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Acids. — Acidum Citricum.

PART in.

Officinal preparations. Tinctura Camphorce composita, L. D.Tinctura Opii ammoniata, E.

ACIDUM CITRICUM, Lond. Citric Acid.

" Take of lemon juice a pint; prepared chalk, an ounce,or a quantify sufficient to saturate the juice; diluted sulphuricacid, nine fluid ounces. Add the chalk by degrees to the lemonjuice heated, and mix them; then pour off the liquor. Washthe citrate of lime which remains, in repeated portions ofwarm water, and then dry it. On the dried powder pourthe diluted sulphuric acid, and boil for ten minutes; expressthe liquor-strongly through a linen cloth, and filter it throughpaper. Evaporate the filtered liquor with a gentle heat, sothat crystals may form as it cools. To obtain the crystalspure, dissolve them in water a second and a third time; filtereach solution, boil it down, and put it apart to crystallize."

Si/n. Acidc Citrique (F.), Acido Citrico (I.)

This process, which was contrived by Scheele, will sel-dom require to be performed by the apothecary, as the crys-tallized acid is now manufactured very pure, and sufficientlyreasonable on the great scale.' The theory of the process isvery simple. The lime of the chalk unites with the citric acidthat exists ready formed in the lemon juice, and produces aninsoluble citrate of lime, which precipitates united with someof the mucilaginous and extractive matter of the juice. Theseare separated by repeated washings; and the sulphuric acid,which is added to the dried citrate, decomposing it, owing tothe superior affinity of the sulphuric acid for lime, a sulphateof lime forms while the citric acid is disengaged. The crystalsof the first crystallization are dark coloured; which is partlyowing to a portion of mucilage that still adheres to the criticacid, and partly to the excess of sulphuric acid acting on thecitric acid and decomposing a portion of it. The repeatedcrystallizations free the crystals from this dark colour; but asit is of some importance to avoid any hurtful excess of sulphu-ric acid, and as the strength of lemon juice is variable and un-certain, it is better to determine the quantity of acid requiredby the quantity of chalk employed. For this purpose a por-tion of the sulphuric acid intended to be used must be pre-viously saturated with the chalk, and the weight of the portionemployed accurately ascertained; by the knowledge of whichthe exact quantity of sulphuric acid required to decompose thecitrate may be found. According to the experiments of Proust 9,94 ounces of lemon juice saturate 4 ounces of chalk with citricacid, and produce 7J ounces of dry citrate, which require fortheir decomposition, and the complete saturation of the lime

1 The principal manufacturer in London, is Mr. Coxwell, of Fieet Street.' Journal dc Physique, lii. 366.

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