Preparation

Though the solid citrate of magnesia may be prepared without difficulty by a direct combination of its constituents, yet it has not been introduced into our officinal code, nor indeed into extensive use; a preference being given to the liquid form, in which the accessaries of the preparation, as officinally directed, recommend it strongly to a squeamish taste and delicate stomach.

According to the directions in the second edition of the U. S. Pharmacopoeia of 1850, and the present edition, the solution is prepared by dissolving magnesia in an excess of solution of citric acid; filtering into a strong glass bottle of the capacity of twelve fluidounces, into which a portion of syrup of citric acid had been previously introduced; then adding a little bicarbonate of potassa, with sufficient water nearly to fill the bottle, which must be tightly corked, and secured by twine; and, lastly, shaking occasionally till the bicarbonate is dissolved. The first liquid introduced into the bottle, after the syrup of citric acid which is intended to give an agreeable flavour to the preparation, is a solution of citrate of magnesia with an excess of citric acid, which excess reacts upon the subsequently introduced bicarbonate of potassa, producing a portion of citrate of potassa, while the separated carbonic acid is retained in the liquid, and renders it brisk and effervescing. The preparation, therefore, is simply a solution of citrate of magnesia, with a little citrate of potassa, in water impregnated with carbonic acid, and agreeably flavoured. The substitution of bicarbonate of potassa, at the close of the process, for the carbonate of magnesia formerly employed, is to render the liquid clear; and the resulting citrate of potassa is not injurious.

Properties

The solution of citrate of magnesia thus prepared is a colourless liquid, effervescing when poured out of the bottle, and of an agreeable acidulous taste. When long kept, it is apt to deposit an insoluble form of citrate of magnesia, and to be weakened proportionably. The citrate of magnesia contained in the solution is probably the tribasic citrate, consisting of one equivalent of acid and three of base; and the quantity in each bottle is about an ounce of the anhydrous salt. A similar preparation may be made by dissolving the solid citrate in water, flavoured with lemon syrup, and then introducing, from the fountain, as much carbonic acid water as may render the solution sufficiently effervescent.

Medical Properties and Uses

This is an elegant aperient, having the usual properties of the saline cathartics, and in large doses capable of operating efficiently. it is, however, incapable of supplying the place of sulphate of magnesia as an antiphlogistic remedy, and should not be substituted for it in cases requiring energetic treatment. But, when the object is simply to evacuate the bowels, and at the same time produce a moderately refrigerant effect, it may be employed, with propriety, in all cases in which its excess of acid may not be contraindicated. it is peculiarly adapted to cases in which the stomach is very delicate or irritable, and is usually preferred by patients to any other saline cathartic, on account of the agreeable taste. For full purgative effect, the whole contents of a bottle containing twelve fluidounces are required; but simply as an aperient, one-half, or one-third of the quantity will generally be sufficient. When the contents of the bottle are only partially removed, it should be immediately well corked, and kept in an inverted position, standing upon the cork.