CANE SUGAR. 



The following method of filtration, is not inferior to the above : a number 

 of filters are folded from papers about 3 cm. in diameter; one of these is 

 grasped with the point of a small forceps and held in the material to be 

 tested ; a clear solution passes into the interior of the filter from which one 

 drop is removed with a pipette such as is used for filling fountain pens. 

 Ling and Eendle 10 use an indicator prepared as follows : 

 One grm. each of ferrous ammonium sulphate and ammonium sulpho- 

 cyanide are dissolved in 20 c.c. of water, to which is added 5 c.c. of hydrochloric 

 acid ; if necessary this is decolourized with a little zinc dust and the solution 

 preserved out of contact with air ; a cupric salt gives an intense red 

 colouration with this indicator ; the indicator becomes coloured after standing, 

 but may be decolourized several times by the addition of zinc dust, its 

 delicacy eventually becoming impaired. 



Pavy's Method. 11 In Pavy's modi- 

 fication of Fehling's method, ammonia 

 is added with the object of keeping in 

 solution the cuprous oxide. The solu- 

 tion is prepared by adding to 120 c.c. of 

 Fehling's solution (10 c.c. = '05 grm. 

 glucose) 300 c.c. of ammonia -880, and 

 making up to 1 litre ; 50 c.c. of this 

 solution are reduced by -025 grm. of 

 glucose. The determination is made as 

 follows: The apparatus (Fig. 258\ 

 consists of a flask holding about 150 c.c , 

 into the neck of which is fitted a rubber 

 stopper carrying three tubes, one of 

 which communicates with a reservoir of 

 ammonia by a rubber tube carrying a 

 screw clamp ; a second communicates 

 with a burette containing the glucose 

 solution, the flow of which is controlled with a screw clamp ; the third serves to- 

 carry away and condense under water the escaping fumes of ammonia. The 

 whole apparatus is supported by suitable holders ; 50 c.c. of Pavy's solution are 

 placed in the flask and boiled, ammonia being allowed to drip slowly into the 

 flask. As soon as the air is expelled, indicated by the almost complete absorp- 

 tion of the escaping bubbles, the glucose solution is slowly added and continued 

 until the blue colour is discharged ; the last few drops should be allowed to flow 

 very slowly. The addition of the ammonia must be continued throughout the 

 experiment, and as, notwithstanding the partial expulsion of the air on boiling,, 

 the reduced copper reoxidizes, the experiment should be made as expeditiously as is 

 compatible with accuracy. The above remarks in describing Fehling's method as- 

 regards standardization, conditions of experiment, &c., are equally applicable here. 



FIG. 258. 



466 



