CH. V.I FEHLING'S METHOD. 141 



62. Fehling's method. 



Preparation of solution. See Ex. 97, p. 106. 



Method. With a pipette measure 10 cc. of freshly prepared 

 Fehling's solution into a small flask. Add 40 cc. of distilled water, 

 heat the mixture till it boils and keep it boiling the whole time. Run 

 in the sugar solution from a burette, 0-5 to I cc. at a time, allowing 

 the mixture to boil for about 15 sees, between each addition. A red 

 precipitate of cuprous oxide forms and the intensity of the blue in 

 the supernatant fluid decreases. Continue to add the sugar till this 

 is completely removed. This is best determined by holding the 

 flask by the rim at the neck and viewing it by transmitted light. If 

 an excess of sugar be added a yellow or brown colour appears due to 

 the formation of caramel by the action of the alkali on the sugar. 



If less than 5 cc. of the sugar are used, the solution must be 

 diluted till about 10 cc. are necessary. Thus if 2-5 cc. are used 

 in the first rough titration, the sugar should be diluted I in 4, by 

 taking 25 cc. and adding water till the volume of the solution is 

 100 cc. The burette is washed out and filled with this diluted 

 solution and the process repeated. But this time run in nearly the 

 whole of the sugar solution judged necessary at such a rate that the 

 mixture does not go off the boil. Then add o-i to 0-2 cc. at a time 

 till the reduction is complete. This titration should be repeated at 

 least once more. 



Calculation. 10 cc. of Fehling's solution are reduced by 0-05 gram, glucose. 



Example. 1-5 cc. of the original solution necessary. 



Sugar diluted i in 7 (10 cc. sugar made up to 70 cc.) 

 10-2 diluted sugar solution required for 10 cc. Fehling's. 

 10-2 cc. dil. sugar = -05 gm. glucose. 



05 x 100 . 

 ioo cc. = I0 . 2 



TOO cc. original sugar = ' 5 x IO x 7 



10-2 



= 3'43 per cent. 



162 A. Ling's method. 



Preparation of the indicator. Dissolve 1-5 gram, ammonium 

 thiocyanate and i gram, ferrous ammonium sulphate in 10 cc. water 

 at about 45 C. and cool at once. Add 2*5 cc. of concentrated 



