ACIDUM PHOSPHORICUM DILUTUM - DILUTE PHOSPHORIC ACID, H,PO4,=98.

This acid is rather widely diffused, being found free or combined with alkaline and earthy bases in soils, and in many vegetables and fruits, such as wheat, potatoes, rice, lemons, etc., also in fish, and in the bones, nerves, and flesh of animals, and according to Marcet, more in the lungs than in other parts; also in the urine and other secretions.

Preparation

The officinal (tribasic) acid is prepared by distilling phosphorus with dilute nitric acid by the aid of gentle heat; some of the latter acid passes over in vapor, and therefore the distillate is returned to the retort occasionally in order to prevent loss: it is finally concentrated to a syrupy consistence (heat being used to get rid of nitrous fumes), and the resulting strong phosphoric acid is diluted with water to a sp. gr. of 1.08, which represents about 14 per cent. of acid.

Characters And Tests

Tribasic or orthophosphorous acid is a colorless, inodorous liquid, of acid, not unpleasant taste, and even when concentrated, not corrosive, nor coagulating albumen. It gives with ammo-nio-nitrate of silver, a canary-yellow precipitate of phosphate.

There are two other forms of phosphoric acid, not officinal, but of much interest to the chemist. If heat be applied to the common acid it loses water, and its first anhydride (pyrophosphoric or bibasic acid, H1P2 O7) is produced: by a higher heat, more water is driven off, and its second anhydride, glacial or metaphosphoric or monobasic acid HPO3, is formed. This is in the U. S. Pharmacopoeia, and is a colorless, ice-like, deliquescent solid: it is the only form which coagulates albumen.

All soluble phosphates give a white crystalline precipitate with sulphate of magnesia, after the addition of sal-ammoniac and liquor ammoniae (ammonio-magnesian phosphate, or "triple phosphate" - MgNH1PO4).

Absorption And Elimination

Phosphoric acid is readily absorbed by the stomach. Ordinary doses combine with alkalies - potash or soda salts - probably displacing them from combination with weaker acids, lactic or carbonic, and forming phosphates; after larger or poisonous doses, Hoffmann states that he has found it free in the blood or loosely combined with albumen (Journal de Chemie, June, 1868).

As phosphate it is mainly eliminated in the urine, and Bocker found the excretion of potash phosphate especially increased under its use: some acid may possibly be eliminated in a free state.

Physiological Action

The action of phosphoric acid bears a general resemblance to that of sulphuric acid, but in medicinal doses it is less liable to irritate the stomach or interfere with digestion, and it exerts a more stimulating effect on the general system: it has a more pleasant taste than the other inorganic acids. The pharmacopoeial solution does not coagulate albuminous tissues, and like oxalic and tartaric acid only coagulates egg-albumen after addition of chloride of sodium or other neu-tral salt.

Circulatory System

The effect of moderate doses of phosphoric acid is stimulant, but of large doses, especially when injected into the blood-current, depressant. Two cub. centim. of a four per cent. solution given to a frog, increased the pulse-frequency, and the direct application of acid to the frog's heart at first strengthened though it afterward weakened the contractions; after death, the heart-muscle was non-excitable (Munk and Leyden). In warm-blooded animals, after the subcutaneous injection of about 8 grammes, slowness, weakness, and irregularity of heart-beat occurred, with retarded respiration, lowered temperature, prostration, and death (Meyer).

After injections of phosphoric acid into the jugular vein, the blood-pressure and the pulse-frequency are lowered, although after small quantities they quickly rise again. Pavy found that he could inject 8 or 10 dr. of the pharmacopoeial solution into the jugular of a dog without causing death, and if, in any animal, the maximum amount compatible with life was injected, the urine and the arterial blood became highly charged with sugar ("Guy's Hospital Reports," vol. vii., 1861). We may connect this result with the fact that phosphoric acid acts even more powerfully than hydrochloric in diminishing the alkalinity of blood (Walter), while, on the other hand, injections of soda prevent the production of artificial diabetes; but the full bearing of such facts is not yet known. Injections of acid into the carotid caused primary slowing of pulse with secondary quickening before death, strong inspiratory cramp, convulsions and coma (quoted by Husemann). After death from excessive quantities, ecchymoses were almost always found in the lungs, and the blood was altered, being dark but fluid, and not easily coagulable, sometimes gelatinous. The effect on the blood is not always the same: thus, Pavy in one experiment found the "large venous trunks in the liver plugged with coagulated blood" after an injection of 30 dr. of acid into the duodenum; and Gubler says, "introduced into the veins of animals, phosphoric acid coagulates the blood and causes death in a few minutes:" this depends on the dose and concentration. Neumann states that the corpuscles are not destroyed by the acid, but may be much altered in form and vital properties.

The action upon man is of more practical interest, but very few investigations have been made with phosphoric acid. Bobrick records a rise of the pulse from 70 to 90 beats per minute, but in the course of an hour it fell to 66; this was after a dose of 1/2 oz. A rigor also occurred, the cause of which is not easy to trace, but it was followed by a pleasant sensation of warmth. Dr. J. B. Andrews (N. Y.) administered doses of from 1 to 3 dr., and investigated the effect by means of sphygmograms taken at intervals of from fifteen minutes to one hour. He says, "Within the first interval there is an increase in the force of the pulsations, though there is little change in the number during the whole time of experimentation. The increase is most marked after the lapse of from one to two hours, and it is not till after several hours that the pulse returns to its normal condition. The first experiment I made upon myself, beginning with 20 drops, and continuing the use of the remedy in increased doses till the amount of 4 dr. was reached. The sensations experienced from 40 min. to 3 dr. were those of moderate alcoholic stimulation, slight pain through the frontal region, and a buoyancy and lightness of feeling rather agreeable. ... In the pulse-traces, additional force is manifest in the heart's action in all cases, and in the general appearance of weakly persons placed on acid treatment the same fact is apparent - the congestion of the extremities and lips has soon given place to a more natural color" (American Journal Insanity, October, 1869).