ESTIMATION OF CARBOHYDRATES 223 



(f) Final Titration. 



10 c.c. of Fehiing's solution are diluted as before and the diluted 

 glucose solution carefully added to the boiling liquid. It is advisable 

 to run in at once a little less than the amount required to decolorise 

 the solution entirely (say 8 c.c.), and then to add cautiously cri to 0-2 

 c.c. at a time until there is complete decolorisation, always allowing time 

 for the reduction to occur. This final titration should be repeated 

 running in practically all the glucose solution necessary at one time, 

 and then completing with O'l c.c. at a time. Suppose icri c.c. were 

 insufficient, but 10-3 c.c. too much, as seen by a faint yellow colora- 

 tion of the solution, then IO'2 c.c. is the proper value. 



Soxhlet carried out altogether 5 or 6 titrations, adding more or 

 less than the exact amount of glucose solution at once, and thus deter- 

 mined the limits of too much and too little until they approached one 

 another and differed by only 0*1 c.c. 



(d) The Determination of the End Point. 



The great difficulty of the estimation is the determination of the 

 end point, i.e. when the blue colour is completely discharged. The 

 eye by itself is not very sensitive, but the first trace of yellow in the 

 solution can generally be seen. When the sugar solution is added 0*1 

 c.c. at a time, this amount, or 0*2 c.c., can be deducted, depending on 

 the observer's judgment. ( 



Lavalle has suggested that the dilution of the Fehiing's solution be done 

 with caustic soda solution instead of with water. The cuprous oxide either 

 settles better or stays in solution, depending on the amount used ; but the 

 result is not so accurate in the presence of excess of caustic soda. 



(e) Use of Indicators, 



(I) Potassium Ferrocyanide. 



Both Fehling and Soxhlet used indicators to determine the end 

 point. A small quantity of the solution is removed and filtered if 

 necessary, or a drop may be taken and tested by adding some acetic 

 acid to acidify it and potassium ferrocyanide. A brown coloration or 

 precipitate of copper ferrocyanide shows that copper is still present in 

 the solution. No colour is formed when the reduction is complete. 



In the case of the estimation of glucose in urine the indicator cannot 

 be used, since the ammonia which is formed dissolves some of the 

 cuprous oxide and a colour is given with the ferrocyanide. 



