44 PHYSIOLOGICAL CHEMISTRY. 



Starch may be recognized by a number of chemical tests, the best of 

 which are the following: 



Ex. Boil a small amount of starch with water so as to make a thin 

 paste. Allow this to cool, and add a few drops of an aqueous, or alcoholic 

 solution, of iodine. A deep blue color is formed, which disappears on 

 boiling the mixture. This test is exceedingly delicate and characteristic, 

 and serves for the detection of minute traces of iodine as well as starch. 

 The blue color is destroyed by alkalies or much alcohol as well as by 

 heat. 



Ex. That starch is insoluble in cold water may be shown by stirring 

 some with water in a beaker, allowing to settle, and pouring the liquid 

 through a paper filter. The filtrate tested with the iodine solution does 

 not give a blue color. Use the potato starch of the experiment for this 

 test. 



When boiled with dilute acids starch is converted into sol- 

 uble compounds. The nature of these compounds depends 

 on the acid used and on the duration of the heating. By pro- 

 longed heating glucose is the main product of the reaction, as 

 already illustrated, but various intermediate steps may be rec- 

 ognized, maltose and forms of dextrin being readily demon- 

 strated. With strong acids the results are quite different. 

 With sulphuric acid the reaction is completely destructive, 

 water, carbon dioxide and sulphurous acid from reduction 

 being formed. Strong nitric acid acts as an oxidizing agent 

 and by proper manipulation oxalic acid may be obtained in 

 quantity as a product of the oxidation. 



Ex. Add 15 cc. of strong sulphuric acid to a gram of starch in a flask 

 holding about 200 cc. Heat to the boiling point and observe that a black 

 mass is soon produced. By prolonged heating this is further decomposed, 

 while fumes of sulphurous oxide escape, leaving finally a colorless liquid. 



Ex. Add 15 cc. of strong nitric acid to one gram of starch in a flask 

 holding 200 to 300 cc., place this on a sand-bath in a fume chamber and 

 apply heat. After a time copious red fumes are given off. Remove the 

 lamp and allow the reaction to continue until the fumes cease to be evolved. 

 Finally, transfer the liquid to a porcelain dish and evaporate to a small 

 volume. On cooling, a crystalline residue remains which consists mainly 

 of oxalic acid. 



When carefully heated starch may be largely converted into 

 a form of dextrin, which, as will be fully explained later, is 

 one of the important stages in the common transformations of 



