Hale — Initiative Action of Iodine, etc. 379 



Aet. XXXII. — The Initiative Action of Iodine and of Other 

 Oxidizers in the Hydrolysis of Starch and JDextrins / by 

 F. E. Hale. 



[Contributions from the Kent Chemical Laboratory of Yale University — CVIL] 



Frequent mention is made in the literature of the produc- 

 tion of a red color, as well as a blue, when starch is used as an 

 indicator in iodometric titration, and numerous formulae 

 have been given for making a starch solution warranted to 

 give a blue color with iodine, and to keep without spoil- 

 ing. An investigation of a loss of iodine, in titrimetric 

 processes found to be connected with the formation of a red 

 color, has led not only to the elimination of the loss, but also 

 to a probable explanation of the cause of the red color, of the 

 loss of iodine, and of their mutual relation. 



The first loss of iodine was noticed in titrating a decinormal 

 solution of arsenite with a decinormal solution of iodine. The 

 amount of iodine needed was greater when an ordinary starch 

 solution* was used than it was when no indicator was used. 

 The starch solution was made by grinding 5 grm. of starch 

 paste with a few cubic centimeters of cold water with the 

 addition of O'Ol grm. of mercuric iodide, pouring into a liter 

 of boiling water, and boiling five to ten minutes. Only the 

 clear supernatant liquid was used. An abundance of red was 

 produced which slowly faded, and only the production of a 

 permanent blue was taken as a reading. 



The reading without starch was taken when the first tinge 

 of yellow appeared, and to a skilled eye this reading is very 

 sharp, and comes upon the addition of a single drop of deci- 

 normal iodine. The most important phenomenon, now notice- 

 able, is that if a cubic centimeter of the starch solution be 

 added after the yellow has appeared, a fine blue is produced 

 without even a tinge of purple. This fact shows that some 

 cause for the production of the red lies in the titration. It 

 excludes any explanation of the loss of iodine by the forma- 

 tion of iodate. It excludes any explanation of the red color 

 by some arsenic acid compound with starch when titrating an 

 arsenite solution, or by some antimonic acid compound with 

 starch when using a tartar emetic solution. The last state- 

 ment is made because a red color has been produced by the 

 action of arsenic acid upon certain sugars, but arsenious acid 

 and salts of arsenic acid did not produce such a red color with 

 the sugars. Moreover starch failed to give any such color 

 with arsenious acid, arsenic acid, or salts of arsenic acid.f 



* G. Gastine., Zeitschr. anal. Chem., 1889, 339. 



f Zeitschr. anal. Chem., 1882, 124. (Original not accessible.) 



Am. Jour. Scl— Fourth Series, Vol. XIII, No. 77. — May, 1902. 

 26 



