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.