A crystalline precipitate of nitrate of urea may give rise to error if the urine be very concentrated or the experiment conducted in the cold. This may be distinguished by its disappearance on warming or by the microscope. The action of the nitric acid on the coloring matter of the urine, forming a dark band at the point of junction, may obscure the reaction, but with care will not give rise to mistakes.
Another test recently introduced, which presents some advantages over the nitric acid, and is certainly quite as delicate, consists in a saturated solution of common salt in water acidulated with about 5 per cent. of the dilute hydrochloric acid of the Pharmacopoeia. This solution should be used exactly in the manner described for nitric acid. There is no change of color at the line of junction, and no precipitate takes place there except albumen or peptone, or resins when they have been administered. The opaque line of precipitate may, if the amount of albumen present be small, require a short time to form, so that in cases of doubt it is well to allow the test-glass to stand for a few minutes. It will, however, show very distinctly in any cases in which nitric acid shows any precipitate. The line does not, however, increase in thickness and density in proportion to the amount of albumen so exactly as that produced by nitric acid, so that the brine test is not so useful for approximately quantitative use as the nitric acid, although fully as delicate. If it be desired to distinguish peptone from albumen, it may be done by a comparison of this test with the nitric acid, which does not throw down peptone. If a deposit occur, which may consist of resin, the addition of more urine will dissolve it if resin, while albumen will not be affected.
Picric acid is a delicate and often a convenient test. The dry acid may be dissolved in the urine, or a saturated solution used into which the urine may be slowly dropped, each drop making a slight whitish cloud as it slowly falls through the yellow solution.
The iodo-hydrargyrate of potassium is perhaps the most delicate test of all: Potassii iodidi, 3.32 gm.; Hydrarg. bichlor., 1.35 gm.; Acidi acetici, 20 c.c.; Aq. destill. q. s. ut fiat 100 c.c.—Tauret's test. It may be used in the same way as the nitric acid or brine, or simply intermixed. Its only disadvantage is that it throws down alkaloids, but as this will not happen unless the alkaloid be taken in large quantity—as might happen, for instance, in the case of quinine—the chances of error from this source are not very great if this peculiarity be borne in mind.
Ferrocyanide of potassium in an acid solution has recently been proposed as a convenient test. It may be made up into pellets with citric acid or used in the same combination in the form of papers.
The phenic-acid test is prepared as follows:
| Ac. phenic. glacial. (95 per cent.), | drachm ij; |
| Ac. acet. puri., | drachm vij; |
| M. Add liq. potassæ, | ounce ij-drachm vj. |
| Millard. |
This is said to be very delicate, but the writer has no experience with it.
Tungstate of sodium is another recent addition to the list, which it is evident is already long enough for practical purposes.
Several of the tests mentioned have recently been prepared in the form of papers saturated with known quantities of the reagent and dried. They may be carried in the pocket-book and applied at the bedside, if desired, in a test-tube small enough to be very conveniently carried in the vest pocket. The iodo-hydrargyrate is perhaps the most useful. It is the most delicate, and a plan has been proposed for making with it a quantitative estimate of considerable accuracy by means of a standard solution or piece of gray glass adjusted by such a solution, with which the precipitate produced can be compared as to its opacity.