Variations in the standardization of iodine solutions for analytical purposes

The official methods require that iodine be standardized against arsenic trioxide and sodium thiosulfate against potassium dichromate. Theoretically, such thiosulfate and iodine solutions should be and frequently are equivalent. However, in a sufficient number of instances it was noted that the correspondence was not unity. Investigation indicated that such iodine solutions when first acidified, then made neutral and standardized against arsenic trioxide, showed no variation when compared with standard thiosulfate. The difficulty was found to be in the storage container and it is postulated that iodate formation is responsible for the variation noted. Examples of discrepancies in analysis as a result of variation were demonstrated. It is recommended that additional precautions be observed in the storage of iodine solutions and that either iodine solutions be required to show no variation in standardization or that a double standard be established, depending upon the particular conditions of use intended.