=194. Colorimetric Estimation of Starch.=—The production of the intensely blue color which starch gives with iodin has been used not only as the basis of a qualitive method, but also of many attempts at quantitive determination. These attempts have, as a rule, been attended with very unsatisfactory results, due both to the extraordinary delicacy of the reaction and to the fact that starches of different origin do not always give exactly the same intensity of tint when present in the same quantity. At the present it must be admitted that little should be expected of any quantitive colorimetric test. In case such a test is desired the procedure described by Dennstedt and Voigtländer may be followed.[162] A weighed quantity of the starch-holding material, containing approximately half a gram of starch, is placed in a two liter flask and boiled with a liter of water. After cooling, the volume is completed to two liters and the starch allowed to subside. Five cubic centimeters of the clear supernatant liquor are placed in a graduated cylinder holding 100, and marked in half cubic centimeters. One drop of a solution of iodin in potassium iodid is added and the volume completed to the mark. A half gram of pure starch is treated in the same way and different measured portions of the solution treated as above until the color of the first cylinder is matched. From the quantity of pure starch in the matched cylinder the quantity in the sample is determined. The test should be made in duplicate or triplicate. If a violet color be produced instead of a blue, it may be remedied by treating the sample with alcohol before the starch granules are dissolved. =195. Fixation of Iodin.=—In addition to forming a distinctive blue color with iodin, starches have the power of fixing considerable quantities of that substance. The starches of the cereals have this power in a higher degree than those derived from potatoes. In presence of a large excess of iodin the starches of rice and wheat have a maximum iodin-fixing power of about nineteen per cent of their weight. When only enough of iodin is employed to enter into combination the percentage absorbed varies from nine to fifteen per cent. The absorption of iodin by starches is a matter of importance from a general chemical standpoint, but as at present determined has but little analytical value. It is evident, however, that this absorption must take place according to definite chemical quantities and the researches of investigators may in the future discover some definite quantitive method of measuring it.[163]