Origin and Preparation. Citric acid exists in various acid fruits, as the sour orange, lemon, lime, tamarind, etc.; but it is in the form of lemon-juice that it is most frequently employed. The juice of the lime, which is the fruit of Citrus acris of Miller, may be considered as identical, and used with it indiscriminately.

Lemon juice (Limonis Succus, U. S., Br.) is obtained from the lemon (Limon, U. S.), or fruit of Citrus medica, by simple expression. it is of a sharply acid, but agreeable taste, which, as well as its medical virtues, it owes to the citric acid contained in it. According to the experiments of Dr. H. Bence Jones, it contains about 6 per cent. of the pure anhydrous acid (Lond. Med. Times and Gaz., Oct. 1854, p. 408); but the proportion varies exceedingly. Other constituents are mucilage, extractive, and a very small proportion of saline matter, of which citrate of potassa constitutes by far the largest part. Some importance has been attached to this latter ingredient; but, when it is considered that an ounce of the juice contains, according to Dr. Jones, less than two grains of the salt, it must be admitted that any views of its operation, founded upon this constituent, must rest on a very narrow basis. For medical purposes, it is generally best to employ the juice in a fresh state; but, as lemons cannot always be had, it is important to be able to preserve it, especially for use on long voyages. When the juice cannot be obtained unaltered, recourse may be had to the pure acid.

Citric acid is separated from lemon or lime juice by boiling the juice, saturating it completely with carbonate of lime, washing the insoluble citrate of lime thus procured, and decomposing it with dilute sulphuric acid, which forms an insoluble sulphate of lime, and leaves the citric acid in solution. This is then obtained, in a crystallized state, by concentrating the solution sufficiently, and allowing it to cool.

Properties

The crystals of citric acid, when perfect, have the form of rhomboidal prisms with dihedral summits; but they are seldom regular, as kept in the shops. They are transparent, colourless, without smell, extremely sour, fusible in their own water of crystallization, very soluble in water, and less soluble in alcohol. The watery solution becomes mouldy by keeping. As tartaric acid is often substituted for, or fraudulently mixed with the citric, it is important to be able to detect its presence. This may be done, with great facility, by making a strong solution of the suspected acid in water, and then adding a rather strong solution of carbonate of potassa. if the citric acid be pure, there will be no precipitate; if tartaric acid be present, there will be a deposit of bitartrate of potassa. Citric acid is wholly dissipated by a red heat; and lime or other fixed impurity may thus be detected. The crystals consist of 12 eqs. of carbon, 5 of hydrogen, and 11 of oxygen, with 4 eqs. of water.

A substitute for lemon-juice, sufficiently near it in acid strength, may be made by dissolving an ounce of the crystals in a pint of water; and the flavour may be improved, by rubbing up four drops of oil of lemons with the acid before dissolving it.

Effects on the System

So far as the action of remedial doses is concerned, the effects of citric acid are the same as those of the vegetable acids in general, already detailed. But there is some doubt in relation to the operation of very large quantities. Mitscherlich found that a drachm of citric acid acted violently on a rabbit, without killing it; but that two drachms proved fatal to a large animal of the same species in an hour and a half. Dr. Christison, on the contrary, gave a drachm to a cat with perfect impunity. No positive inference can be drawn from these results as to the effect of large doses on man. As they differed so much in two different species of the inferior animals, there is no reason to suppose that the action of the acid in either of them is a proper measure of its influence on the human subject. By Orfila it is ranked with the irritant poisons. it cannot, however, be very poisonous, if given largely diluted; for Dr. Babington, of London, gave six ounces of lemon-juice, equivalent to about three drachms of the crystallized acid, three times a day in acute rheumatism, not only without unpleasant effects, but with great advantage. The only unequivocal effect, uniformly observed, was a diminution in the frequency and strength of the pulse, and the heart's action. it had no laxative effect, and no other action on the kidneys than to increase the discharge, through the influence of the quantity of liquid. {Lancet, Nov. 1851, p. 431.) Nevertheless, until further experiment has decided the question, it would be safest to avoid very large doses, especially in concentrated solution. From the experiments of Mitscherlich on rabbits, it is inferred that the blood is rendered thinner and less coagulable by its use, and the powers and actions of the heart reduced, without any inflammation of the stomach, even from poisonous doses. (Lond. Med. Times and Gaz., loc. citat.) Taken too freely and too long, it disturbs digestion, and tends to impair nutrition.

Therapeutic Application

in the form of lemonade, or added to barley-water, or other slightly nutritive drinks, lemon-juice is much used, as a cooling and refreshing beverage, in inflammatory and febrile diseases, and is no doubt positively beneficial as well as agreeable. it suits an irritated stomach better than any other of the vegetable acids.

Within a few years it has been employed by some as the main agent in the cure of inflammatory rheumatism, having been first used for this purpose by Dr. Owen Rees, of London, who gave one or two fluid-ounces of lemon-juice from four to six times a day, and found it very effectual. Other practitioners have since used it with the same favourable results. Dr. Wm. Pepper, of Philadelphia, found it greatly to relieve the disease in a week or less time; and the patients were generally cured in less than two weeks. (Trans, of Col. of Phys. of Philad., N. S., i. 124.) By some it has been given much more largely; and by Dr. Babington, as above stated, in doses of six ounces three times a day. Other practitioners have been less successful with it; and my own experience has not been specially favourable, though too limited to authorize a decided opinion upon that ground alone. My impression is that it acts favourably as an arterial sedative in the disease, in a manner similar to nitrate of potassa, without having any peculiar or specific influence over it; and it must be remembered, in estimating its efficiency in acute rheumatism, that this disease generally terminates favourably of itself, and often ends in recovery, under an expectant plan of treatment, within the period in which it is claimed that lemon - juice cures it.