CIT′RIC ACID

H 3 C 6 H 5 O 7 ,H 2 O. Syn. Acid of lemons , Concrete a. of l. ; Ac′idum limo′nis , Acidum cit′ricum (B. P.), L.; Acide citrique , Fr.; Citron ens aüre , Ger. An acid peculiar to the vegetable kingdom. It is obtained in large quantity from the juice of lemons and other fruits of the genus Citrus ; it is also found in gooseberries, currants, cranberries, whortleberries, cherries, .; and Dr Wright has lately found it in great abundance in unripe mulberries, in conjunction with malic acid. When currants or gooseberries are employed as a source of citric acid, they are first subjected to pre ure, and the juice so obtained from them is then fermented. The fermented liquor is next submitted to distillation, and the alcohol collected. The residue in the retort containing the citric acid is saturated with chalk, and the resulting citrate of lime is decomposed by means of sulphuric acid. 100 lbs. of the fruit are said to yield 10 lbs. of spirit and 1 lb. of acid. Prep. The citric acid manufacture consists in separating it from the mucilage, sugar, and other foreign matter with which it is combined in the juice of lemons and limes. 1. (Ph. L. 1836,—Scheele’s proce .) Take of lemon juice 4 pints; prepared chalk, 4 1 ⁄ 2 oz.; diluted sulphuric acid, 27 1 ⁄ 2 fl. oz.; distilled water, 2 pints. Add the chalk by degrees to the lemon juice, made hot, and mix well; set by, that the powder may subside, and afterwards pour off the supernatant liquor. Wash the precipitated citrate of lime frequently with warm water; then pour upon it the diluted sulphuric acid, mixed with the distilled water, and boil the whole for 15 minutes in gla , stoneware, or lead; pre the mixture strongly through a linen cloth, and filter it. Evaporate the filtered liquor with a gentle heat, and set it aside, that crystals may form. To obtain the crystals pure, di olve them in water a second and a third time; filter each solution, evaporate, and set it apart to crystallise. 2. (Ph. L. 1851.) Merely placed in the materia medic a. 3. (Ph. E. 1841.) Similar to that of Ph. L. 1836, except that the washed citrate of lime is ordered to be squeezed in a powerful pre , and that the filtered solution of citric acid is ordered to be tested with nitrate of baryta, and if the precipitate is not nearly all soluble in nitric acid, to add a little citrate of lime to the whole liquor, till it stands this test. 4. (Ph. D. 1826.) Same as that of Ph. L. 1836. 5. (Ph. D. 1851.) Included in the materia medic a. 6. (P. B. 1867.) Differs from the proce of the Ph. L. 1836 in some unimportant detail only. 7. (Dr Price.) The crude juice is saturated with ammonia, pota a, or soda (carbonates), or with the ammoniacal product distilled from gas-liquor; chalk, 150 parts, or hydrate of lime, 90 parts, are then added for every 192 parts of citric acid contained in the liquor, and the whole stirred well together; heat is next applied, and the ammonia distilled into another quantity of lemon juice; the citrate of lime thus obtained is then decomposed with dilute sulphuric acid, and the whole proce conducted as before. When pota a or soda is used the distillation is omitted, and the expre ed liquor, after filtration, used to decompose fresh lemon juice. 8. (Ordinary manufacturing proce .) To crude lemon or lime juice, mixed with water, is added ground chalk; the precipitate is washed to free it from the impurities di olved in the water, and afterwards decomposed by sulphuric acid. If the citric acid is not sufficiently white, it is decolorised by digestion with animal black. 9. (Kuhlman.) This chemist proposes saturating the hot lemon juice as far as po ible with very finely divided barium carbonate, and afterwards completing the neutralisation with barium hydrate or sulphide. The precipitated barium citrate is then to be washed, and decomposed with the requisite quantity of sulphuric acid. The advantage of barium over lime as a precipitant is the more ready crystal lis ability of the citric acid from the solution thus obtained. Sulphate of baryta is absolutely insoluble in solution of citric acid, whilst sulphate of lime is not; and the presence of the latter impedes the crystallisation of the acid. Obs. If the lemon or lime juice be allowed to ferment a short time, the mucilage and other impurities will, to a certain extent, separate and subside. See Concluding Remarks . Prop., Uses, . Citric acid forms rhomboidal prisms, which are clear, colourle , odourle , sour, and deliquescent in a moist atmosphere. It is an agreeable acid, at once cooling and antiseptic. It is much used in medicine as a substitute for lemon juice, and to form effervescing draughts, citrates, . 17 gr. citric acid, in crystals, or 1 ⁄ 2 fl. oz. of lemon juice, are equivalent to— | 25 | gr. | bicarbonate of potash; | | 20 | ” | carbonate of potash; | | 15 | ” | carbonate of ammonia; | | 20 | ” | bicarbonate of soda; | | 35 | ” | carbonate of soda. | The bicarbonate of pota a is that generally preferred for making saline draughts with citric acid; and when flavoured with a little tincture of orange peel and simple syrup, or syrup of orange peel alone, it forms a most delicious effervescing beverage. Citric acid in pure crystals or in lime juice is much used by the calico-printer, being the best known ‘resistant’ for iron and alumina mordants. Pur. Citric acid is frequently met with adulterated with tartaric acid; the fraud is easily detected by di olving the acid in a little cold water, and adding to the solution a small quantity of acetate of potash. If tartaric acid be present, a white, crystalline precipitate of cream of tartar will be produced on agitation. When pure it is devoid of colour, is entirely, or almost entirely, decomposed by heat. It is soluble in water and in spirit, and what is thrown down from its watery solution by acetate of lead is di olved by nitric acid. No salt of pota ium precipitates anything with citric acid except the tartrate. When a few drops of a solution of citric acid are added to lime water, a clear liquid results, which, when heated, deposits a white powder, soluble in acids without effervescence. By the action of nitric acid citric acid is converted into oxalic acid. When the crystals of citric acid are very deliquescent, the presence of free sulphuric acid may be suspected. This latter may be detected with facility by di olving the citric acid in a little water, strongly acidifying the solution with hydrochloric acid, and adding chloride of barium, when, if sulphuric acid be present, an insoluble precipitate of sulphate of barium will fall down after a short time. Oxalic acid is sometimes present in citric acid, the cause of its presence being explained further on. To test for it proceed as follows: Di olve a small quantity of the citric acid in water, and add to the solution an exce of ammonia; acidify with acetic acid, filter, and test the filtrate with calcium sulphate. Estim. See Acidimetry and Lime juice . Tests. See above . Concluding Remarks. The preparation of citric acid has now become an important branch of chemical manufacture, from the large consumption of this article in various operations in the arts. In conducting the different steps of the proce some little expertne and care are, however, nece ary to ensure succe . The chalk employed, which should be dry, and in fine powder, is added to the juice from a weighed sample, until the latter is perfectly neutralised, and the quantity consumed is exactly noted. The precipitated citrate of lime is next thoroughly washed with water, and the sulphuric acid, diluted with 6 or 8 times its weight of water, whilst still warm, is poured upon it, and thoroughly mixed with it. The agitation is occasionally renewed for 8 or 10 hours or longer, when the solution of citric acid is poured off, and the residuum of sulphate of lime thoroughly washed with warm water, the washings being added to the liquid acid. This last is then poured off from the impurities that may have been deposited, and evaporated in a leaden boiler, over the naked fire, or by high-pre ure steam, until it acquires the gravity of 1·13, when the proce is continued, at a lower temperature, until a syrupy aspect is a umed, and a pellicle appears on the surface of the liquor. Without great care at this part of the proce the whole batch may be carbonised and spoiled. At this point the concentrated solution is emptied into warm and clean crystallising ve els, set in a dry apartment, where the thermometer does not fall below temperate. At the end of 4 days the crystals are found ready for removal from the pans. They are thoroughly drained, redi olved in as little water as po ible, and after being allowed to stand for a few hours to deposit impurities, again evaporated and crystallised. The acid of the second crystallisation is usually sufficiently pure for the market; when this is not the case a third, or even a fourth, crystallisation must be had recourse to. The mother-liquors from the several pans are now collected together, and a second or third crop of crystals obtained from them, by evaporation as before. A frequent cause of difficulty in obtaining crystals from the solutions is the employment of too little sulphuric acid to decompose the whole of the citrate of lime; the consequence of which is that a little of that salt is taken up by the free citric acid, and materially obstructs the crystallisation. Forty parts of dry sulphuric acid are required to decompose 50 parts of chalk. Commercial sulphuric acid (oil of vitriol) is usually of the sp. gr. of 1·845, and it therefore requires 49 lbs. of this acid for every 50 lbs. of chalk employed in the proce . In practice it is found that a very slight exce of sulphuric acid is preferable to a preponderance of undecomposed citrate of lime. The first crop of crystals is called ‘brown citric acid,’ and is chiefly sold to the calico-printers. Sometimes a little nitric acid is added to the solution of the coloured crystals, for the purpose of bleaching them, but in this way a minute quantity of oxalic acid is formed. A more general plan is to bleach the citrate of lime by covering it with a weak solution of chloride of lime, exposing it in shallow ve els to the sun’s rays, and rewashing it before decomposing it with sulphuric acid. A safer plan is to di olve the crude citric acid, digest with animal charcoal, and again concentrate the solution to the crystallising point. When the aqueous solution of citric acid obtained, as already described, is concentrated by boiling in an open evaporating pan, the acid is not only liable to suffer partial decomposition by its long exposure to the air, but it not unfrequently acquires a brown colour from the carbonisation those portions of the liquid undergo which are in contact with the bottom of the pan, which being heated by high-pre ure steam frequently reaches a temperature exceeding 200° F. This latter result is brought about in consequence of the slight movement in the dense acid liquor in the pan. To remedy the lo and inconvenience arising from the employment of the open evaporating pan, some years back Mr Pontifex devised an apparatus which effects the evaporation of acid liquor in vacuo (and therefore out of contact with air), and at a temperature never exceeding 130 F°. Moreover, in Mr Pontifex’s boiler the time nece ary for the concentration of the citric-acid liquor is diminished to about an eighth, and as the strong ebullition keeps the liquid in constant motion its charring is entirely prevented. Mr Row says that lemon juice may be purified to a great extent by diluting it with water until it contains about 12 oz. of acid to the gallon, and then filtering from the flocculent precipitate of mucilage thus thrown down. The citrate of lime obtained from juice so treated is comparatively pure. Good lemon juice yields about 6 1 ⁄ 2 % of crystallised lemon acid; 2 galls. yield fully 1 lb. of crystals. See Lemon juice , Lime juice . .