201, 202. ESTIMATION. 215 



tion of cyanide of mercury by glucose as recommended by Knapp 1 

 ( 84), and shows that by neither method can saccharose be accur- 

 ately distinguished from glucose. 



O. Schmidt distinguishes between grape- and cane-sugar by 

 adding basic acetate of lead and ammonia, and warming. The 

 white precipitate that first appears turns red if grape-sugar is 

 present; cane-sugar does not yield this reaction. 2 



201. Estimation of Glucose in Presence of Saccharose. In titrating 

 glucose in the presence of saccharose, it is advisable, according to 

 Eumpf and Heinzerling, 3 to conduct the experiment with the 

 utmost rapidity, as the error caused by the dextrin is then but 

 very small. 



Barfoed detects grape-sugar in the presence of dextrin by means of 

 an aqueous solution of crystallized acetate of copper (1 in 15), 

 containing 1 per cent, of acetic acid. To the liquid under 

 examination a few drops of the reagent are added, the whole 

 boiled for an instant and cooled. If grape-sugar is present, 

 cuprous oxide gradually separates out.* Cane-sugar, milk-sugar, 

 and gum behave like dextrin. These statements have been 

 confirmed by Miiller. 5 



202. Detection of Dextrin in Presence of Cane-sugar. Scheibler 6 

 has shown that a 20 per cent, aqueous solution becomes cloudy 

 on the addition of 4 volumes of 90 to 95 per cent, alcohol if the 

 sugar is contaminated with as little as even -5 per cent, of dextrin. 

 A second test of Scheibler's, depending upon the use of iodine, 

 may be passed over here, as it succeeds with impure dextrin only. 

 The presence of dextrin and cane-sugar in solution together may 

 often be inferred if, after inversion, Fehling's solution shows less 

 grape-sugar than could have been anticipated by calculation from 

 the rotatory power before inversion ; or if the rotatory power 

 after inversion is less than that before inversion (taken as due to 

 pure cane-sugar) would have led us to expect. (See 207.) 



203. Gravimetric Estimation of Glucose. Mulder recommends 



1 Annal. d. Chem. und Pharm. cliv. 252 (Pharm. Journ. and Trans. [3], i. 

 301, 1870). 



2 Annal. d. Chem. und Pharm. cxix. 102, 1861. 



3 Zeitschr. f. anal. Chem. ix. 358, 1870. Compare also Barfoed, ibid. xii. 

 29, 1873 (Year-book Pharm. 176, 1874). 



4 Zeitschr. f. anal. Chem. xii. 27, 1873 (Journ. Chem. Soc. xxvi. 1163). 



5 Zeitschr. f. anal. Chem. xviii. 601 (Year-book Pharm. 74, 76, 1879). 



6 Zeitschr. f. anal. Chem. x. 372, 1871. 



