Urea can be estimated volumetrically by the method of Liebig, which depends on the power of mercuric nitrate to give a precipitate with it. The sulphates and phosphates must be first removed by the addition of 40 cc. of a mixture of 1 volume saturated barium nitrate and 2 volumes saturated solution of caustic baryta, to 40 cc. of urine. This is filtered, and from the filtrate an amount corresponding to 10 cc. urine is taken. Into this known volume of urine a standard solution of mercuric nitrate (of which 1 cc. corresponds to 1 centigramme of urea) is dropped until a sample drop of the liquid, mingled on a watch glass with a drop of concentrated sodium carbonate solution, gives a yellow color, which indicates that some free mercuric nitrate is present. «For every cubic centimetre of the standard mercuric solution used, there is one centigramme of urea in the sample of urine; a reduction of 2 cc. should be made from the mercuric solution used in the experiment, on account of the chlorides, which are present in tolerably constant amount.
Another simple and more accurate method consists in mixing known quantities of urine and sodium hypobromite (NaBrO) with excess of caustic soda. The urea is decomposed in the presence of this salt, and free nitrogen evolved - CON2H1 + 3(NaBrO) + 2(NaOH) = 3NaBr + Na2C03 + 3H20 + 2N.
The quantity of urea may be determined by ascertaining the volume of nitrogen, which can be measured directly in a graduated tube. 37.5 cc. of N represents O.I gramme of urea at ordinary temperature and pressure.
Uric acid, of which the formula is C5H1N403 or C3H203 (NH.CN)2, is only present in extremely small quantities in the normal urine of mammalia, but in birds, reptiles and insects it forms the chief ingredient of the renal secretion. It is sparingly soluble in water, and insoluble in alcohol and ether. However, in solutions of the neutral phosphates and carbonates of the alkalies it combines with some of the base, so as to form acid salts, and at the same time converts the neutral into acid phosphates, to which, as has been already stated, the urine owes its acid reaction. These salts are more soluble in warm than in cold water, and hence generally fall as a sediment when the urine cools. Uric acid is readily converted into urea by oxidation, and is probably one of the steps in the formation of urea generally occurring in the body during the gradual oxidation of the proteid bodies.
The presence of uric acid may be recognized by the murexide test. The substance to be tested is gently heated in a flat capsule with some nitric acid. A decomposition occurs, N and C02 going off, urea and alloxan remaining as a layer of yellow fluid. If this be cautiously evaporated, and a drop of ammonia added, a striking purple red color is produced, which the addition of potash turns violet.
The amount of uric acid normally follows pretty closely the variations in urea, but is usually only about 8 grains (.5 gramme) per diem. In certain diseases the quantity may be much increased. For the quantitative estimation, which is seldom decided by the practitioner, the student must consult the text-books of physiological chemistry.
Kreatinin (C4H7N30) is always present in urine, probably being formed from kreatin by the loss of one molecule of water. About 15 grains (1 gramme) is excreted per diem.
Xanthin (C5H1N402) also occurs in urine, but in extremely small quantities.
Hippuric acid (C9H9N03) is a normal constituent of human urine, occurring, however, in very small quantities. On the other hand, it is one of the most important nitrogenous constituents of the urine of the herbivora, where it takes the place of uric acid. Its presence depends on the existence of certain ingredients (benzoic acid, etc.) in the food, which are capable of combining with glycin, and forming a conjugated acid, a molecule of water being formed at the same time, thus -
C7H602 + C2H5N02 == C9H9N03 + H20.
The amount of hippuric acid increases with increased consumption of vegetable food, in the cellulose of which the materials exist that are required for its formation. The union of glycin and benzoic acid may take place in the liver, for, after removal of that organ, benzoic acid injected into the veins appears unchanged in the urine; but the extirpated kidney is also said to be capable of effecting this synthesis.
Oxalic acid (C2H204) occurs often, but not constantly, in the urine. It is generally united with lime. It is said to appear in greater quantity, together with an excess of uric acid, after meals, and therefore to be related to the production of the latter in the body; but it probably is chiefly derived from oxalates contained in some materials taken with the food.
It appears probable that the color of the urine depends on the presence of small quantities of distinct substances which have different origins in the body. Three such have been described, and may be taken provisionally to represent our knowledge of the subject: -
1. Urobilin, which is an outcome of the coloring matter of the bile, and therefore a remote derivative of the coloring matter of the blood, is frequently present in the urine. It is probably the same as hydrobilirubin, some of which is occasionally absorbed from the intestinal tract and eliminated by the kidneys.
2. Urochrome is said to be the special pigment of the urine. It oxidizes on exposure, forming a reddish substance that gives the dark color to some urinary sediments (Uroerythrin).
3. A certain material (Indican) capable of producing Indigo, is commonly present in the urine of man, and in greater quantity in that of some animals, particularly the horse. It is supposed to be formed from the indol that arises from the putrefactive changes consequent on the pancreatic digestion. The indol is absorbed and unites with sulphuric acid to form Indican, which is a yellow substance. Under certain conditions it can be converted by oxidation into indigo-blue.
The urine is the great outlet for all inorganic salts. The most important of these are -
Common salt (NaCl), of which a very variable but always considerable amount passes away in the urine. The average quantity excreted per diem may be said to be about half an oz. (15 grammes). It depends greatly on the quantity taken with the food, and falls during starvation, but does not completely disappear. It is said that if absolutely no common salt be taken with the food the quantity of NaCl excreted diminishes greatly, and albumin appears in the urine about the third day. The amount of salt eliminated follows, with striking accuracy, the changes that take place, at different times and under different circumstances, in the quantity of urea excreted. These facts seem to indicate that there is some relationship between the secretion of the two bodies, or that sodium chloride participates in the chemical changes of the nitrogenous tissues. In many diseases there occur variations in the quantity of common salt in the urine which can hardly be explained by the change in or absence of food.
About 60 grains (3 to 4 grammes) of phosphoric acid is excreted daily in the urine, being combined with alkalies to form salts, viz., potassium, sodium, calcium, and magnesium phosphates.
Nearly 40 grains (2 to 3 grammes) of sulphuric acid, as sulphates of alkalies, are daily got rid of in the urine.
The acid comes partly from the food, but chiefly from the oxidation of the sulphur contained in the proteids of the tissues.
A considerable quantity ofpotassium, sodium, calcium, and magnesium, combined as already mentioned, or with chlorine, is contained in the urine.
Small traces of iron are also always present in the urine.
The urine also contains free C02, N, and some O. 1oo volumes of gas pumped out of fresh urine have been found to consist of -
C02 = 65.40 per cent. N = 31.86 " O = 2.74 "