[2] Benedict's solution has the following composition:
Copper sulphate, 17.8 gm.
Sodium citrate, 178.0 gm.
Sodium carbonate (anhydrous), 100 gm.
Distilled water to 1000 c.c.
Quantitative Sugar Tests.
(1) The Fermentation Test:—The fermentation test is the simplest quantitative test for sugar, and is quite accurate enough for clinical work. It is performed as follows: The specific gravity of the 24° urine is taken, and 100 c.c. of it put into a flask, and a quarter of a yeast cake crumbled up and added to it. The flask is then put in a warm place (at about body temperature) and allowed to remain over night. The next morning a sample of the fermented urine is tested for sugar. If no sugar is present the urine is made up to 100 c.c. (to allow for the water that has evaporated) and the specific gravity taken again. The number of points loss in specific gravity is multiplied by .23, and this gives the percentage of sugar in the urine.
(2) Benedict's Test:—The best quantitative test for dextrose (excepting polariscopic examination, which is too complicated for ordinary work) is Benedict's test.
It is performed as follows: Measure with a pipette 25 c.c. of Benedict's solution into a porcelain dish, add 5 or 10 gm. (approximately) of solid sodic carbonate, heat to boiling, and while boiling, run in the urine until a white precipitate forms.
Then add the urine more slowly until the last trace of blue disappears. The urine should be diluted so that not less than 10 c.c. will be required to give the amount of sugar which the 25 c.c. of reagent is capable of oxidizing.
Calculation: 5, divided by the number of c.c. of urine run in, equals the per cent. of sugar.
Benedict's quantitative solution is prepared as follows: Dissolve 9.0 gm. of copper sulphate in 100 c.c. distilled water. (The copper sulphate must be weighed very accurately.) Dissolve 50 gm. anhydrous sodic carbonate, 100 gm. sodic citrate, and 65 gm. of potassium sulpho cyanate in 250 c.c. of distilled water.
Pour the copper solution slowly into the alkaline citrate solution. Then pour the mixed solution into the flask without loss, and make up to 500 c.c.; 25 c.c. of this solution is reduced by 50 mgm. of dextrose, 52 mgm. of levulose or 67 mgm. of lactose.