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and filtered, and neutralized with powdered chalk and a little milk of lime; the precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid, the solution filtered, evaporated to remove calcium sulphate and concentrated, preferably in vacuum pans. The acid is thus obtained in colourless rhombic prisms of the composition C6H8O7 + H2O. Crystals of a different form are deposited from a strong boiling solution of the acid. About 20 gallons of lemon juice should yield about 10 lb of crystallized citric acid. The acid may also be prepared from the juice of unripe gooseberries. Calcium citrate must be manufactured with care to avoid an excess of chalk or lime, which would precipitate constituents of the juice that cause the fermentation of the citrate and the production of calcium acetate and butyrate.

The synthesis of citric acid was accomplished by L. E. Grimaux and P. Adam in 1881. Glycerin when treated with hydrochloric acid gives propenyl dichlorhydrin, which may be oxidized to s-dichloracetone. This compound combines with hydrocyanic acid to form a nitrile which hydrolyses to dichlor-hydroxy iso-butyric acid. Potassium cyanide reacts with this acid to form the corresponding dinitrile, which is converted by hydrochloric acid into citric acid. This series of operations proves the constitution of the acid. A. Haller and C. A. Held synthesized the acid from ethyl chlor-acetoacetate (from chlorine and acetoacetic ester) by heating with potassium cyanide and saponifying the resulting nitrile. The acetone dicarboxylic acid, CO(CH2CO2H)2, so obtained combines with hydrocyanic acid, and this product yields citric acid on hydrolysis.

Citric acid has an agreeable sour taste. It is soluble in ¾ths of its weight of cold, and in half its weight of boiling water, and dissolves in alcohol, but not in ether. At 150°C. it melts, and on the continued application of heat boils, giving off its water of crystallization. At 175° C. it is resolved into water and aconitic acid, C6H6O6, a substance found in Equisetum fluviatile, monks-hood and other plants. A higher temperature decomposes this body into carbon dioxide and itaconic acid, C5H6C4, which, again, by the expulsion of a molecule of water, yields citraconic anhydride, C5H4O3. Citric acid digested at a temperature below 40°C. with concentrated sulphuric acid gives off carbon monoxide and forms acetone dicarboxylic acid. With fused potash it forms potassium oxalate and acetate. It is a strong acid, and dissolved in water decomposes carbonates and attacks iron and zinc.

The citrates are a numerous class of salts, the most soluble of which are those of the alkaline metals; the citrates of the alkaline earth metals are insoluble. Citric acid, being tribasic, forms either acid monometallic, acid dimetallic or neutral trimetallic salts; thus, mono-, di- and tri-potassium and sodium citrates are known. On warming citric acid with an excess of lime-water a precipitate of calcium citrate is obtained which is redissolved as the liquid cools.

The impurities occasionally present in commercial citric acid are salts of potassium and sodium, traces of iron, lead and copper derived from the vessels used for its evaporation and crystallization, and free sulphuric, tartaric and even oxalic acid. Tartaric acid, which is sometimes present in large quantities as an adulterant in commercial citric acid, may be detected in the presence of the latter, by the production of a precipitate of acid potassium tartrate when potassium acetate is added to a cold solution. Another mode of separating the two acids is to convert them into calcium salts, which are then treated with a perfectly neutral solution of cupric chloride, soluble cupric citrate and calcium chloride being formed, while cupric tartrate remains undissolved. Citric acid is also distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas silver citrate is reduced only after prolonged ebullition.

Citric acid is used in calico printing, also in the preparation of effervescing draughts, as a refrigerant and sialogogue, and occasionally as an antiscorbutic, instead of fresh lemon juice. In the form of lime juice it has long been known as an antidote for scurvy. Several of the citrates are much employed as medicines, the most important being the scale preparations of iron. Of these iron and ammonium citrate is much used as a haematinic, and as it has hardly any tendency to cause gastric irritation or constipation it can be taken when the ordinary forms of iron are inadmissible. Iron and quinine citrate is used as a bitter stomachic and tonic. In the blood citrates are oxidized into carbonates; they therefore act as remote alkalis, increasing the alkalinity of the blood and thereby the general rate of chemical change within the body (see Acetic Acid).

CITRON, a species of Citrus (C. medica), belonging to the tribe Aurantieae, of the botanical natural order Rutaceae; the same genus furnishes also the orange, lime and shaddock. The citron is a small evergreen tree or shrub growing to a height of about 10 ft.; it has irregular straggling spiny branches, large pale-green broadly oblong, slightly serrate leaves and generally unisexual flowers purplish without and white within. The large fruit is ovate or oblong, protuberant at the tip, and from 5 to 6 in. long, with a rough, furrowed, adherent rind, the inner portion of which is thick, white and fleshy, the outer, thin, greenish-yellow and very fragrant. The pulp is sub-acid and edible, and the seeds are bitter. There are many varieties of the fruit, some of them of great weight and size. The Madras citron has the form of an oblate sphere; and in the “fingered citron” of China the lobes are separated into finger-like divisions formed by separation of the constituent carpels, as occurs sometimes in the orange.

The citron-tree thrives in the open air in China, Persia, the West Indies, Madeira, Sicily, Corsica, and the warmer parts of Spain and Italy; and in conservatories it is often to be seen in more northerly regions. Sir Joseph Hooker (Flora of British India, i. 514) regards it as a native of the valleys at the foot of the Himalaya, and of the Khasia hills and the Western Ghauts; Dr Bonavia, however, considers it to have originated in Cochin China or China, and to have been introduced into India, whence it spread to Media and Persia. It was described by Theophrastus as growing in Media, three centuries before Christ, and was early known to the ancients, and the fruit was held in great esteem by them; but they seem to have been acquainted with no other member of the Aurantieae, the introduction of oranges and lemons into the countries of the Mediterranean being due to the Arabs, between the 10th and 15th centuries. Josephus tells us that “the law of the Jews required that at the feast of tabernacles every one should have branches of palm-tree and citron-tree” (Antiq. xiii. 13. 5); and the Hebrew word tappuach, rendered “apples” and “apple-tree” in Cant. ii. 3, 5, Prov. xxv. 11, &c., probably signifies the citron-tree and its fruit. Oribasius in the 4th century describes the fruit, accurately distinguishing the three parts of it. About the 3rd century the tree was introduced into Italy; and, as Gallesio informs us, it was much grown at Salerno in the 11th century. In China citrons are placed in apartments to make them fragrant. The rind of the citron yields two perfumes, oil of cedra and oil of citron, isomeric with oil of turpentine; and when candied it is much esteemed as a dessert and in confectionery. The lemon (q.v.) is now generally regarded as a subspecies Limonum of Citrus medica.

Oribasii Sardiani, Collectorum Medicinalium Libri XVII. i. 64 (De citrio); Gallesio, Traité du citrus (1811); Darwin, Animals and Plants under Domestication, i. 334-336 (1868); Brandis, Forest Flora of North-West and Central India, p. 51 (1874); E. Bonavia, The Cultivated Oranges and Lemons, &c., of India and Ceylon (1890).

CITTADELLA, a town of Venetia, Italy, in the province of Padua, 20 m. N.W. by rail from the town of Padua; 160 ft. above sea-level. Pop. (1901) town, 3616; commune, 9686. The town was founded in 1220 by the Paduans to counterbalance the fortification of Castelfranco, 8 m. to the E., in 1218 by the Trevisans, and retains its well-preserved medieval walls, surrounded by a wet ditch. It was always a fortress of importance, and in modern times is a centre for the agricultural produce of the district, being the junction of the lines from Padua to Bassano and from Vicenza to Treviso.

CITTÀ DELLA PIEVE, a town and episcopal see of Umbria, Italy, in the province of Perugia, situated 1666 ft. above the sea,