396 Hale — Initiative Action of Iodine and other 



iodides are formed together. The arsenious acid in alkaline 

 solution fades the blue first, leaving the red to fade more slowly. 

 This is also true of sulphurous acid. In alkaline solution a 

 very dilute solution of sulphurous acid may be made to fade 

 the blue leaving a trace of the red, but the endurance of the 

 red is usually but momentary as the reaction is so very rapid ; 

 but in acid solution sulphurous acid fades the red first and the 

 blue last, even when abundant red is present such as is caused 

 by the permanganate experiment with impure starch or amidu- 

 lin. This latter fact may explain why in titration of iodine by 

 thiosulphate one is not bothered with the production of red hues. 



The presence of amidulin in the impure starch cannot be 

 said to be absolutely established, since it has not been chemi- 

 cally isolated from the solution and examined by itself, but 

 the circumstantial evidence seems substantial. 



1st. Impure starch causes a loss of iodine which is accom- 

 panied by the production of a compound coloring red with 

 iodine when used in a titration of arsenite solution or tartar 

 emetic by iodine. 



2d. The same red-coloring compound is produced from 

 impure starch by boiling with potassium permanganate and 

 acid potassium carbonate or by treatment with hydrogen 

 dioxide and acid potassium carbonate, hot or cold. 



3d. Pure starch neither causes a loss of iodine nor is it acted 

 upon in presence of acid potassium carbonate by iodine, hydro- 

 gen dioxide, or potassium permanganate, to form the red- 

 coloring compound. 



4th. The red-coloring compound found in the impure starch 

 was proven to have the same power of rotating polarized light 

 as erythrodextrin, and pure erythrodextrin caused no loss of 

 iodine in regular titration with arsenite solution. 



5th. A compound coloring blue with iodine whose iodide 

 was not precipitated by sulphuric acid was found in the impure 

 starch. 



6th. The latter impurity, the compound coloring blue, is 

 intermediate between starch and erythrodextrin, since its 

 iodide is more stable than erythrodextrin iodide and more sol- 

 uble than starch iodide. 



7th. Pure amidulin causes a loss of iodine in titration of 

 arsenite solution, or of tartar emetic, by iodine solution, and is 

 acted upon in the presence of acid potassium carbonate by 

 iodine, hydrogen dioxide, and potassium permanganate, as 

 above to form a compound coloring red with iodine. 



8th. Amidulin iodide is not precipitated by sulphuric acid. 



9th. Amidulin is intermediate between starch and erythro- 

 dextrin both because its iodide is more stable than erythro- 

 dextrin iodide and more soluble than starch iodide and because 

 it is intermediate in saliva digestion of pure starch. 



