Flow injection for iodide determination at the ppb level

A flow-injection system for iodide determination has been developed based on the Sandell and Kolthoff reaction in which iodide catalyses the reaction of Ce(IV) and As(III). The decrease in Ce(IV) concentration measured colorimetrically in the range 410–430 nm, after a fixed time interval, is thus dependent on the iodide concentration. The method is simple and sensitive to very low concentrations of iodide. The detection limit is 0.8 μg I− L−1 (S/N = 3). The calibration graph was found to be linear over a range of 0–7 μg L−1. The effects of some interferents, including halides and others that are found in urine and milk samples (glucose, ascorbic acid, thiocyanate ion, bicarbonate ion and calcium ion), were studied. The presence of fluoride ion, thiocyanate ion, or oxidizable organic molecules caused serious positive errors. Sulfate ion was the only interferent examined that caused a negative error, but the effect of sulfate can be ignored when its concentration is less than 10⁵ mg L−1. © 1999 John Wiley & Sons, Inc. Lab Robotics and Automation 11: 37–44, 1999