Chemical Reaction of Urine. The liquid is tested by litmus paper, red and blue, weakly impregnated. The normal reaction is determined by the food: the urine of carnivora and sucking herbivora is acid turning blue litmus red: the urine of vegetable feeders is alkaline turning reds blue. In the horse the alkalinity is mainly due to excess of lime bicarbonate, passing, with standing, into lime carbonate, the carbon dioxide being derived from organic acids (lactic, malic, citric, etc.), by oxidation. The hippurates are also alkaline in reaction. In dogs the acidity is due to lime and soda phosphates, sulphates, urates and oxalates.
Pathologically we find the urine strongly alkaline from the evolution of ammonia from urea, in fermentations occurring with prolonged retention in the bladder or in cystitis. The urine is acid even in herbivora in all fevers in which appetite is lost or seriously impaired, and in which the metabolism is excessive.
Chemical Changes in the Urine in Disease. Sodium Chloride, is present in large amount in health (horse 25 to 35 grammes, dog 0.25 to 5 grammes daily) is diminished in fever, anæmia, visceral and exudative inflammations. It is increased during the absorption of false membranes and exudates. It is thrown down by adding solution of nitrate of silver, the curdy white precipitate being insoluble in nitric acid.
Phosphates of lime, soda, potash and, scantily, of magnesia are normally present (horse 0.08 to 0.60 gramme phosphoric acid daily) and are present in excess in digestive disorders and in malnutrition of bones (rachitis, osteoporosis and rheumatoid arthritis). The alkaline phosphates are very soluble and never precipitated. Earthy phosphates dissolve in acid urine, but are precipitated from alkaline. To a little of the urine add a few drops of acetic acid, followed by a few drops of uranium acetate. A yellow precipitate of uranium and ammonium double phosphate is thrown down.
Indican (C8H7NSO4) is formed from indol which passes successively through the forms of indoxyle and indoxylid potassio-sulphate. This is normally present in the urine, the horse excreting 1 to 2 grammes daily, the dog 0.15 gramme. It is present in excess in intestinal indigestions, constituting indicanuria. It is tested by adding a drop of muriatic acid and one of a solution of chloride of lime to the urine, when it will show a blue ring, the depth of which indicates the relative amount.
Urea (CON2H4) the principal waste product of nitrogenous matter, is always present in considerable amount. The sound horse may eliminate 100 to 200 grammes daily, the dog 5 to 180 grammes. It is present in excess in all fevers and inflammations unless urination is suspended or impaired, in cryptogamic diuresis, in mellituria, uræmia, nephritis and cystitis. Test: The addition to a filtered solution of urine, freed from phosphates, of solution of acid nitrate of mercury, precipitates it as nitrate of urea. A simpler test is to add to a drop or two of urine on a glass slide a drop of nitric acid and heat gently. The nitrate of urea is precipitated in the characteristic rhombic or hexagonal crystals as seen under the microscope. Heat urea crystals in a test tube: biuret is formed and ammonia escapes. Add a trace of a copper sulphate solution and a few drops of a 20 per cent. solution of caustic potash: a rose-red color is produced—the biuret reaction.
Uric Acid (C5H4N4O3). Traces only of this are found in the normal herbivorous urine, yet it is more abundant when on a full dry grain diet, on milk (suckling) or on animal food. The dog kept on animal food has a large amount.
Pathologically it is produced in the dog and even in the horse in fever, overwork and starvation, the animal living on his own tissues. Interference with oxidation in the lungs seems to produce it as an arrest in the transformation of albuminoids to urea. The neutral urate of soda remains in solution: the acid urate of soda is precipitated. Test: To the urine add one-fourth its volume of muriatic acid and set aside for 24 hours in a cool place. On the bottom and sides of the glass and on the surface of the liquid will be found the yellowish red acicular crystals of uric acid.
Hippuric Acid (C9H9NO3) is normally present in all urine, but is especially abundant in that of herbivora. The horse eliminates 60 to 160 grammes daily. It has been found to be increased by feeding on dandelion, carrots, clover, asparagus, apples, plums, benzoic acid, oil of bitter almonds, toluol, cinnamic or kinic acid. It is absent in sucking calves, and horses fed on grain devoid of husk. Pathologically it is increased in hyperthermia, icterus, some liver diseases and diseased kidneys. Test: Precipitate any albumen by nitric acid and boiling, then add hydrochloric acid which precipitates the hippuric acid in long needle-like crystals. Heated in a small glass tube it forms an oily liquid, and heated to redness gives off an odor of hydrocyanic acid (nitro-benzol) and carbon is left. This distinguishes alike from uric acid and benzoic.
Phenol is produced by intestinal fermentation. The horse normally excretes about 3 grammes daily. Pathologically it appears in excess in indigestions, abscesses, softened discharging tubercle, pyæmia, and septicæmia. Test: Dilute solutions of ferric salts give a blue coloration.