun to use hexanol (C₆H₁₄O) as a relatively benign alternative. Hexanol is also an organic solvent (an alcohol) that readily dissolves iodine and is immiscible with water. The end product of our extraction procedure is nearly pure iodine dissolved in hexanol. At relatively large concentrations in the solvent (> a couple hundred μg/L) dissolved iodine imparts color to the solution: purple in the case of carbon tetrachloride or chloroform, and brown in the case of hexanol. Iodine also produces distinct peaks in the UV spectra of these solvents, a fact that has been studied for more than a century. This feature has been exploited to quantify tiny amounts (1 μg/g) of iodine dissolved in ethanol (Kljubin et al. 2016). UV spectrophotometry is non-destructive and the sample can be recovered after analysis. A final step in AMS target preparation is to add silver or silver nitrate to the solution, to precipitate iodine as silver iodide. We provide examples that show how iodine spectra can be used to quantify the amount of total iodine in a variety of sample types, monitor the progress of successive extractions with the solvent, and ensure complete conversion of iodine to silver iodide with the addition of silver.