GRAVIMETRIC ESTIMATIONS 103 



Weight of precipitate < -03 gram. Weight of precipitate > 0-3 gram. 



Furfural = (a + 0-0052) x 0-517 Furfural = (a + 0-0052) x 0-518 



Pentose = (a + 0-0052) x 1-017 Pentose = (a + 0-0052) x 1-0026 



Pefitosane = (a + 0*0052) x 0-8949 Pentosane = (a + 0-0052) x 0-8824 



According to Boddener and Tollens * a considerable saving 

 in time may be effected by precipitating the phloroglucide in 

 hot solution, i.e. between 80 and 85. The reaction then 

 takes place according to the equation 



C 6 H 4 2 + C 8 H 6 3 = C U H 4 2 + 3 H 2 



so that tlje precipitate actually weighs less than the one pro- 

 duced in the cold ; the precipitation is, however, complete in 

 from one and a half to two hours. The weight of furfural 

 corresponding to the precipitate so obtained may be calculated 

 by adding *ooi (to allow for the phloroglucide remaining in 

 solution) and multiplying the resulting figure by 0^571 . The 

 number so obtained if multiplied by I -935 gives the correspond- 

 ing amount of pentose or if multiplied by 1 703 gives the amount 

 of pentosane. The method is, however, not suitable if it is 

 desired to estimate the methyl-pentosanes as distinct from the 

 pentosanes, in which case Krober's method as modified by 

 Ellettf and Mayer J should be employed. 



Estimation of Glucose. 



Several methods have been devised for employing Fehl- 

 ing's solution for the gravimetric estimation of sugars, each 

 method only giving accurate results if careful attention is given 

 to all details of manipulation. 



According to Allihn two solutions are required : 



A. Prepared by dissolving 34-6 grams of pure crystallized 

 copper sulphate in water and making up the volume to 500 c.c. 



B. Containing 173 grams of Rochelle salt and 125 grams 

 of potassium hydroxide dissolved in water and made up 

 to 500 c.c. 



Thirty c.c. of each of the two solutions A and B are mixed 

 together in a 300 c.c. beaker, diluted with 60 c.c. of water and 



* Boddener and Tollens:' " J. Landw.," 1910, 58, 232. 

 tEllett: " J. Landw.," 1905, 53, 13. 

 % Mayer: "J. Landw.," 1907, 55, 261. 



Soxhlet : " J. f. prakt. Chem.," 1880, [2], 21, 227 ; Allihn : id., 1880, [a], 

 22, 63 ; Pfliiger : " Pfliiger's Archiv," 1898, 69, 399. 



