Citric Acid, the acid which gives to the fruits of the citron family their peculiar sour taste. It may be extracted from many fruits and vegetables, as oranges, currants, gooseberries, strawberries, raspberries, whortleberries, tamarinds, onions, and potatoes. The red elder berry contains so much of it that it has been proposed to substitute it in part for limes and lemons, from which the acid is now wholly prepared. The manufacture of citric acid is conducted at but few large establishments; but on a small scale it may be prepared by the same process that is adopted in the large way. Lemon juice is imported in a concentrated state, produced by evaporation at a gentle heat. It consists of citric acid 6 to 7 per cent., alcohol 5 to 6, and the remainder water, inorganic salts, etc. By some it is allowed to partially ferment, for the purpose of evaporating the clear liquor from the mucilage; or it is clarified in the usual method by the use of albumen in the form of the white of egg. Carbonate of lime in fine powder is then gradually added, and stirred in so long as the effervescence continues. Citrate of lime (an insoluble powder) forms, and after being separated by drawing off the watery liquor is well washed with warm water.
This salt is then intimately mixed with strong sulphuric acid diluted with six parts of water. After some hours the citrate is decomposed, the sulphuric acid having taken up the lime and formed an insoluble sulphate, setting the citric acid free. This, separated by decanting and filtering, is evaporated in leaden pans till it attains the specific gravity 1.13. The evaporation is afterward continued by a water or steam bath till the liquor begins to be sirupy, or to be covered with a thin pellicle. It is then instantly removed from the fire, and put aside to crystallize, the mother liquor after a few days being evaporated as above, and again set to crystallize, and so on as long as clean crystals are obtained. To obtain pure citric acid, all the crystals should be redissolved and recrystal-lized, it may be several times, and the solution digested with bone black. A gallon of lemon juice should make about eight ounces of crystals; more than this is sometimes obtained. Attempts have been made to prepare the citrate of lime for exportation instead of the lemon juice, to reduce the cost of transportation; but its liability to ferment, which destroys the citric acid by converting it into acetic and butyric acids, prevents this method from being employed.
Citric acid was first obtained by Scheele in 1784. The crystals are transparent, in the form of rhombic prisms, with summits of four trapezoidal faces. The salt is soluble in water, warm or cold, as also in alcohol, but not in ether. Its specific gravity is 1.6. Its taste is intensely acid, and almost caustic, but still agreeable. It is distinguished from other vegetable acids by its depositing an insoluble citrate when heated with lime water, by the form of its crystals, and also by forming a deliquescent salt with potassa. The crystals are combinations of 1 equivalent of citric acid with either 3, 4, or 5 of water, according to the method and temperature by which they have been obtained. The anhydrous acid, as it exists in the citrate of silver, is represented by the formula C6H5O7; but it has never been isolated. - Citric acid is much used at sea as an antiscorbutic, but the raw lemon juice is thought to be more efficient. English vessels are all required by law to carry a certain quantity of the latter for each man employed. It goes to the manufacture of citric acid, as already stated, in a concentrated form; but that intended for the use of ships is protected from fermentation by the addition of some spirit.
Citric acid is largely adulterated, sometimes to an almost incredible percentage, with tartaric acid. Acetate of potassa, added to its solution in cold water, will, if tartaric acid be present, throw down a white crystalline precipitate of bitartrate of potassa (cream of tartar). It sometimes also retains some sulphuric acid. Besides its use as an antiscorbutic, citric acid is employed instead of lemon juice for the preparation of refreshing drinks, and in the arts it is of value to calico printers.