142 THE CARBOHYDRATES. [CH. V. 



hydrochloric acid. The solution thus obtained has invariably 

 a brownish-red colour, due to the presence of some ferric salt. Add 

 zinc dust, in small portions at a time, till the fluid is just colourless. 

 On standing for some time the red colour reappears, and must be 

 removed again by a trace of zinc dust. But the delicacy of the 

 indicator is impaired by being decolourised several times. When 

 this indicator is treated with a cupric salt, the colourless ferrous 

 thiocyanate is oxidised to the red ferric thiocyanate. 



Method of analysis. 10 cc. of Fehling's solution and about 

 30 cc. of water are boiled in a flask and the sugar solution is run in 

 from a burette as described above in Fehling's method. The 

 indicator is not used till the blue colour has nearly disappeared. 



Then place a drop of the indicator on a white slab. Transfer 

 a drop of the mixture from the flask to the middle of the drop of 

 the indicator as rapidly as possible by means of a glass tube. If 

 a red colour appears immediately on touching the drop the reduction 

 is not completed. More sugar must be added and a fresh drop of the 

 indicator used as before till no colour or only a faint tinge of red is 

 obtained. If less than 5 cc. of the sugar solution are necessary to 

 complete the reaction, the solution must be diluted till about 10 cc. 

 are required, as described above in Fehling's method. 



Special precautions. Use a glass tube, not a rod, for transferring 

 the drop. 



Do not put your finger on the top of the tube. Dip it in the 

 flask and transfer it immediately to the indicator. The flask may 

 be taken off the boil for an instant while this is done. 



Do not stir the drops on the slab. 



Wash the tube before using it again. 



Calculation of results. This is the same as in Fehling's method. 



163. The estimation of cane sugar by Benedict's method. 



Measure 50 cc. of the solution with a pipette into a flask. Add 

 10 cc. of N. hydrochloric acid. Boil over a free flame and keep 

 the mixture very gently boiling for three minutes. Cool under the 

 tap, neutralise by the addition of 10 cc. of N. sodium hydroxide. 

 Transfer quantitatively to a 100 cc. volumetric flask and make up 



