Determination of High Iodine Levels by ICP-OES after Separation from Excess Phosphate by Co-precipitation

For determination of iodine, dissolution or oxidation of an organic matrix by acid K-chlorate in dilute nitric acid to obtain non-volatile iodate immediately, is a method of choice. Optical emission spectroscopy is usually sensitive enough to control iodized table salt. Spectral interference by P upon the most sensitive I-emission line, e.g. in mineral feeds and marine algae, was overcome by co-precipitation of P with Al- or Fe hydroxide, leaving iodate (and also borate) in solution. After addition of Al or Fe nitrate or sulfate, ammonia was added to reach pH 8.8, and the resulting precipitate separated by cetrifugation and decantation. Al-hydroxide was more efficient, and its precipitation was facilitated by adding 10-30 mg of a natural clay to 5 mL sample. Correct results with materials from former ring tests were obtained down to 15 mg/kg by an ICP 20 years of age.