When the addition of more chalk produces no effervescence, the agitating gear is stopped, and the contents of the tub are allowed to settle; the clear liquor, containing much soluble impurity, is run away by means of a tap. The citrate of lime is now washed rapidly, but thoroughly, with warm water, the contents are well stirred up, again allowed to settle, and the washings again run off; this process is continued till the citrate is thoroughly cleansed. It is then ready for decom position, which is carried on in the same vessel. The proportion of sulphuric acid required to effect this is about 9i parts of strong acid, diluted with six times its weight of water, to 'very 10 parts of chalk previously used. The acid is run iu while still hot., and the mixture kept in a state of agitation for about twelve hours, or until the whole of the citric acid is decomposed. This operation complete, the whole contents are run off, while still well mixed together, into a shallow leaden vat, placed immediately beside tho decomposing tub, and connected with the bottom of the latter by means of a leaden pipe. The heavy sulphate of lime, which may afterwards be sold as manure, sinks immediately to the bottom of this vat, leaving the citric acid liquor free to flow iuto the concentrating vessel placed nt its side; this vessel is made of wood, lined on the inside with lead, and furnished with a leaden coil, which lies at the bottom of the pan, and through which steam is constantly passing. In order to render the concentration more speedy, the wooden sides of the pan enclose a row of metal pipes, through which also steam is made to pass. The steam is withdrawn as soon as a thin film appears on the surface of the evaporating liquid, and care must be taken that this point is not passed. Ou withdrawing the steam, the concentrated acid is run or pumped into a convenient cistern, and from this it is ladled into canvas bags suspended from a wooden frame, beneath which are placed rows of circular leaden basins; the liquor runuing through is retained in these basins, all mechanical impurities being left behind in the bags. As soon as the crystals cease to form, the mother-liquors are poured back into the concentrating pans, and the citric acid is carefully detached from the basins. The article thus obtained is sufficiently pure for ordinary purposes, and represents the citric acid of commerce.
During the process of evaporation in the leaden vats, the concentrated liquor invariably becomes contaminated with more or less lead. When the acid is used for the preparation of aerated waters, this becomes a serious difficulty, owing to the poisonous nature of lead compounds; it has bceu proposed to obviate this by employing vessels of wood or earthenware.
All solutions of organic acids are liable, during concentration by heat, to undergo a process of decomposition, by which carbon is liberated, and the article more or less damaged thereby. Hem, much care is necessary in order to prevent the heat from becoming too great, if the lipter bs evaporated in open steam pans, and the difficulty experienced in avoiding this contingency was for many years a constant source of annoyance to manufe,cturera of citric acid. An arrangement by which the liquid is concentrated in a vacuum, at a constant temperature, was invented and patented by Pontifex, in 1856. This apparatus is known as the Pontifex vacuum pan, and though a com plicated and expensive piece of machinery, seema to be the only effectual means of overcoming this troublesome obstacle. It will be fully described and illustrated in the article on tartaric acid.
Owing to this liability of citric acid to decomposition, a cheap and easy method of converting the len3on and lime juice into some stable citrate, before exportation, would be an inestimable advantage, both to the exporters of the juice and the manufacturers of the acids. Attempts have been made to effect this, but hitherto without much success, and the subject is still worth the attention of those interested in this important branch of manufacture. Lime as abase for citric acid is out of the question, as, unfortunately, citrate of linae is aa unatable as the acid itself. Perret has employed magnesia as the base with a considerable amount of auccess, though the time occupied in effecting the crystallization of the tri-basic citrate is a manifest disadvantage. Another matter to which the manufacturer would do well to give his attention, is the objectionable waste of time in the processes of evaporating, filtering, eic.; the attainment of more economy in this respect is much to be desired, and would amply repay careful consideration.
A patent was granted a few years ago for a method of producing citric acid, in conjunction with tartaric acid and alcohol, and in a state of extreme purity, from an African plant called the tamarin.ha; the plant is cut while green, and subjected to pressure, by which means the juice or sap is extracted ; the three above-named substances are obtained separately from the sap by sub sequent chemical processes. We do not know whether this method of preparing citric acid has been found successful on a commercial scale, but it is scarcely probable.
The following method of obtaining citric acid is due to Dr. Price ;—The crude juice is saturated NNith an alkaline carbonate, or with the ammoniacal product distilled from gas-liquor : 150 parts of chalk or 90 parts of milk of lime are now added for every 192 parte of citric acid contained in the juice. Heat is applied, and the ammonia distilled off into another portion of the juice. The dccouiposition of the citrate of lime by sulphuric acid, and the subsequent processea, are carried on in tbe usual way.
Citric acid is constantly adulterated with tartaric acid ; the fraud is readily detected, if tartar be present in any quantity, by dissolving a portion in water and adding a small quantity of a strong solution of acetate of potash, or other potash salt, when a crystalline precipitate of cream of tartar will show itself on the sides of the vessel.
The first crape of citric fluid crystals, which are brownish in colour, are used largely by the calico-printer aa a " resistant " for iron and alumina mordants ; when the acid is required for other purposes, it is necessary to subject it to one, or even two Or three more crystallizations, accordiog to the degree of purity wished for. The above application of the acid is the most important, though citric acid is also widely used in the manufacture of aerated waters (see " Aerated Waters"), to which it imparts an agreeable and cooling acerbity. It is used medicinally as an antiseptic and aeorbutic, the place of lemon-juice.
The manufactures of citric and tartaric acida are usually carried on in the same works, and are, in the main, identical. To avoid repetition, further detailing of the particulars of tbe manufacture of the acid under consideration has been deferred for treatment under the latter acid.