Vortex-assisted liquid-liquid microextraction procedure for iodine speciation in water samples

A novel, sensitive and selective method for the spectrophotometric determination of the basic inorganic iodine species was developed based on the vortex-assisted liquid-liquid microextraction (VALLME) of an ion pair formed between colorless tetrabutylammonium cation (TBA) and triiodide anion (I3−). Molecular iodine was determined spectrophotometrically at 295 nm by the measurement of the intrinsic triiodide absorbance after the addition of an excess of TBA iodide to the sample and following VALLME of the ion-pair TBA-I3 with amyl acetate. The procedure for iodate determination includes the formation of iodine as a result of iodate reduction with TBA-I in acid solution. The new approach was introduced in order to determine iodide or total inorganic iodine content. N-Bromosuccinimide (NBS) was used for the oxidation of iodide to iodate. The excess NBS was removed with oxalic acid. Afterwards, the analysis proceeded as for iodate. The experimental conditions were optimized and found to be: 0.04 mol L− 1 HCl; 0.12 mmol L− 1 NBS (only for iodide, total iodine); 1 mmol L− 1 and 1.4 mmol L− 1 TBA-I for iodide and iodate determination, respectively; 150 μL of amyl acetate as the extraction solvent; vortexing, 20 s at 3000 rpm; and centrifugation, 2 min at 3000 rpm. The calibration plot was linear in the range 3.8-63 (R2 = 0.994) and 5.2-88 (R2 = 0.998) μg L− 1 of iodide (as I−) and iodate (as IO3−), respectively. The calibration curve for iodine determination (as I2) is satisfactorily described by the second-order equation (R2 = 0.999) in the concentration range 6.4-76.2 μg L− 1 (as I−). The limit of detection (LOD) was 1.1, 2.3 and 1.5 μg L− 1 of iodide, iodine (as I−) and iodate, respectively. The suggested procedure was successfully applied for the determination of iodide, iodate and iodine in salt, sea and mineral water samples. The accuracy of the method was assessed through comparison with reference methods and by recovery testing.