The rôle of water in the starch iodine reaction.

Abstract

A number of investigators 1 , 2 , 3 have reported that “dry” starch will not react with iodine to produce the well-known blue substance, starch iodide.

Starch is a very hygroscopic material, 4 , 5 and like other hygroscopic substances, when in contact with the atmosphere, will tend to absorb water until the vapor pressure of the absorbed water is in equilibrium with the vapor pressure of the surrounding air. 6 Many of the researches involving “dry” starch were carried out with starch in more or less complete moisture equilibrium with the atmosphere, hence by no means really dry.

The conditions of preparation, storage, and other treatment which affect moisture content, of the starch used should be fully described in work of this sort.

Since the literature indicates that starch at a certain degree of dryness will no longer yield blue starch iodide when treated with iodine, it seemed worth-while to determine approximately the water concentration necessary that this reaction may take place.

In the following investigation the starches used were prepared according to the method of Alsberg and Rasks 7 and ground after the method of Alsberg and Perry, 8 and then stored for varying periods of time in reagent bottles at room temperatures. Such preparations were of course in moisture equilibrium with the air in the containing vessel.

In ground starch the comparatively dense outer layer of the starch grain, which as Denniston 9 pointed out, reacts more slowly with iodine than the inner portion of the grain, is cracked and chipped, and are more favorable for rapid formation of starch iodide than when the intact grain is used.