CITRIC acid, in chemistry, is found in the juice of lemons and limes, and is that which gives it the sour taste. It is mixed, however, with mucilaginous and extractive matter. Scheele found that it could not be obtained pure and crystal lized by mere evaporation of the lemon juice, and that even the addition of alco hol did not separate completely the fo reign matter. The process he followed is, to saturate the expressed juice of the lemon, by the addition of chalk. The citric acid, combining with the lime, forms an insoluble compound, which of course precipitates. This is well washed with warm water, until the water pass off co lourless; and in this way the mucilage and extractive Matter are abstracted. The citrate of lime is then subjected to the action of as much sulphuric acid, previ ously diluted, as is sufficient to saturate the lime of the quantity of chalk that has been employed. The citric acid is disen gaged and dissolved by the water ; the mixture is boiled for a few minutes, to facilitate the precipitation of the sulphate of lime, and is then filtered. The filtered liquor is evaporated to the consistence of syrup, and sulphate of lime separated daring the evaporation being withdrawn; and, on cooling and standing for some time, the citric acid is obtained in needle like crystals.
Citric acid exists in a number of other fruits, from which it may be extracted, and much, it is said, of what is at present found in the shops is prepared from the juice of the lime. From Vauquelin's an alysis of the pulp of the tamarind, it ap pears to be the chief acid constituent of that fruit ; one pound of the common pre pared pulp of the shops containing an ounce and a half, with smaller quantities of malic and tartaric acids. This acid is very soluble in water. At a moderate temperature, 100 parts of water dissolve 75 parts, cold being produced (luring the solution ; at 212° it dissolves twice its weight of it. Like the other vegetable acids, its solution undergoes spontaneous decomposition, though not very readily. The more powerful acids decompose it, though with some difficulty. Concen trat sulphuric acid converts it into ace tic acid. Scheele remarked, that nitric acid did not convert it, as it did some of the other vegetable acids, into oxalic acid ; but Fourcroy and Vauquelin have found that, when acted on by a large quantity of nitric acid for a long time, it affords a small portion of oxalic, with a larger portion of acetic acid.
Citric acid combines with the alkalies and earths, forming salts denominated citrates. The citrate of potass is very
soluble, and does not crystallize but with difficulty, and is deliquescent : its taste is purely saline, and rather mild. It con tains 55.55 of acid, and 44.55 of alkali. Citrate of soda is likewise very soluble, requiring little more than its weight of water for its solution : it crystallizes in six-sided prisms, and the crystals are slightly efflorescent. Their taste is faintly saline ; the proportions of the solid salt are 60.7 of acid, and 39.3 of soda. Ci trate of ammonia is equally or even more soluble than the others, and does not crys tallize but when its solution is much con centrated : the form of its crystals is an elongated prism. It consists of 62 of acid, and 38 of ammonia. The earthy citrates are in general less soluble. When the so lution of barytes is poured into the acid, a precipitate, soluble in the liquid by agita tion, is formed : when the whole is satu rated, the salt is deposited at first in the form of a powder, which is covered after wards with a kind of crystalline efflores cence, and which a large quantity of water dissolves. It consists of 50 of acid, and 50 of base. When the citric acid is saturated by lime, small crystals are deposited, which are very sparingly so luble : 100 parts contain 62.66 of acid, and 37.34 of lime. When saturated by magnesia, the concentrated solution does not easily crystallize regularly, but rather assumes the state of a white, opaque, and somewhat spongy salt. The proportions of the salt, are 66.66 of acid, and 33.34 of base.
Vauquelin has likewise examined the action of citric acid on the metals. It does not dissolve silver ; but it com bines with its oxide, and forms a salt, insoluble, of a harsh and strong metallic taste, and which, like the other salts of silver, is blackened 'by light : it is also decomposed by heat, sometimes leav ing metallic silver intermixed with char coal. It consists of 36 of acid, and 64 of oxide.
Citric acid, in its crystallized state, can be preserved for any length of time out decomposition ; and a grateful lemon.. ade may be prepared from it, by dissolving 30 or 40 grains in a pint of water, with the addition of a little sugar ; and to communi cate flavour,a little lemon peel,or powder, formed by rubbing sugar on the fresh le mon. The lemon juice may be regarded as a specific in scurvy, and there is every probability that the crystallized citric acid may be equally effectual.