"Take of Lemon juice, four pints; prepared chalk, four ounces and a half; diluted sulphuric acid, twenty-seven fluid ounces and a half; distilled water, two pints. Add the chalk by degrees to the lemon juice heated, and mix them; set by, that the powder may precipitate; then pour off the supernatant liquor. Wash the citrate of lime frequently with warm water; then pour on it the diluted sulphuric acid and the distilled water, and boil for fifteen minutes; press the liquor 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, dissolve them in water a second and a third time; filter each solution, evaporate, and set it apart to crystallize."

Dublin.

"Take of lemon juice any quantity; prepared chalk, as much as may be required. To the lemon juice, heated, add gradually the chalk; then pour off the liquid from the residue of the citrate of lime. Wash this many times in hot water, and then dry it. To the dried powder add eight times the quantity of diluted sulphuric acid, as of chalk previously added; boil the mixture, express it strongly through cloth, and filter the liquid through paper. Evaporate the filtered liquor until crystals form on cooling. The crystals may be purified by repeated solutions and crystallizations."

Syn. Acide Citrique (F.), Acido Citrico (I.).

This process, which was contrived by Scheele, should never be performed by the apothecary, as the crystallized acid is now manufactured very pure, and sufficiently reasonable, on a great scale.1 The theory of the process is very simple. The lime of the chalk unites with the citric acid which exists ready formed in the lemon juice, expels the carbonic acid, and produces an insoluble citrate of lime, which precipitates. Any mucilage it retains may be removed by repeated washings; and the sulphuric acid, which is added to the dried citrate, decomposing it, owing to the superior affinity of the sulphuric acid for lime, an insoluble sulphate of lime forms, and is precipitated, while the citric acid is disengaged, remains in a solution, and is crystallised by evaporation. The crystals of the first crystallisation are dark coloured; which is partly owing to a portion of mucilage which still adheres to the citric acid, and partly to the excess of sulphuric acid acting on the citric acid and decomposing a portion of it.

The repeated crystallisations free the crystals from this dark colour; but as it is of some importance to avoid any hurtful excess of sulphuric acid, and as the strength of lemon juice is variable and uncertain, it is better to determine the quantity of acid required by the quantity of chalk employed. For this purpose a portion of the sulphuric acid intended to be used must be previously saturated with the chalk, and the weight of the portion employed accurately ascertained; by the knowledge of which the exact quantity of sulphuric acid required to decompose the citrate may be found. According to the experiments of Proust1, 94 ounces of lemon juice saturate 4 ounces of chalk with citric acid, and produce 7 1/2 ounces of dry citrate, which require for their decomposition, and the complete saturation of the lime they contain, 20 ounces of diluted sulphuric acid, composed of one part of the common acid, and three parts of water, or of a specific gravity of 1.15. To ascertain, however, the exact point of saturation of the lime with the sulphuric acid, take a little of the clear supernatant fluid, filter it, and add to it a few drops of acetate of lead; if no sulphuric acid be present, citrate of lead only will be formed, soluble in nitric acid, which does not dissolve sulphate of lead.2

1 The principal manufacturer in London is Mr. Coxwell, of Fleet Street.

Qualities. - Pure citric acid is in white, transparent, and persistent crystals. The primary crystal is a right rhomboic prism; in general, however, it is variously modified. It is inodorous; has an extremely acid, almost caustic taste; and reddens strongly the vegetable blues. In a damp air, the crystals absorb moisture. Seventy-five parts of water at 60° Fahrenheit dissolve one hundred parts of this acid; and at 212° twice their weight. The weak solution, when long kept, is liable to become mouldy and to undergo spontaneous decomposition. Citric acid combines with the alkalies, earths, and metallic oxides, and forms citrates. The sulphuric and nitric acids decompose it. Its components, according to Berzelius, are 41.369 carbon, 3.800 hydrogen, and 54.831 of oxygen; according to Dr. Prout, they are carbon 34.28, oxygen 22.87, water 42.85. In its crystallized state the crystals contain of citric acid 76.32, water 23.68, in 100 parts3; in equivalents it consists of 4 of carbon = 24.48 + 2 of hydrogen = 2 + 4 of oxygen = 32, making the equivalent of the anhydrous acid 58.48: but the crystallized acid containing 1 1/2 eq. of water = 12.5, its equivalent is 70.98. Citric acid is sometimes adulterated with tartaric acid; or with citrate of lime.

The first is discovered by adding to the solution of the acid to be tested a solution of any salt of potassa, when an insoluble bitartrate, in small brilliant crystals will be formed if the tartaric acid be present; the second is detected by dissolving the crystals in water, saturating the solution with ammonia, and adding to it some oxalate of ammonia, which will instantly precipitate the lime, if present. If it be mixed with sulphate of potassa, the solution will yield a precipitate with chloride of barium, which is insoluble in hydrochloric acid. Any substance remaining when the acid is subjected to the action of fire, is an impurity.

1 Journal de Physique, iii. 366.

2 Citric acid may be also obtained from the juice of the cranberry, vaccinium oxycoccus; bird-cherry, prudus padus; bitter-sweet, solatium dulcamara; and dog-rose, rosa canina.

3 Phillips's Translations of Pharm. p. 23.

Medical properties and uses. - The solution of this acid in water, in the proportion of nine drachms and a half of the crystals to Oj. of water, answers nearly all the purposes of recent lemon juice for forming the common effervescing draught with carbonate of potassa. The following table shows the quantity of citric acid required to saturate one scruple of the alkaline salts mentioned in it: -

A solution of Э j. in Oj. of water, sweetened with sugar that has been rubbed on fresh lemon peel, forms a grateful refrigerent beverage, resembling lemonade; and which is equally useful in febrile and inflammatory complaints. It is said that the crystallized acid is not so useful in scurvy as the juice of the fresh fruit. Citric acid is incompatible in formulae with the alkalies, the alkaline and earthy carbonates and acetates, the earthy and alkaline sulphurets, soaps, and tartrate and bitartrate of potassa.