STAECH. 7 



9. Prove that starch can be decomposed by acids or 

 ferments by means of the following: 



In about 100 cubic centimeters of water in a porce- 

 lain dish boil enough starch, previously moistened with 

 cold water, to make a thin paste. Add about 10 cubic cen- 

 timeters of dilute sulphuric acid and boil, stirring at first 

 until the liquid becomes thinner. Keep the solution up to 

 its original volume by the addition of water. If this is not 

 done the strong acid will turn the liquid brown or black. 

 From time to time remove a portion, cool, and test with 

 iodin. When the iodin gives a red color the starch has 

 been converted into dextrin. When no color appears on 

 the addition of iodin it has been changed to glucose. Test 

 a portion for glucose by adding an equal volume of sodium 

 hydrate solution, then, drop by drop, cupric sulphate solu- 

 tion till a deep-blue color is produced. Heating this will 

 give a yellow or red precipitate, showing the presence of /? 

 glucose. This is known as Trommels test for glucose. 



10. To a clear solution of starch add a solution of 

 tannic acid. A yellowish precipitate forms which dissolves 

 on heating. 



11. Try Trommer's test with the starch solution. It 

 does not respond. 



* 12. Add gradually to the remainder of the solution 

 which has been boiled with the acid, while it is still hot, 

 powdered calcium or barium carbonate until it is neutral. 

 Filter and evaporate the nitrate to dryness on the steam- 

 bath. 1 Glucose remains: examine its properties and pre- 

 serve it for subsequent tests. 



1 To evaporate a liquid on a steam- or water-bath the 

 evaporating dish in which it is contained can be heated by stand- 

 ing it on a beaker of boiling water. This removes all danger of 

 burning the residue. 



