VOLUMETRIC ESTIMATIONS 



99 



In making up the mixture B great accuracy is not essential. 

 For titration 25 c.c. of A (very accurately measured) are 

 mixed with 25 c.c. of B. The complete reduction of 50 c.c. 

 of Pavy's solution is effected by "025 gram of glucose. 



Pavy's solution may also be prepared from Fehling's 

 solution as follows : 120 c.c. of Fehling's are mixed with 

 300 c.c. of strong ammonia ('880) and 400 c.c. of 12 per cent 

 potash solution. The mixture is then made up with distilled 

 water to one litre. 



Method. Fit a 250 c.c. flask with a well-fitting cork bored 

 with two holes, one to contain an 

 outlet tube and the other the nozzle 

 of the burette. Pour into the flask 

 50 c.c. of Pavy's solution and 50 c.c. 

 of distilled water ; mix thoroughly 

 and introduce a little powdered glass. 

 Dilute the sugar solution with a 10 

 per cent solution of ammonia, in order 

 that 50 c.c. shall be about equivalent 

 to 50 c.c. of the Pavy solution. Bring 

 the Pavy solution to the boil by 

 means of a small flame, and run in 

 the sugar solution I c.c. at a time. 

 Having thus roughly ascertained the 

 amount of sugar required, accurate 

 readings are to be obtained by running 

 in nearly all the requisite sugar at 

 once, and then drop by drop until the 

 end point is reached. 



FIG. 3. 



i The following precautions are very important : 



1. The operation must be carried out rapidly, else all the 

 ammonia is driven off and the cuprous oxide is precipitated. 



2. The Pavy solution must be boiling throughout the 

 titration, else air will enter the flask, owing to the lowered 

 temperature, and the solution of cuprous oxide will be oxi- 

 dized. 



III. ESTIMATION BY MEANS OF BENEDICT'S SOLUTION. 



In this method the difficulty of the red precipitate of 

 cuprous oxide obscuring the end point is overcome by carry- 



7* 



