

CH. V.] GLUCOSE. 105 



Glucose (dextrose or grape sugar) C 6 H 12 O 6 . 



94. Preparation o! glucose from starch. 



To 700 cc. of distilled water add 40 cc. of pure HC1 and boil. 

 Mix 100 grams, of potato starch with 200 cc. of cold water, and 

 slowly stir this into the boiling mixture. Wash in the remainder 

 of the starch with another 100 cc. of water. Boil under a reflux 

 condenser for 3 hours. To the hot solution add solid lead carbonate, 

 a little at a time, till effervescence ceases (about 100 grams, are 

 usually required). Cool and filter. Evaporate the filtrate to a 

 thin syrup. Add an equal volume of hot 95 per cent, alcohol. 

 Filter. Evaporate the filtrate to a thick syrup. Treat this with 

 twice its volume of 93 per cent, alcohol and allow the solution to cool 

 slowly. If a syrup falls out of solution on cooling, the alcohol is too 

 strong, and a few drops of water should be added and the solution 

 again heated to redissolve it. When the cooled solution no longer 

 deposits any syrup add a crystal of glucose and set aside to crystal- 

 lise. 



After crystallisation is complete, which may take six or seven 

 days, drain the crystals and dry by spreading them on a porous 

 earthenware plate. To recrystallise dissolve the dried crystals in 

 half their weight of water and add to the resulting syrup twice its 

 volume of boiling 93 per cent, alcohol. Set the alcoholic solution 

 aside to crystallise, and dry the resulting crystals as before. 



Unless directions to the contrary are given use a 0-2 per cent, solution of 

 glucose for the following exercises. 



95. Boil 3 cc. with I cc. of 5 per cent, sodium hydroxide. The 

 solution turns yellow. 



NOTE. The yellow colour is due to the formation of caramel (a condensa- 

 tion product) by the hot alkali. 



96. Treat two or three cc. of 5 per cent, caustic soda with 

 four or five drops of a i per cent, solution of copper sulphate. A 

 blue precipitate of cupric hydroxide, Cu(OH) 2 is formed. Add to 

 the mixture an equal bulk of the sugar solution. The precipitate 

 dissolves. Boil the solution for a short time. The blue colour dis- 

 appears, and is replaced by a yellow or red precipitate of cuprous 

 oxide, Cu 2 O (Trommer's test). 





