Fehling’s Test for Sugar.—Fehling’s alkaline solution and Fehling’s copper solution must be kept in separate bottles until ready for use. Then about 2 c.c. of Fehling’s alkaline solution is poured into a test tube and 2 c.c. of Fehling’s copper solution is added. This is diluted with hydrant water to 8 c.c. Half of this quantity is sufficient for the test. The upper half of the solution is boiled over flame (gently agitated while heating), and while still boiling a few drops of urine are added. If no change appears, it is boiled again and a few drops more of urine are added. If a reddish precipitate appears, sugar is present. The chemical reaction taking place is the reduction of copper sulphate to cuprous oxide. Sometimes a partial reduction occurs when urates are in excess, but once having seen the real reduction, a partial one cannot mislead the examiner.
Haines’s Test.—Pour 1 teaspoonful of Haines’s solution into a test tube and boil gently over a Bunsen burner; add 6 or 8 drops of urine and again heat to boiling. A yellow or red precipitate will indicate the presence of glucose.
QUANTITATIVE TEST FOR SUGAR
Benedict’s Test.[110]—The simple quantitative test for sugar is the one devised by Benedict. This is simpler than the polariscopic examination and better suited for ordinary use.
Place 5 c.c. of Benedict’s quantitative solution in a small dish, add a little less than one-fourth of a teaspoonful of sodium carbonate and one-eighth of a teaspoonful of talcum and add 10 c.c. of water. Dilute urine (1 part urine to 9 parts water) except where the qualitative test showed a low percentage of sugar, that is, when the precipitate turns green instead of yellow, in which case it will be unnecessary to dilute the urine. Place dish over burner and bring the contents to a boil. Pour the urine into a graduated pipette. Now add the urine drop by drop to the contents in the dish until the blue color entirely disappears. This test should be done over several times to assure an accurate calculation. The calculation is made as follows: 5 c.c. of Benedict’s quantitative copper solution are reduced by 0.01 gram of glucose, consequently the quantity of undiluted urine required to reduce 5 c.c. Benedict’s solution contains 0.01 gram of glucose.
| 0.01 | × 100 = per cent. |
| x | |
| x = c.c. of undiluted urine. | |
Example; 1500 c.c. urine in 24 hours. 5 c.c. used to reduce (decolorized) Benedict’s solution.
| 0.01 | × 100 = 0.2 per cent. |
| 5 |
1500 × 0.002 (0.2 per cent.) = 3 grams of sugar in 24 hours.