ORGANIC ANALYSIS. On the Volumetric Determination of Starch by Means of Iodine Solution. Fannie T. Littleton. (Amer. Chem. Jour., 1897, xix., 44-49.)-The object of this investigation was to determine whether small quantities of starch could be determined colorimetrically by means of the iodine reaction, as has been stated by several workers in this direction. A standard solution of iodine was prepared, which contained iodine dissolved in potassium iodide in the proportion of 1 molecule to 1
... 1 molecule to 1 molecule, and this was used in conjunction with a standard solution of wheat starch containing 0.1 gramme per litre. Temperature was found not to have much effect when the rise was not more than 5" C" but above this the colour gradually became fainter, and almost disappeared when the temperature had risen to 15 degrees. Potassium hydrate, when added to the extent of 0.002 gramme, caused 1 gramme of starch to dissolve rapidly, and did not affect the colour of the starch iodide solution if the alkali were exactly neutralized before the addition of iodine. Solutions of various kinds of starch were then compared, an exceptionally pure specimen of arrowroot starch being taken as the standard, and the heights of the columns of corresponding colour being compared with this. The strengths (which varied inversely as the heights of the columns) were as follows : Arrowroot, 100; potato, 97.06; wheat, 94.81; tapioca, 90.41; and rice, 83.83. These results were about what one would expect from the known relative size of the starch granules. Hence, rice starch, with its smaller granules and consequently probable larger amount of cellulose, should not show as high a percentage of true starch as arrowroot with its larger granules. Experiments made to determine whether the quantities of starch and iodine required to produce the maximum colour had a definite ratio showed that this was the case. Thus 0,007 gramme of starch required 0.00448 grainme of iodine, and 0.001 gramme required 0.00064 gramme ; but this only held good when a solution of iodine of a single strength was employed, for when the same quantity of starch solution was tested with iodine solutions of different strengths, the results were quite irregular. This was ascribed to the dissociating effect of the additional water contained in the weaker iodine solutions, and this view was confirmed by other experiments described in the paper. Therefore, in using the method, it will be-necessary to compare substances whose percentages of starch do not differ widely. It is further shown that chloroform will extract iodine from moist starch iodide, although the contrary is stated in Thorpe's " Dictionary of Applied Chemistry," iii., 565. The general conclusion arrived at is that, on account of the easy dissociation of the so-called starch iodide, the method is only available with substances containing u, large amount of starch, and the colorimetric comparisons must then be made under conditions very nearly identical. This limits very considerably the application of the process in the examination of food material, although in occasional cases it may be of value.