PROBLEM 27 57 



TESTS FOR ORGANIC NUTRIENTS 



Problem 26 : To test for starch. 



Materials. Cornstarch, iodine solution, 1 and test tube. 



Method and Observations. Add a small bit of starch to a 

 test tube containing a little cold water. Add a few drops of iodine 

 solution. Note the result. Make a little starch paste by heating 

 with water ; cool, add iodine as before. Do you get the same 

 result? 



NOTE. The presence of starch and no other known substance is shown by a change 

 to a deep blue color on addition of iodine. 



Now mash up a little of the bean cotyledon and add iodine. 

 Is there any starch in the bean? Work out in experiment 

 form. 



Conclusion. How would you detect the presence of starch in 

 a substance? 



Problem 27: To test for grape sugar. 



a. Test with Fehling's Solution 



Materials. Glucose, Fehling's solution, 2 test tubes, Bunsen 

 burner. 



Method. Place in a test tube a little glucose and water, 

 add an equal amount of Fehling's solution. Heat to the boil- 

 ing point. 



Observations. What color changes take place? 



NOTE. If the color of the mixture becomes brick red upon heating, then grape 

 sugar is present. 



Conclusion. Is grape sugar present in the substance tested? 



1 Iodine solution may be made by adding a few crystals of iodine to enough 95 

 per cent alcohol to dissolve it well. Or to 1 gram of iodine crystals, add f gram of 

 potassium iodide, and dilute to a dark brown color in 35 per cent alcohol. 



2 Fehling's solution may be made as follows : Add 35 g. of copper sulphate to 

 500 c.c. of water. Solution No. 1. 



To 160 g. caustic soda (sodium hydroxide), and 173 g. Rochelle salt, add 500 c.c. 

 of water. Solution No. 2. 



For use mix equal parts of solutions 1 and 2. This may also be obtained of 

 druggists, in tablet shape. 



