3!0 PHYSIOLOGICAL CHEMISTRY. 



percentage of sugar is low, and which usually becomes yellowish 

 upon standing. If the sugar present exceeds 0.06 per cent this 

 precipitate generally forms at or below the boiling-point, whereas 

 if less than 0.06 per cent of sugar is present the precipitate forms 

 more slowly and generally only after the solution has cooled. The 

 greenish precipitate obtained with urines containing small amounts 

 of sugar may be a compound of copper with the sugar or a com- 

 pound of some constituent of the urine with reduced copper oxide 

 instead of being a precipitate of cuprous hydroxide or oxide as is 

 the case when the original Fehling solution is reduced. 



Benedict claims that, whereas the original Fehling test will not 

 serve to detect sugar when present in a concentration of less than 

 o.i per cent that the above modification will serve to detect sugar 

 when present in as small quantity as 0.015-0.02 per cent. The 

 modified solution used in the above test differs from the original in 

 that 100 grams of sodium carbonate is substituted for the 125 

 grams of potassium hydroxide ordinarily used, thus forming a 

 Fehling solution which is considerably less alkaline than the orig- 

 inal. This alteration in the composition of the Fehling solution 

 is of advantage in the detection of sugar in the urine inasmuch as 

 the strong alkalinity of the ordinary Fehling solution has a tendency, 

 when the reagent is boiled with a urine containing a small amount 

 of dextrose, to decompose sufficient of the sugar to render the de- 

 tection of the remaining portion exceedingly difficult by the usual 

 technique. Benedict claims that for this reason the use of his mod- 

 ified solution permits the detection of smaller amounts of sugar than 

 Joes the use of the ordinary Fehling solution. Benedict has fur- 

 ther modified his solution for use in the quantitative determination 

 of sugar (see page 368). 



Second Modification. 1 Very recently Benedict has further modi- 

 fied his solution and has succeeded in obtaining one which does not 

 deteriorate upon long standing. 2 The following is the procedure 



1 Private communication from Dr. S. R. Benedict. 



2 Benedict's new solution has the following composition : 



Cupric sulphate 17.3 gm. 



Sodium citrate 173.0 gm. 



Sodium carbonate (anhydrous) 100.0 gm. 



Distilled water to 1000.0 c.c. 



With the aid of heat dissolve the sodium citrate and carbonate in about 600 

 c.c. of water. Pour (through a folded filter if necessary) into a glass graduate 

 and make up to 850 c.c. Dissolve the cupric sulphate in about 100 c.c. of water 

 and make up to 150 c.c. Pour the carbonate-citrate solution into a large beaker 

 or casserole and add the cupric sulphate solution slowly, with constant stirring. 

 The mixed solution is ready for use, and does not deteriorate upon long standing. 



