A Modification of the Method for Determining Methionine in Proteins

In 1932 (1) the author published a method for determining methionine in proteins based on the recovery of methyl iodide. Alcoholic silver nitrate was used as absorbant and the excess of silver nitrate was titrated with thiocyanate after filtering off the silver iodide. This method was quite satisfactory and has been used recently by Barritt (2) for the determination of methionine in wool. Certain difhculties in the handling of the alcoholic silver nitrate and in the treatment of the silver iodide were experienced, however, and therefore a search was made for a better absorbant. My attention was called to the new method of Viebock and Schwappach as modified by Clark (3), in which a mixture of glacial acetic acid, potassium acetate, and bromine is used to absorb methyl iodide in the determination of methoxyl groups. Methyl iodide is converted to iodate and the iodine liberated therefrom by potassium iodide and sulfuric acid is titrated with thiosulfate. This is considerably more accurate than the silver titration, and much simpler. A complication arises, however, if the hydriodic acid used contains hypophosphite. During the heating the excess of hypophosphite is converted to phosphine, which is oxidized by the bromine to phosphate, and the scrubbers become clogged. In order to prevent this, a scrubber containing saturated mercuric chloride solution is introduced into the absorption train between the cadmium sulfate (or chloride) scrubber, and the acetic acid absorbers. In order to eliminate the small quantities of methyl iodide held back by these first two scrubbers, they are immersed in warm water kept heated by the stream which runs through the condenser.