22 LABORATORY EXERCISES. 



10. TO TEST FOODS FOR GRAPE-SUGAR. 



Materials : Grape-sugar, corn-starch, white of egg, mutton tallow 

 raisins, onions, grapes, granulated sugar, and other foods; Fehling's 

 solution ; * test-tubes, alcohol-lamp. 



A. Method of applying Fehling's test. 



1. Dissolve a small amount of grape -sugar 



(glucose) in water in a test-tube. 



a. What is the difference in the effect of 



water on grape-sugar and on starch ? 



b. Add some Fehling's solution * and boil. 



What changes do you notice? 



2. Into the first of three test-tubes put some 



white of egg mixed with water; into a 

 second tube some corn-starch ; and int-> 

 a third tube some mutton tallow (fat). 

 Add Fehling's solution to each and boil, 



a. Does any change take place in the 



color of the Fehling's solution? 



b. Do any of the colors resemble at all 



* To make a quart (1000 cc.) of Fehling's solution dissolve 3 tea- 

 spoonfuls (34.64 grams) of pure pulverized copper sulfate (blue vit- 

 riol) in a little less than a half -pint of water (200 cc.). Make a 

 second solution by dissolving in a pint (500 cc.) of water twelve 

 heaping teaspoonfuls (150 grams) of Rochelle salt and 3 (5-inch) 

 sticks of caustic soda (50 grams). Mix the two solutions thor- 

 oughly, and dilute with enough water to make a quart (1000 cc.). 

 Fehling's solution does not keep for any great length of time, and 

 hence must be made up fresh a short time before it is needed. It 

 is more convenient to prepare it in small quantities from the tab- 

 lets that may be obtained of druggists. Before making any tests 

 boil a small quantity of the Fehling's solution in a clean test-tube. 

 If it retains its transparent blue color it is ready for use; otherwise 

 a fresh supply must be prepared. 



