CARBOHYDRATES AND RELATED BODIES. 37 



strong and must be diluted with at least an equal volume of water before 

 beginning another test. If, on the other hand, the liquid is still blue, add 

 2 cc. more of the sugar solution, boil again for two minutes and allow 

 to settle. If the color is now yellow an approximate value for the amount 

 of sugar in the solution becomes known, but if still blue, the operation of 

 adding solution and boiling must be continued until, after settling, a yellow 

 color appears. Approximately 250 mg. of glucose is required to reduce the 

 Fehling solution taken, and this must be contained in the sugar solution 

 added. From this preliminary experiment calculate the amount of sugar 

 present in each cubic centimeter. 



Ex. With the data obtained in the above experiment as a basis, make 

 now a new sugar solution, having a strength of about I per cent. Measure 

 out 50 cc. of the Fehling's solution, heat to boiling and run the new sugar 

 solution from the burette as before, the first addition being about 20 cc. 

 Boil and note the color after settling and then cautiously continue the 

 addition of sugar solution, a few tenths of a cc. at a time, boiling after 

 each addition, until the blue color gives place to a yellowish green and 

 then, by the addition of a drop or two, to a pale yellow. 



Sometimes the final disappearance of the copper from the solution is 

 determined by filtering a few drops through a very small filter and add- 

 ing a drop of acetic acid and a drop of ferrocyanide solution to the filtrate, 

 when the characteristic reddish color is given if a trace of copper is pres- 

 ent. 



To determine cane sugar by the Fehling's solution it must first be con- 

 verted or " inverted " into a mixture of glucose and fructose. If the sugar 

 is in the dry condition the inversion can be accomplished as follows : 

 Weigh out 9.5 gm., dissolve in 700 cc. of water, add 20 cc. of normal 

 hydrochloric acid and heat for 30 minutes on the water-bath. Then neu- 

 tralize with 20 cc. of normal sodium hydroxide solution and make up to 

 1000 cc. on cooling. This gives now a i per cent solution, which is em- 

 ployed as given for glucose, using the factor 4.94 instead of 4.74 as the 

 amount of sugar oxidized by each cc. of the copper solution. On comple- 

 tion of the experiment calculate 95 parts of cane sugar for each 100 parts 

 of invert sugar found. 



The attention of the student is directed to the fact that malt sugar and 

 milk sugar may be determined by the aid of Fehling's solution without 

 previous inversion. This should be verified by experiment. 



Method by Use of Ammoniacal Copper Solutions. The 



determination of glucose in pure aqueous solution by the above 

 method is simple and accurate, but in liquids containing other 

 organic matters the precipitate sometimes fails to settle read- 

 ily, so that the recognition of the end point is difficult. This 

 is often the case with urine and other physiologically important 

 liquids. Advantage may be taken of the fact that cuprous 



