CITRIC ACID. (Fn., Acide citrique ; Gm., Citronsdure.) Formula This acid was isolated and distinguished from tartaric acid, which it closely resembles, by Scheele in 1784. The citric acid of commerce consists of beautiful white crystals, prismatic in form, and, according to an analysis by Dr. Ure, of the following composition : carbon, 33.00 per cent. ; hydrogen, per cent ; oxygen, 62.37 per cent. Citric acid exists in the juice of many fruits, especially in lime juice and lemon juice, from which it is obtained on a large scale. The juice of gooseberries and currants has also been used as a source of this acid.
The outlines of the manufacture of citric acid from lemon and lime juice, as carried on at the present time, are as follows :—After clarification, the juice is heated to about 100° C., and powdered whiting (carbonate of lime) is added until the liquor is saturated, a point readily determined by its ceasing to effervesce ; the whiting should be added in small quantities, suitable to the amount of liquor under treatment, and the mixture is kept constantly agitated by machinery until the whole of the citric acid present has been converted into insoluble citrate of lime. When this is the case, the mixture stands until the citrate of lime has settled, when the supernatant liquid is run off, and the residue well washed by adding and decanting cold water, the agitating apparatus being set in motion after each addition. The washed citrate is next decomposed by means of hot sul. phuric acid, sulphate of lime, and free citric acid being formed. The former is got rid of by running the contents of the vessel into a settling tank close at hand, in which the heavy sulphate is retained, while the solution containing the citric acid flows into vessels in which it is concen trated by steam-heat. The concentrated citric liquor is pumped into a cistern, from which it is ladled into filters, made usually of canvas ; the filtrate runs into crystallizing pans placed beneath, in which it stands until the crystals cease to form. The mother-liquors are run back into the concen trating pan.
If a very pure article be required, it is customary to place a small quantity of animal charcoal in each of the filters; this serves to decolorize the liquid, and to free it from insoluble foreign matter. Tho crystals may be dissolved and re-crystallized until they are obtained perfectly pure and of large size ; hut when ordinary commercial citric acid is required, one process of crystalliza tion is enough.
The lime juice from which the acid is prepared is imported from Sicily, the south of Italy, and from the West Indies. After removing the seeds and peel, the fruit is strongly expressed, and the juice collected ; it is evaporated in copper pans until it has a density of about when it is a thin, dark-brown, syrupy liquid, containing about 32 per cent. of free citric acid. An instrument termed citronuter is sometimes used to measure the amount of citric acid contained in the juice, hut this method is not to be relied on, owing to the variableness of the quantity of insoluble and saccharine matter present in the sample, as well as to the fact that during the concentration of the juice, part of the acid is invariably d, composed and carbon thereby set at liberty ; the dark colour of the juice is also due to the presence of free cerbou. It is imported into this country in casks containing alma 100 gallons.
The vessel in which the decomposition takes place is a wooden tub, conical in form, and of any convenient size ; this tub is fitted with suitable agitating gear, worked by machinery above. The juice is run in from a cistern, by means of a metal pipe provided with a stop-cock, having been previously heated to about 100° C. Small portions of common whiting, finely:ground, are added successively, the contents being well agitated the while, until the mixture ceases to effervesce. It should be observed that the reaction with MMus or turmeric affords no indication of the point at which all the citric acid is converted into citrate of lime, owing, it is said, to the formation of an acid citrate, and also to the presence of phosphoric acid, which is always to be found in the crude lime juice; these bodies are with difficulty neutralized by chalk, and render the mixture distinctly Reid when considerably more chalk has been added than is sufficient to combine with the whole of the citric acid. The liquid may be, and sometimes is, neutralized by the addition of milk of lime, but the practice is objectionable, and has been discontinued by the best manufacturers, on account of the mucilage precipitated by the lime, which hinders the filtration and crystallization of the concentrated liquor. It was formerly the custom to get rid of these mucilaginous matters by sub jecting the crude juice to a process of fermentation, but this has been generally given up as unneee,sary.