The microdetermination of iodine in biological materials with special reference to the combustion of samples in the Parr oxygen bomb.

The most troublesome obstacle to the quantitative determination of the very minute quantities of iodine which are normally present in biological materials has been the destruction, without loss of iodine, of the relatively large amounts of organic matter present. The procedures which have been used to destroy the organic matter include fusion with alkali, combustion in a closed system, and digestion with sulfuric acid or hydrogen peroxide. Von Kolnitz and Remington (1) have critically reviewed all of these methods and point out serious objections to each. Leipert (2) developed a wet combustion method in which the organic matter is oxidized and the iodine is converted to iodine pentoxide by means of chromium trioxide in a sulfuric acid medium. Chromium trioxide usually contains iodine and is difficult to purify. Matthews, Curtis, and Brode (3) devised a method for preparing chromium trioxide of low iodine content. The chromium trioxide method is not directly applicable to biological materials of low iodine content such as food. For the analysis of liquid biological samples we have adapted and improved the acid permanganate method used by Riggs and Man (4) for determination of iodine in blood. Potassium permanganate is easily purified and is an excellent oxidizing agent. It serves also to indicate conveniently the amount of oxidizing agent required for complete oxidation of the organic matter, and in the distillation procedure the decoloration of the permanganate color is assurance of complete reduction of both the iodine and permanganate. However, the permanganate method is also not applicable directly to biological samples of low iodine content, such as food, which necessitate the oxidation of considerable organic matter. For such samples some type of closed combustion is usually employed. McClendon was the first to make closed combustions on biological materials in a bomb (5), but shortly discarded the bomb for a silica combustion tube (6). The latter method finally developed to the use of a platinum combustion tube with