ESTIMATION 137 



of Cu, the amount of copper equivalent to each cubic centi- 

 metre of the permanganate may be calculated. Tables have 

 been drawn up giving the copper equivalents of glucose, in- 

 vert sugar, mannose, galactose, sorbose, rhamnose, arabinose, 

 xylose, maltose and lactose ; these may be found in the 

 original paper of Bertrand or in the text-book of " Practical 

 Biological Chemistry," by Bertrand and Thomas.* 



B. GRAVIMETRIC METHODS. 

 Estimation of Pentoses. 



The ease with which furfural can be produced from pen- 

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

 due to Krober t : — 



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 preventer, such as is used in a 

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

 hydrochloric acid (sp. gr. i-o6, containing about 12 per cent 

 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 

 filter paper next to a drop of aniline acetate solution ; § if no 



* Bertrand and Thomas : " Practical Biological Chemistry." Trans- 

 lated by Colwell, London, 1920. 



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

 " Zeit. physiol. Chem.," 1902, 36, 239. 



JThe amount chosen should be sufficient to produce from -03 to 

 0-3 gram of phloroglucide. 



§ This is best prepared, according to ToUens, by shaking up equal 

 volumes of aniline and water in a test tube and adding glacial acetic acid 

 drop by drop until the turbid solution suddenly becomes clear. 



