THE CARBOHYDRATES. 297 



complete reduction takes place, the amount of sugar contained in a 

 given specimen of urine can be readily calculated according to the 

 following equation : 



5 

 y : 0.05 : : 100 : x ; and x = , 



y 



in which y indicates the number of cubic centimeters of urine re- 

 quired to reduce the 10 c.c. of Fehling's solution, and x the amount 

 of sugar contained in 100 c.c. of urine. 



As the best results are obtained only if from 5 to 10 c.c. of urine 

 are used in one titration, it is usually necessary to dilute the urine 

 to the required degree ; in the determination of this point the specific 

 gravity may serve as a guide. As a general rule, urines of a specific 

 gravity of 1.030 should be diluted five times, and if the density is 

 still higher ten times. To be certain that the proper degree of dilu- 

 tion has been reached, 5 c.c. of Fehling's solution are treated with 

 1 c.c. of the diluted urine, a little caustic soda solution and distilled 

 water being added to make in all about 25 c.c. This mixture is. 

 thoroughly boiled; if the fluid still remains blue, another 1 c.c. of 

 diluted urine is added, and so on, until the last two tests differ by 

 1 c.c. of urine, the last cubic centimeter added causing a separation 

 of cuprous oxide. In this manner the percentage of sugar may be 

 approximately determined. Albumin, if present, must first be re- 

 moved by boiling. 



10 c.c. of Fehling's solution, diluted with 40 c.c. of water, are 

 placed in a porcelain dish and boiled. While boiling, the diluted 

 urine is added from a burette, 0.5 c.c. at a time, when, as a rule, 

 the precipitated cuprous oxide will rapidly settle, so that gradually 

 a white bottom may be seen through the blue field, the color of 

 which becomes less and less intense upon the further addition of 

 urine until finally the solution is almost colorless. When this point 

 is reached, the urine is added drop by drop until decolorization 

 is complete. The degree of dilution multiplied by 5 and the result 

 divided by the number of cubic centimeters of diluted urine em- 

 ployed will then indicate the percentage-amount of sugar. 



Unfortunately, it is at times difficult to determine exactly the point 

 when all the copper has been reduced i. e., the point at which the 

 blue color has entirely disappeared. As this is approached droplets 

 of the hot mixture may be transferred by means of a glass rod to a 

 piece of filter-paper that has been soaked with a solution of potas- 

 sium ferrocyanide. So long as unreduced copper is present reddish 

 brownish rings result. The solution is carried to the point where 

 this no longer occurs. The immediate result, however, counts ; on 

 prolonged exposure to the air some of the copper in suspension be- 

 comes oxidized and gives rise to the color also. 



To obviate this difficulty it has also been suggested to double the 

 amount of alkali in the Fehling's solution, in which case no separa- 

 tion of cuprous oxide occurs ; the solution is then titrated to the 

 point of decolorization, but remains clear. 



