A Method for the Determination of StcD-cJi. 173 



able. It seemed desirable to develop a method, if possible, 

 which would prove sufficiently accurate for such purposes. 



The work along this line up to date has been confined to 

 starch solutions, the thought being that any method giving 

 accurate results on such solutions might be made applicable 

 to the estimation of starch in various food products by suit- 

 able modifications. 



Upon investigation it was found that when equal volumes 

 of a starch solution were treated with equal volumes of iodine 

 solution in the presence of potassium iodide and in amounts 

 greater than the equivalent of the starch present, the latter 

 was completely precipitated, and upon filtering and washing 

 with potassium iodide solution, filtrates were obtained con- 

 taining equal quantities of iodine. These facts suggested the 

 possibility of a volumetric method similar to that used in the 

 determination of the iodine number of fats. Such a method, 

 however, would obviously involve the removal of the halo- 

 genated starch before titration. 



A number of trials were made upon starch solutions of 

 different concentrations to determine whether the absorption 

 of iodine was affected by concentration, and also to determine 

 whether duplicate determinations would give concordant re- 

 sults. 



Some of the results on duplicate determinations were as 

 follows: For a 1 percent starch solution the titration figures 

 for unabsorbed iodine were 3.35 and 3.20 ; for a 0.5 percent 

 solution, 8.50 and 8.50; for a 0.1 percent solution, 4.72 and 

 4.80 ; and for a 0.05 percent solution, 17.83 and 18.40. These 

 results were sufficient to show this fact — that in the case of 

 any one solution of starch, equal quantities of starch absorb 

 equal quantities of iodine, and further, that unabsorbed iodine 

 could be recovered. The point as to whether concentration 

 has any effect upon the absorption was readily determined by 

 reduction to a common basis of the results obtained for ab- 

 sorbed iodine in the individual cases. It was found that with 

 sufficient care in precipitation and washing of precipitates, 

 especially in the case of the more concentrated solutions of 

 starch, results were obtained which approximate the figures 

 0.00125, this representing the grams equivalent of starch 

 per 1 cubic centimeter of 1 500 normal iodine solution. 



This factor was used in calculating some results obtained 

 by the following method : 



