134 



THE CARBOHYDRATES 



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 i 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. 

 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 oxidized. 



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- 

 ing out the reduction in the presence of potassium thiocyanate 

 whereby the cuprous oxide is converted into an insoluble 



Fig. 3. 



