Formation of volatile iodine compounds under hot concentrated acid conditions (nitric acid or aqua regia) and in diluted acid solutions with or without thiocyanate

It is reported that iodine volatilization can occur in any elemental analysis of total iodine by ICP-MS. This problem affects the accuracy of the results, and it has been neither rationalized nor solved up to now. In this work, the formation of volatile iodine compounds in concentrated acid solutions (nitric acid or aqua regia) under microwave heating was studied by UV-vis spectrophotometry, linear sweep voltammetry, and cyclic voltammetry. It was evidenced that molecular iodine (I2) can unexpectedly form in concentrated hot HNO3 solutions, irrespective of the starting iodine compound (iodide, iodate, periodate, 3-iodo-L-tyrosine, 3,5-diiodo-L-tyrosine dehydrate). I2 is produced by the nitrogen oxides existing in these conditions. The formation of volatile iodine is minimized in aqua regia, as chloride is able to keep iodine in solution due to the formation of charged chloro-iodo complexes (e.g. I2Cl–). The dilution of the concentrated acid solution, required prior to the ICP-MS analysis, causes the disruption of I2Cl– so that I2 can again volatilize. To avoid this, 0.1 M thiocyanate can be added, as it forms a strong I2SCN– complex which keeps I2 in solution as ion. Also other iodine species possibly occurring in the explored conditions, iodide and iodate, were demonstrated to be converted to I2SCN– in diluted HNO3 solutions and in the presence of 0.1 M thiocyanate. Recovery tests demonstrated that iodine volatilization is minimized if samples containing iodine are treated in aqua regia and, after dilution, they are added with 0.1 M thiocyanate