One of the best methods for the accurate and quick estimation of the amount of sugar in urine is, perhaps, the volumetric, devised by Fehling, who employed a standard copper solution, known as ‘Fehling’s solution,’ of the following composition:
| Sulphate of copper | 901⁄2 | grains. |
| Neutral tartrate of potash | 364 | ” |
| Solution of caustic soda, sp. gr. 1·12 | 4 | fl. oz. |
| Add water to make up exactly | 6 | ” |
Of this solution 200 grains are exactly decomposed by one grain of sugar.
The following is the mode of performing the analysis given by Dr Roberts[249]:—Measure off 200 grains of the above standard solution in a 200-grain tube, pour this into a flask, and add about twice its volume of water; then place over a spirit lamp to boil. While the copper solution is being heated the urine to be analysed should be diluted with water to a known degree. In the case of ordinary diabetic urines the best dilution is one in ten. This is obtained by carefully filling a 6-ounce measure with water to the depth of 41⁄2 ounces, and then adding urine so as to make up exactly 5 ounces. The mixture will then contain exactly one tenth of urine (when the quantity of sugar in the urine is very small, a dilution of one in five, or even the undiluted urine may be employed). The next step is to fill a burette (which must be graduated to grains) with the diluted urine to 0. Then proceed to add it in successive small portions to the boiling copper solution until the blue colour has entirely disappeared. After each fresh addition from the burette, the mixture should be raised to the boiling point, and then allowed to stand a few seconds, so that the precipitated copper may subside, and the observer may see, by holding the flask between the eye and the light, whether the mixture still retains any blue colour.
[249] ‘Urinary and Renal Diseases,’ by Dr W. Roberts.
As soon as the blue colour has disappeared the analysis is complete, and the quantity of diluted urine may be read off. The percentage of sugar in the urine can now be readily calculated. Suppose 125 grains had been added from the burette, this represents one tenth, or 12·5 grains of undiluted urine, and contains exactly one grain of sugar; by dividing 12·5 into 100 the per-centage of sugar is obtained, or 100⁄12·5 = 8; the urine contains 8 per cent. of sugar.
Another process for the quantitative determination of sugar in urine, called by its author, Dr Roberts, “the differential density method,” is based upon the loss of density, experienced by diabetic urine, after all the sugar has been removed by fermentation. Dr Roberts says repeated examples derived from diabetic urine so treated, together with corresponding experiments made with solutions of sugar of known strength in normal urine, and in pure water, as well as theoretical calculation have warranted the conclusion, that the number of degrees of density so lost indicates as many grains of sugar per fluid ounce.
The method, which is extremely simple, is thus performed:—Into a 12-ounce bottle measure 4 fluid ounces of the diabetic urine, and drop into it a piece of fresh German yeast, about as large as a cobnut or walnut; insert a cork in the bottle, and let the cork have a nick cut in the side, to allow of the escape of the carbonic acid. Then fill an ordinary 4-ounce bottle with the same sample of urine, omitting to add any yeast, and cork it in the ordinary manner. Place both bottles in a warm situation, where the temperature is about 80° or 90° Fahr., for twenty or twenty-four hours; at the end of which time, the fermentation being over, the scum will either have cleared off or subsided. The fermented urine is then poured into a proper urine-glass, and its specific gravity ascertained.
The specific gravity of the unfermented companion portion is also taken, and by comparing the two results the loss of density is thus arrived at. Before the respective densities are taken it is best to remove the two samples to a cool place, where they should remain for two or three hours, in order that they may acquire the temperature of the surrounding air.
The two following examples may serve as illustrations of the method.