The photometric determination of minute amounts of iodine in silicate rocks

A rapid and sensitive method for the determination of minute amounts of iodine in silicate rocks has been developed. A rock was fused with potassium hydroxide, and the cake was treated with water and sulfuric acid. The iodine in the solution was reduced to iodide by sodium sulfite, and was then oxidized to free iodine by sodium nitrite. The free iodine thus obtained was extracted with carbon tetrachloride and then back-extracted with a sodium hydroxide solution. The iodine in this aqueous layer was determined photometrically by a method based on the catalytic effect of iodine on the color-fading of ferric thiocyanate. By the proposed method, amounts of iodine as low as 0.007 mu g in a 1-g sample of silicate rocks can be determined. Application to some igneous rock samples gave results with a relative standard deviation of less than 7.2%, except for very low concentrations.