222 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(a) Preliminary Rough Estimation. 



10 cc. of Fehling's solution (i.e. 5 c.c. of each) are measured out 

 Avith a pipette into a porcelain basin or small flask, diluted with about 

 -40 c.c. of water and raised to the boiling-point. 



The sugar solution is run in from a burette, I c.c. at a time, whilst 

 the Fehling's solution is kept gently boiling the whole time. 



The reduction must be allowed to complete itself before adding afresh 

 quantity of the sugar solution. 



It is noted when the blue colour of the solution has entirely dis- 

 appeared. The solution may become slightly yellow, due to the action 

 ^of the alkali of the Fehling's solution upon excess of the sugar solution. 



An idea of how much sugar is present in the solution is thus ob- 

 tained 



(b) Dilution or Concentration of the Sugar Solution. 



Since the method is only accurate if the concentration of the 

 sugar is between 0-5 and I per cent, the sugar solution must be diluted 

 or concentrated. 



It is best to have as nearly as possible 10 c.c. of the diluted or con* 

 centrated sugar solution = 10 c.c. of Fehling's solution. If less than 

 10 ac. of the glucose solution have been used, a known volume of the 

 solution is diluted ; if more, a known volume is concentrated to a smaller 

 volume, e.g. 100 c.c. to 30 c.c. 



Suppose 3 c.c. of the sugar solution were sufficient; then 3 c.c. 

 should be diluted to 10 c.c. 



It is more convenient to dilute a larger quantity : 30 c.c. are 



measured out with a pipette into a 100 c.c. measuring flask, the flask is 



; filled to the mark with water and the contents are mixed : or 30 c.c. 



;are measured into a dry flask and 70 c.c. of water are added with 



a clean pipette. 



The burette is carefully rinsed out with the diluted sugar solution 

 and the final titration carried out. 



