GRAVIMETRIC ESTIMATIONS 83 



2. The Pavy solution must be boiling throughout the 

 titration, else air will enter the flask, owing to the lowered 

 temperature, and the solution of cuprous oxide will be oxi- 

 dized. 



B. GRAVIMETRIC METHODS. 



Estimation of Pentoses. 



These compounds may, according to Neuberg,* be esti- 

 mated by conversion into their diphenylhydrazones, 



a,H,„04NN(C,;Hfl)2 



and subsequent weighing ; this method is, however, not always 

 suitable. 



The ease with which furfural can be produced from pen- 

 toses has led to the following method of estimation, which is 

 due to Krober j- : — 



A weighed quantity of substance + (usually about 5 grams) 

 is placed in a 300 c.c. flask provided with a cork bored with 

 two holes, through one of which passes a tap-funnel, and 

 through the other a splash preventor, such as is used in a 

 Kjeldahl distillation. Through the tap-funnel 100 c.c. of 

 hydrochloric acid (sp. gr. i •06, containing about 12 percent 

 HCl) are then added, and the contents of the flask are dis- 

 tilled briskly ; when 30 c.c. have passed over, the distillation is 

 interrupted and the contents of the receiver are poured into a 

 beaker with a 400 c.c. graduation mark ; a fresh quantity of 

 30 c.c. of hydrochloric acid (sp. gr. i -06) is now added through 

 the tap-funnel, and the distillation is continued until 30 c.c. 

 more have distilled over ; the new distillate is again transferred 

 to the beaker, 30 c.c. more acid are added to the flask, and the 

 whole process is repeated ; altogether about a dozen distilla- 

 tions, each lasting ten minutes, are required to carry over the 

 last traces of furfural. In order to ascertain whether the dis- 

 tillate still contains furfural, a drop of the liquid is placed on a 



* Neuberg : " Ber. deut. chem. Gesells.," igoo, 35, 2243 ; see also Mauren- 

 brecher and ToUens : " Ber. deut. chem. Gesells.," 1906, 39, 3578. 



t Krober : "J. Landw.," 1900, 48, 357, and 1901, 49, 7. 



X The amount chosen should be sufficient to produce from -03 to 0-3 gram of 

 phloroglucide. 



6 * 



