72 SUBSTANCES SOLUBLE IN WATER. 



EXAMINATION FOR GLUCOSES, SACCHAROSES, ETC. 



83. Glucoses. The alcoholic extract may, as mentioned in 

 70, contain a small amount of glucose which, if present, should 

 be quantitatively estimated. But, as was there observed, the 

 whole of the glucose is not usually removed by cold alcohol, and 

 the remainder must be looked for in the aqueous extract. If no 

 tannic acid or other substance that reduces Fehling's solution is 

 present, the glucose may be estimated in part of the aqueous 

 extract in 71 by direct titration. 1 But if the glucose is ac- 

 companied by other substances that also reduce the salt, these 

 must be removed before the estimation can be made. They 

 may be avoided by using the filtrate from the mucilage 

 precipitate ( 73), or from the dextrine group ( 76) ; the 

 alcohol must be removed by evaporation, the residue dissolved in 

 water and made up to a known volume. (Of. 197.) If such 

 substances as tannic acid, etc., have to be removed, it is best to 

 precipitate a portion of the aqueous extract with basic acetate of 

 lead and remove the excess of lead with sulphuric acid before 

 determining the sugar. 



/ Instead of keeping the alkaline copper solution recommended 

 by Fehling ready for use, I keep separate solutions of the three 

 salts of which it is composed, viz., 34*639 grams of crystallized 

 sulphate of copper, 173 grams of Rochelle salt, and 120 grams of 

 caustic soda, each in a litre of water. 10 cc. of each of these solu- 

 tions with 20 cc. of water represents 10 cc. of alkaline copper solu- 

 tion diluted with 4 volumes of water as recommended by Fehling. 

 Of the three solutions the sulphate of copper alone requires to be 

 accurately measured. 



The titration is made as follows : the alkaline copper solution 

 is brought to the boil in a white porcelain dish and the sugar 

 solution (previously made up to a known volume) added from a 

 burette until the blue colour has completely disappeared. 10 cc. 

 of copper solution indicate 0'05 gram grape-sugar. Should the 

 final disappearance of the blue colour be concealed by dark 

 colouring matter, etc., in the solution, a few drops may be rapidly 



1 On the estimation of sugar with copper solution, see Fehling, Annalen d. 

 Chem. u. Pharm. Ixxii. 106, cvi. 75 (Pharm. Journ. Trans. [1], ix. 419) ; 

 Graeger, N. Jahr. f. Pharm. xxix. 193 ; O. Schmidt, ib. 270 ; Stsedeler u. 

 Krause, Annalen d. Chem. u. Pharm. Ixix. 94 ; Pellet. Journ. de Pharm. et 

 de Chimie, 4te Se*rie, xxvii. 460 (Journ. Chem. Soc. xxxiv. 612). 





