A new method for indirect determination of iodide and thiosulfate in table salt and milk based on a combination of solid-phase extraction and flame atomic absorption spectrometry.

A new indirect method for the determination of iodide and thiosulfate ions in table salt and milk by flame atomic absorption spectrometry was described. This method is based on the reduction of chromium (VI) to chromium(III) with the reducing action of iodide and/or thiosulfate, separation of unreacted Cr(VI) as its 1,5-diphenilcarbazide complex on a column filled with Amberlite XAD-16, elution of the complex by 10 mL of 0.05 mol L-1 H2SO4 in methanol and determination by flame atomic absorption spectrometry. Amount of the analytes were calculated from the amount of Cr(VI) reacted with the analytes. The optimum conditions for the determination of iodide and thiosulfate ions, including pH and volume of sample solution were examined. The effect of interfering species on the recovery of the iodide and thiosulfate ions was also investigated. The precision of the proposed method is good as it provides relative standard deviation value of 3.5% for thiosulfate and 4.5% for iodide during five replicate determinations of 10 µg mL-1 of thiosulfate and 15 µg mL-1 of iodide, respectively. The accuracy of the procedure was tested by analyzing spiked real samples and certified reference sample (BCR 150 Milk powder). The procedure described was successfully applied for the determination of iodide in table salt and milk and, thiosulfate in table salt. Iodide and thiosulfate ions have been determined in real samples with relative error below 15%.