Spectrophotometric determination of iodate in iodides of alkaline metals
Conditions for interaction of the iodate ions with iodide and methylene blue as a chromogenic reagent were studied. The interaction of iodate with an excess of iodide depends on the medium acidity. In dilute acids, the reaction of with I- is accompanied with a release of iodine which oxidizes the dye. The observed weakening of the color which is proportional to the concentration of iodate ions in the solution, results from the oxidative destruction of methylene blue. Spectra of methylene blue, conditions of the redox interaction, as well as the effect of the solution acidity and reagent concentration on the completeness of the reaction were studied. The light absorption of the solutions was measured at a wavelength of 664 nm. The reaction of the free iodine formation is rapid and quantitative at pH 4 - 5. The desired acidity of the medium was attained using 1 M solution of acetic acid. The optimal concentration of methylene blue was determined. The results of the study formed a base for developing a technique of iodate ion determination in iodides of alkaline metals. The analyzed solutions were prepared with addition of sodium acetate to prevent iodide oxidation by atmospheric oxygen. To eliminate the interfering effect of Fe (III), sodium pyrophosphate (which should be introduced after acidification of the solution and isolation of iodine) was used. The accuracy of the technique was proved in spike tests on the model solutions and real samples by the method of sample weight variation. The developed technique was tested in determination of the iodate ions in cesium and sodium iodides. The relative standard deviation does not exceed 15%. The sensitivity of the developed technique is 10 times higher compared to the technique based on the formation of an iodine-starch complex.