Iodine revisited: If and how inorganic iodine species can be measured reliably and what cause their conversions in water?

This study revisited a list of inorganic iodine species on their detections and conversions under different water conditions. Several surprising results were found, e.g., UV–vis spectrophotometry is the only reliable method for I3– and I2 determinations with coexisting I–/IO3–/IO4–, while alkaline eluent of IC and LC columns can convert them into I– completely; IO4– can be converted into IO3– completely in IC columns and partly in LC columns; a small portion of IO3– was reduced to I– in LC columns. To avoid errors, a method for detecting multiple coexisting iodine species is suggested as follows: firstly, detecting I3– and I2 via UV–vis spectrophotometry; then, analyzing IO4– (> 0.2 mg/L) through LC; and lastly, obtaining I– and IO3– concentrations by deducting I− and IO3– measured by IC from the signals derived from I3–/I2/IO4–. As for stability, I− or IO3– alone is stable, but mixing them up generates I2 or H2OI+ under acidic conditions. Although IO4– is stable within pH 4.0–8.0, it becomes H5IO6/H3IO62– in strongly acidic/alkaline solutions. Increasing pH accelerates the conversions of I3– and I2 into I– under basic conditions, whereas dissolved oxygen and dosage exert little effect. Additionally, spiking ICl into water produces I2 and IO3– rather than HIO. [Display omitted] •An approach for determining multiple coexisting iodine species in water is proposed.•I3− and I2 were able to be completely reduced to I− by IC eluents or LC columns.•IO4− was able to be fully and partially reduced to IO3− by IC and LC, respectively.•Although I− or IO3− is stable, their analyses can be interfered by other iodine species.•Hydrolysis of ICl actually produces I2 and IO3− rather than HIO as reported before.