There are three isomeric conditions of phosphoric acid, identical in composition, but differing in their relation to bases, of which one unites with one equivalent, the second with two eqs., and the third with three; and hence they are distinguished by the names of monobasic, bibasic, and tribasic phosphoric acid; water when combined with these being considered in the state of a base. With these preliminary remarks, I must be content with referring to the U. S. Dispensatory (12th ed.,p. 51), for the chemistry of this acid. Two forms of phosphoric acid are now officinal, the glacial, namely, and the diluted, both of which were introduced into the U. S. Pharmacopoeia, for the first time, at its late revision.

1. Glacial Phosphoric Acid

Acidum Phosphoricum Gla-Ciale. U. S. - Syn. Monobasic Phosphoric Acid. - Metaphosphoric Acid. - Phosphate of Water. This is the first of the three varieties just named, being always combined with one eq. of water, and represented by the formula HO,P05; the anhydrous acid consisting of one eq. of phosphorus and five eqs. of oxygen. It results when the product of the combustion of phosphorus is introduced into water; but is commonly obtained from calcined bones, by treating them with sulphuric acid, which produces a superphosphate of lime, dissolving this out by water, adding carbonate of ammonia, and exposing the resulting phosphate of ammonia first to an evaporating heat, and then to calcination, by which the ammonia is driven off, and the glacial acid is left. (See U. S.D.) This variety, however, of phosphoric acid is generally imported, being seldom made in our chemical laboratories.

The glacial phosphoric acid is a transparent, white or colourless, fusible solid, inodorous, sour to the taste, slowly deliquescent, and soluble in water and alcohol. It is chemically characterized by producing precipitates with soluble salts of lime, baryta, and silver; the precipitate with chloride of barium being redissolved by an excess of the acid; and is distinguished from the other states of the acid by coagulating albumen.

Though capable of producing all the effects of phosphoric acid upon the system, this variety is seldom prescribed, being always first brought to the state of the officinal diluted acid. Indeed, it was introduced into the Pharmacopoeia, as affording a ready means of preparing that officinal.

2. Diluted Phosphoric Acid. Acidum Phosphoricum Dilutum

Acidum Phosphoricum Dilutum. U.S., Br. This is prepared, according to both the United States and British Pharmacopoeias, by carefully heating together phosphorus, nitric acid, and water. The phosphorus is oxidized and converted into phosphoric acid at the expense of the nitric acid; and, having been obtained in a somewhat concentrated liquid form, is afterwards diluted with water to the officinal strength. Our Pharmacopoeia also permits it to be prepared by dissolving a troyounce of the glacial acid in three fluidounces of distilled water, adding a little nitric acid, boiling until the whole of the latter acid is driven off, and then adding distilled water till the diluted acid measures twelve fluidounces and a half. The object of boiling with the nitric acid is to convert the glacial or monobasic into the tribasic, which is the proper officinal acid; the nitric acid appearing to act, in facilitating the change, simply by its presence; as no decomposition takes place. The strength was intended to be as nearly as possible the same as that of the diluted acid, prepared according to the other formula. The sp. gr. of the U. S. diluted acid is 1.056, that of the British officinal preparation 1.08; the latter, therefore, being considerably stronger. This should be recollected in estimating the value of the doses of this acid given by British writers.

Properties

Diluted phosphoric acid is a colourless liquid, without smell, extremely sour, and possessed of strong acid properties. Though much less corrosive, even in a concentrated state, than sulphuric acid, it is yet capable in that state of destroying life. Orfila has seen fatal gastritis produced in a dog by thirty grains of it dissolved in a very little water.

Medical Uses

As a medicine, it has been more used, till of late, on the continent of Europe, particularly Germany, than either in Great Britain or this country. Having never prescribed it, or seen it prescribed, I am not entitled to give an authoritative opinion upon its properties or value as a medicine; but, from all that has been written upon the subject, though some writers claim for it special and extraordinary virtues, there seems to be good reason to believe that it resembles the other acids in its effects, and can do little more than they. It is thought especially to resemble sulphuric acid in tonic virtues, and, though not so energetic, has the advantage that its taste is somewhat more agreeable, and its tendency to produce gastric and intestinal irritation less. It may, therefore, be employed to promote the appetite and invigorate digestion in debilitated states of the system. Scrofulous affections, passive hemorrhages, colliquative sweats, excessive suppuration, low febrile diseases, caries of the bones, ulcerous and eruptive affections with depraved blood, are complaints in which it has been recommended, and in which its tonic properties may have rendered it useful. Its property of dissolving phosphate of lime out of the body has led to its employment in ossification of the heart and blood-vessels, and in cases of phos-phatic deposits in the urine, in the hope that it might dissolve the abnormal bony matter, and calculous formations within the body. In the urinary affection, it may operate beneficially in the same manner as sulphuric acid. It has been supposed to possess the power of greatly reducing vital irritability, and has been employed, in reference to this property, in hysteria and convulsive disorders. On the contrary, from an imagined excitant influence over the generative organs, it has been used in impotence in males. As a local application, it has been especially recommended in caries, in which it corrects the fetor, dissolves and aids in the separation of the dead portions of bone, and otherwise favours the healing process. It is said also to have proved beneficial in offensive cancerous ulcers.

It might be supposed that phosphoric acid would occasionally prove aseful by supplying a deficiency of phosphorus or the phosphates in the tissues. There can be no doubt that it forms with the salifiable bases it meets with in the alimentary canal, as lime, soda, etc., salts which may enter the circulation, and thus produce all the effects which it has of late been somewhat fashionable to ascribe to the phosphates. But when it is considered that there is always more or less of the phosphates in the blood and in the urine, resulting probably from the disintegration of the tissues, and in part, possibly, from the food taken, it is obvious that it is not the presence of the phosphates that is wanted, but the due power of appropriating them. I have never, therefore, been disposed to ascribe much virtue to the phosphates used in medicine, merely with the object of supplying a supposed deficiency of phosphorus in the nervous tissue, or of phosphate of lime in the bones.

The dose for internal use is from ten minims to a fluidrachm, which should he given largely diluted. For external use, the officinal acid may be diluted with nine or ten times its bulk of water.