Concentrations of iodide and total iodine in edible seaweeds harvested on the Galician coast (Northwest Spain)

Abstract

Abstract We developed a rapid and feasible method to determine total iodine and iodide in edible seaweeds by an inexpensive analytical technique, catalytic spectrophotometry. Seaweed samples were solubilized by applying a microwave-assisted alkaline digestion with tetramethylammonium hydroxide (TMAH). The percentage of iodide was measured directly in the alkaline digests by observing the catalytic effect of iodide on the oxidation of As(III) by Ce(IV) in H2SO4/HCl medium. In order to determine total iodine, a microwave-assisted sodium tetrahydroborate (NaBH4) reduction was optimized. After this treatment, all iodine species present in the alkaline digests were reduced to iodide. Microwave-assisted alkaline digestion was performed using 7.5 ml of TMAH and irradiating samples at 670 W for two 5.5 min steps. Microwave-assisted NaBH4 reduction was carried out using 2.5 ml of the alkaline digest, 3.0 ml of a 10% (m/v) NaBH4 solution and 2.0 ml of 5.0 M hydrochloric acid, at a microwave power of 360 W for 3.5 min. The linear calibration ranges were 0.30–20.0 and 0.43–20.0 μg l-1 for iodide determination and total iodine determination, respectively. The limit of detection was 9.2 μg g-1 for iodide and 54.0 μg g-1 for total iodine. Repeatability of the overall procedures, expressed as RSD for eleven determinations, was 10.4% (microwave-assisted alkaline digestion followed by microwave-assisted NaBH4 reduction for the determination of total iodine). Accuracy of the method was assessed by analyzing the NIES-09 (Sargasso) certified reference material. Edible seaweed analysis has shown that most of the iodine is present as iodide in brown seaweed samples (90% of the total iodine), while red seaweeds have iodide percentages lower than 30% and green seaweeds lower than 80%.