GRAVIMETRIC ESTIMATIONS 101 



B. GRAVIMETRIC METHODS. 

 Estimation of Pentoses. 



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

 mated by conversion into their diphenylhydrazones, 



C 5 H 10 4 NN(C 6 H 5 ) 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 f : 



A weighed quantity of substance J (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. 1*06, containing about 12 percent 

 HC1) 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.," 1900, 35, 2243; see also Mauren- 

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



f Krober : " J. Landw.," igoo, 48, 357, and igoi, 49, 7. Tollens, " Z. 

 physiol. Chem.," igoa, 36, 23g. 



+ The amount chosen should be sufficient to produce from '03 to '03 gram of 

 phloroglucide. 



