83, 84. ESTIMATION OF GLUCOSE. 73 



filtered off and tested for copper by the addition of acetic acid 

 and ferrocyanide of potassium. But a slight reaction will 

 generally be obtained as traces of copper remain in solution and 

 the absence of any reddish-brown precipitate after the lapse of a 

 few minutes must be taken as sufficient indication of the termina- 

 tion of the reaction. 



It is well known that the sugar solution should be very dilute ; 

 the best strength is about J per cent. If a preliminary experi- 

 ment shows it to be more concentrated, it should be diluted to 

 about this strength. 1 



The estimation may also be made gravimetrically, by quickly 

 filtering off the cuprous oxide, washing with water, drying and 

 converting into cupric oxide. This method is advisable if, in 

 titrating, the final reaction is obscured, or if the amount of sugar 

 solution available is not sufficient to complete the reduction of the 

 copper-salt taken. 



But it must be borne in mind that by drying the cuprous 

 oxide on the filter and weighing incorrect results would, as 

 Brunner 2 has shown, be obtained, since the alkaline copper solu- 

 tion dissolves cellulose and the filter accordingly loses weight. 

 It is better, therefore, to dissolve the cuprous oxide and deter- 

 mine the copper by the usual methods, or to estimate the excess 

 of copper in the filtrate. 3 317 parts by weight of copper, 

 357 of cuprous oxide or 397 of cupric oxide indicate 180 of 

 glucose, 171 of saccharose or 162 of starch, etc. ( 200). 



84. Other Methods of Estimating Glucose. Glucose may also 

 be estimated by Knapp's reagent, 4 which consists of 10 grams of 



1 Soxhlet Zeitschr. f. anal. Chemie, xviii. 348 (Pharm. Journ. Trans. [3] 

 xi. 720) has shown that the reducing power of the glucose varies with the con- 

 centration of the solutions. In making estimations the sugar solution should 

 therefore be of as nearly as possible the same strength as that used for standard- 

 izing. According to Soxhlet the gravimetric estimation in presence of an 

 excess of copper may be attended with considerable error. But Maercker 

 has shown that satisfactory- results may be obtained by this method also, if the 

 same conditions are always observed. See also Ulbricht, Chem. Centralblatt, 

 1878, 392, 584. 



2 Zeitschr, f. anal. Chemie, xi. 32 (Journ. Chem. Soc. xxv. 928). 



* Compare also Weil, ib. 284 ; Mohr, ib. xii. 296 ; Jean, ib. Ill ; Lagrange, 

 ib. xv. Ill ; Briicke, ib. 100 ; Maschke, ib. xvi. 425. (See Journ. Chem. 

 Soc. xxv. 1121 ; xxvii. 292 ; xxxi. 805, ib. 116 ; xxxii. 930.) 



4 Annal. d. Chem. u. Pharm. cliv. 252 (Pharm. Journ. Trans. [3] i. 301). 

 See also Mertens, Zeitschr. f. anal. Chemie, xiii. 76 ; Brumme, ib. xvi. 121. 

 Knapp's reagent keeps considerably better than Fehling's. 



