ESTIMATION OF CARBOHYDRATES 229 



the flame and allowed to stand for about minute so that the cuprous oxide 

 settles. 



The liquid is filtered by suction through an asbestos filter of a special 

 pattern. It is a glass tube about 14 cm. long constricted near the centre ; the 

 upper portion is about 6 cm. long and 1 7 mm. wide ; the lower portion about 

 8 cm. long with a conical bulb just below the upper portion. The upper 

 part contains the asbestos coarse fibres being put near the constriclion, less 

 coarse particles above, and at the top very fine particles. The whole thick- 

 ness of asbestos is about i cm., the upper portion being from 1-2 mm. thick. 

 A filter flask of about 150 c.c. capacity is usually taken to receive the liquid. 



In filtering, as little as possible of the cuprous oxide is allowed to come 

 upon the filter so as to prevent the formation of a. compact mass, which 

 subsequently dissolves with difficulty. The cuprous oxide is washed with a 

 little hot water, allowed to settle, and the water poured off through the filter. 

 It is not necessary to wash the precipitate absolutely free from alkali and 

 Rochelle salt, but this is advisable. The cuprous oxide in the flask is 

 treated with 5, 10, or 20 c.c. of the ferric__sulphate solution; it dissolves 

 giving a green solution. This is poured upon the filter ; the particles of 

 cuprous oxide dissolve more quickly if the top layer be stirred up with a glass 

 rod. The liquid is received in the filter flask which has been rinsed out, or 

 more conveniently in another clean ftlter flask. A few more drops of the 

 ferric solution may be used if necessary. The conical flask is washed out 

 with water and the washings passed through the filter. 



The solution is titrated with the permanganate solution, the colour change 

 from green to rose with i drop excess of permanganate being noted ; another 

 drop usually shows an intense rose colour. The duration of the operation is 

 15-20 minutes. 



After repeated use the upper layer of asbestos becomes dark in colour ; 

 it is removed, dried and calcined, and returned to its position. 



Calculation of the Results, 



The result is calculated from the equations : 



Cu 2 O + Fe 2 (SO 4 ) 3 + H 2 SO 4 = 2CuSO 4 + 2FeSO 4 + H 2 O 



ioFeSO 4 + 2KMnO 4 + 8H 2 SO 4 = 5Fe 2 (SO 4 ) 3 + K 2 SO 4 + 2MnSO 4 + 8H 2 O 



whence 2 atoms of Cu = molecule KMnO 4 



or 63-6 gm. Cu = 3i'6 gm. KMnO 4 



or 2'oi2 gm. Cu = i gm. KMnO 4 



i c.c. of Bertrand's permanganate = '005 gm. KMnO 4 or *oioo6 gm. Cu 

 i c.c. of *iN permanganate = 0-00636 gm. Cu. 



The standardisation of the permanganate against ammonium oxalate 

 C 2 H 2 O 4 , 2NH 3 + H 2 O is carried out by Bertrand by dissolving -250 gm. 

 in about 50-100 c.c. water, adding 1-2 c.c. of sulphuric acid, heating to 60- 

 80 and titrating with the permanganate, of which about 22 c.c. will be 

 required. 



5 C 2 H 2 4 + 2KMnO 4 + 3H 3 SO 4 = ioCO 2 + 2MnSO 4 + K 2 SO 4 + 8H 2 O 

 whence i mol. oxalic acid or am. oxalate (mol. wt. = 142*1) = 2 atoms of Cu. 



The weight of oxalate multiplied by -$ - or 0-895 gives the amount 



142 ' i 



of copper corresponding to the number of c.c. of permanganate used, from 

 which the value for i c.c. permanganate can be calculated. 



The values for glucose are given in a table compiled by Bertrand (see 

 p. 606). 



