Many workers employ starch-test solution as the indicator of choice in iodometric titrations. The effectiveness of starch solution may depend upon the temperature, the amount of free iodide present, the presence of other salts and acids, the kind of starch used, and the method of preparation of the starch solution. Although the end reaction is usually accompanied by a blue color, or some shade thereof, the color obtained in iodine-starch combinations, depending upon the factors mentioned, may
... at times violet or golden-yellow. Unless freshly prepared, starch solutions will decompose because of bacterial action; furthermore, the presence of suitable preservatives (Samuel, W. A. Chem.-Anal., 1948, 37, 33) results in interference with color reactions in microdeterminations. In studies on the use of minate quantities (ppm) of free iodine in water disinfection, we fouid that starch solution was not a suitable reagent and would not reveal the presence of free iodine in very small amounts. When a-naphthaflavone, xanthones, quinones, and phthaleins are introduced into iodine solutions they usually produce blue-colored addition products. A search of the literature revealed that G. Barger and W. W. Starling (J. chem. Soc. Trans. London, 1915, 107, 411) first noted the production of a blue color with a-naphthaflavone and free iodine. This also was reported by H. Freundlich (Physikat. chem. Grundlagen der Kolloidchcmie. Leipzig, 1924; English translation, Barger G. Reith was the first worker to employ a 0.1% alcoholic solution of a-naphthaflavone as the reagent for titrimetric examinations and for the colorimetric analysis of free iodine solutions. In our studies investigating the use of free iodine in water, especially in swimming pools, it was necessary to have available a very sensitive reagent or indicator to detect free iodine in amounts of less than 1 ppm. MWe found that a 0.1% alcoholic solution of a-naphthaflavoitn was stable and was more sensitive than starch-test soli, tion in revealing minute quantities of free iodine. Concentrations as low as 0.1 ppm were readily detected, whereas starch reagent required at least 10 ppmi of free iodine for the production of-a sensitive color reaction. In determinations where the free iodine concentrations were large (from 10 to 100 ppm), the color produced when~using the' a-nephthaflavone reagent varied front brown and brownish-blue to blue and violet. Quantities of free iodine less than 10 ppm revealed varying degrees of a blue, violet blue, and violet color.