72 SUBSTANCES SOLUBLE IN WATEB. 



EXAMINATION FOE GLUCOSES, SACCHAROSES, ETC. 



§ 83. Gliuxsesl^Th-Q alcoholic extract may, as mentioned in 

 g 70, contain a small amount of glucose whiphjif 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 bis estimated, in part of the aqueous 

 extract in § 71 by direct titration.^ But if the glucose' is ac- 

 companied by other substances, that also reduce ifche 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. (Cf. § 197.) If such 

 substances as tannic acid, eta, 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 

 determiining the sugar. 



Instead of keeping the a.lkaline copper solution recoinmended 

 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 20cc. of water represeats lOcc.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 

 Swicurately 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 iip to, a known volume) added from a 

 burette until the bljie colbui" has completely disappeared. ' 10 cc. 

 of copper solution indicate 0'06 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 



' On the estimation of sugar with copper sohition, see Fehling, Annalen d. 

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

 Craeger, N. Jahr, f. Pharm. xxix. 193; O. Schmidt, ib. 270 ; Staedeler u. 

 Krau8e,.Annalen d. Chem. M. Pharm. Ixix. 94 ; Pelkt, Jburn. de Pharm. et 

 de Chimie, 4te S^rie, xxYJi. 460 (Journ. Chem. Soo. xxxiv. 612). 



