94 THE CARBOHYDRATES 



1. Any substances such as tannins which may have the 

 power of reducing the salts used in titration must be removed. 



2. The strength of the sugar solution must be weak, 

 because the reducing power of sugar varies with the concen- 

 tration, hence it is best to titrate a solution of about the same 

 strength as that used for the standardizing of the Fehling's 

 solution. This necessitates preliminary estimation ; should the 

 strength of the solution be much above this point, add a known 

 volume of water until the strength approximates -5 per cent. 



The titration, which should be completed as rapidly as 

 possible in order to avoid reoxidation of the solution by the 

 air, is performed as follows : 



Five c.c. of each of the solutions A and B are placed in a 

 white porcelain basin and 40 c.c. of water added ; the mixture 

 is then boiled. The sugar solution is placed in a burette and 

 is run into the hot copper solution about 3 c.c. at a time ; after 

 each addition the solution is boiled and the precipitate allowed 

 to settle before the next addition is made. When the blue 

 colour has disappeared, the amount of sugar solution used is 

 noted. 



A second titration is then carried out, and all the sugar 

 required, less I c.c., to effect complete reduction, is run in at 

 once ; should this prove too small an amount of sugar, more 

 is added drop by drop until decolorization results. The 

 process is repeated until two readings are obtained which do 

 not differ one from the other by more than 0-2 c.c., the one 

 being a little too high and the other a little too low ; the mean 

 of these gives the correct result. 



The chief difficulty in the titration lies in the detection 

 of the end point ; this may be ascertained by allowing the 

 precipitate to settle, and then tilting the basin so as to view 

 the clear liquid against the white of the dish. But if the 

 observer's colour-sense be not very critical, an error is easily 

 made, hence various methods have been suggested to determine 

 accurately the end point. 



1. Filter off a small quantity of the solution, acidify it 

 with acetic acid and add a little potassium ferrocyanide ; the 

 presence of unreduced copper is indicated by the formation 

 of a brown coloration or precipitate of copper ferrocyanide. 



2. Ling's reagent consists of I gram of ferrous ammonium 

 sulphate and 1-5 gram of ammonium sulphocyanide dissolved 



