1987 Volume 36 Issue 5 Pages 320-325
A spectrophotometric method was developed for the determination of iodide, which is based on the precipitation of silver iodide and solvent extraction of dicyanoargentate(I) as an ion-pair with Methylene Blue. The recommended procedure is as follows. Pipette 10 ml of a sample solution containing iodide up to 12.7 μg (10-5 M I-) into a 50-ml separatory funnel, and add 1 ml of 4 M sulfuric acid and 1 ml of 0.7 M hydrogen peroxide. After standing for more than 5 min, add 10 ml of carbon tetrachloride and shake the funnel for 1 min to extract the iodine formed. Transfer the organic phase to another 50-ml separatory funnel and add 5 ml of 2×10-4 M L-ascorbic acid. Shake the funnel for 1 min to back-extract the iodine in the organic phase into an aqueous phase. After discarding the organic phase, add 2 ml of 7×10-3 M sulfuric acid, 2 ml of 2.5×10-5 M standard silver(I) sulfate, 1 ml of 7×10-4% himoloc solution and 2 ml of carbon tetrachloride. Shake the mixture for 2 min in order to collect the precipitate of silver iodide on the interface between the aqueous and organic phases. After removing the precipitate with carbon tetrachloride, add 1 ml of 0.7 M hydrogen peroxide, 2 ml of 8×10-3 M sodium cyanide, 1 ml of 1×10-3 M Methylene Blue and 10 ml of 1, 2-dichloroethane. Shake the funnel for 1 min to extract the Methylene Blue-dicyanoargentate (I) ion-pair. After standing for a given period of time, transfer the organic phase to a glass-stoppered tube and add some anhydrous sodium sulfate to remove water droplets. Shake the mixture vigorously by hand untill it becomes transparent and measure the absorbance at 657 nm against dichloroethane using 10-mm glass cells. The calibration graph shows a negative linearity up to 9×10-6 M (1.14 ppm) iodide, and the apparent molar absorptivity for iodide at 657 nm is 1.04×105 1 mol-1 cm-1. The present method was successfully applied to the determination of iodide in various amounts in several natural water samples, and gave a relative standard deviation of 0.52% at 6×10-6 M iodide level.
