132 THE CARBOHYDRATES. [CH. V. 



the exact weight to a milligramme. Add about 50 cc. of distilled water, to 

 which about 3 cc. of pure concentrated sulphuric acid have just previously 

 been added. Warm on a water bath until the solid has completely dissolved. 

 Titrate the warm solution with the permanganate. This must be run in very 

 slowly at first, further additions not being made until the colour has com- 

 pletely faded. The end point is reached when a faint rose colour persists for at 

 least a minute. If A be the weight of ammonium oxalate taken, and P the 

 volume of permanganate required, then i cc. of permanganate corresponds to 



8 95^x A = T mg. copper. 



It is convenient to have T = 10. If T be greater than 10, add 100 

 (T - io)cc. of water to i litre of the solution. If the titration has been con- 

 ducted accurately, i cc. of the permanganate corresponds to i mg. copper. 



The calculation is based on the following equations : 



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



10 FeSO 4 + 2 KMnO 4 + 8 H 2 SO 4 = 5 Feg(SO 4 )a + ICjSC^ + 2 MnSO 4 + 8 H 2 O. 



5 C 2 H 2 O 4 + 2 KMnO 4 + 3 H 2 SO 4 = KjSC^ + 2 MnSO 4 + 8 H 2 O + 10 CO 2 . 



So i mol. of oxalic acid or of ammonium oxalate (C 2 O 4 N 2 H 8 .H 2 O) requires the 

 same amount of permanganate as 2 Fe, which corresponds to 2 Cu. 



2 x 63-6 x A 



So P cc. of permanganate = - = 0-8951 x A gm. Cu. 



1 42* i 



Method. Measure 50 cc. of the copper solution into a 150 cc. 

 flask of " Duro " glass. Add two or three small pieces of broken por- 

 celain to prevent subsequent bumping. Boil by heating on a gauze 

 with a Bunsen. As soon as the solution has commenced to boil run 

 in exactly 10 cc. of the sugar solution, which should be between 

 0-2 and 0-9 per cent, of glucose (see note i). Note the exact time 

 when the boiling recommences. The flame should be moderately 

 high at first, but as soon as the solution recommences boiling after 

 the addition of the glucose, it should be lowered so that it just main- 

 tains gentle boiling. After exactly ten minutes' boiling, stop the 

 reduction by immersing the flask .in cold water. Then cool 

 thoroughly under the tap. 



Filtration of the cuprous oxide. This is done through asbestos 

 by means of a Gooch crucible of 25 to 50 cc. capacity (see fig. 33), or, 

 better, through an asbestos mat supported on a small (15 mm. 

 diam.) perforated porcelain plate, resting in a conical funnel that 

 passes through a rubber stopper fitting the neck of a filtering flask. 

 The preparation of the mat and the subsequent filtration is much 

 facilitated by use of the special pump connexions shewn in fig. 9, p. 74. 

 The mat is prepared as follows : the suspension of well-washed 



