This test can be applied fairly simply, and it gives good results in presence of more than 0.5 per cent. of sugar. But at the same time we cannot use it to detect extremely minute quantities of sugar such as normally occur in human urine, because the urine, as we have already remarked, contains a number of substances which reduce alkaline solutions of copper.
The effect of these substances is sometimes such as to produce the illusion that from 0.3 to 0.5 per cent. of saccharine matter is present. (Neumeister.)
In recent times, for qualitative and quantitative investigation of the grape-sugar, much use has been made of graduated fermentation-tubes.
For this experiment we mix about 10 cubic centimetres of urine with a small quantity of yeast of ascertained weight, and fill the fermentation-tube with the mixture. After the lapse of twenty-four hours, during which the whole is kept in an incubator at a temperature of 30° centigrade, all the grape-sugar will be completely fermented.
From the gaseous fermentation-products of the grape-sugar, which rise into the longer branch of the U-tube, and consist of carbonic acid, we detect the presence of sugar in the urine.
By means of the graduation of the longer branch, we can at once read off the percentage of the sugar. This test is sensitive enough to detect 0.05 per cent. of sugar. It is useful first of all to boil the urine to be tested, in order to remove from it the carbonic acid contained in solution. It is also advantageous to acidify the urine, so that the yeast fungus which flourishes more easily in the acid medium may overpower any gas-producing bacteria, and so avoid a false result.
In 1884, E. Fischer discovered phenylhydrazin, and pointed out the fact that it might be used as a valuable reagent for the sugar in urine.
This preparation has the characteristic peculiarity of forming crystalline compounds with aldehydes and ketones. These crystals, in the cases of the different kinds of sugar, which, as is known, represent the aldehydes and ketones, respectively, are needle-shaped, of a yellow color, with difficulty soluble in water, have a high melting-point, and are called glycosazone.
Jaksch used this property of phenylhydrazin for his phenylhydrazin test. For this experiment he dilutes the urine with an equal quantity of distilled water in a test-tube, and adds twice as much phenylhydrazin hydrochloride as can be taken up on the end of a knife, and double that quantity of sodium acetate. The mixture is well shaken together and left from half an hour to an hour in a boiling water-bath.