3932. Citric Acid

3932.    Citric Acid. This is an agreeable acid, cooling and antiseptic; 20 grains of citric acid are equivalent to 5 fluid drachms lemon juice. When used for making saline draughts, it is preferable to use bicarbonate of potassa as tho neutralizing alkali. Their respective saturating equivalents will be found in Nos. 80 and 81. With tho bases it forma

3933. To Prepare Citric Acid

3933.    To Prepare Citric Acid. Add

41/2 ounces chalk by degrees to 4 pints lemon juice, heated, and mix; set by, that the powder may precipitate; afterwards pour off the supernatant liquor. Wash the precipitated citrate of lime frequently with warm water; then pour upon it 271/2 fluid ounces diluted sulphuric acid and 2 pints distilled water, and boil for 15 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. The preparation of citric acid has become an important branch of chemical manufacture, from the large consumption of this article in various operations in the arts. In conducting this process some little expertness and care are necessary to ensure success. The chalk employed should be dry, and in fine powder, and be added to the juice until it be perfectly neutralized, and the quantity consumed must be exactly noted. The precipitated citrate of lime should be well washed with water, and the sulphuric acid diluted with 6 or 8 times its weight of water, poured upon it while still warm, and thoroughly mixed with it. The agitation must be occasionally renewed for 8 or 10 hours, when the dilute citric acid must be poured off, and the residuum of sulphate of lime thoroughly washed with warm water, and the washings added to the dilute acid. The latter must then be poured off from the impurities that may have been deposited, and evaporated in a leaden boiler, over the naked fire, until it acquires a specific gravity of 1.13, when the process must be continued at a lower temperature until a pellicle appears upon the surface, This part of the process requires great attention and judgment, as, if not properly conducted, the whole batch may be carbonized and spoiled. At this point the evaporation must be stopped, and the concentrated solution emptied into warm and clean crystallizing vessels, set in a dry apartment, where the thermometer does not fall below temperate. At the end of 4 days the crystals will be ready to remove from the pans, when they must be well drained, redissolved in as little water possible, and, after being allowed to stand for a few hours to deposit impurities, again evaporated and crystallized. When the process has been well managed, the acid of the second crystallization will usually be sufficiently pure; but if this be not the case, a third, or even a fourth crystallization must bo had recourse to. The mother liquors from the several pans are collected together, and, by evaporation, yield a second or third crop of crystals obtained by evaporation as before. Citric acid crystallizes with great case, but in some cases, where all the citrate of lime has not undergone decomposition by the sulphuric acid, a little of that salt is taken up by the free citric acid, and materially obstructs the crystallization. This is best avoided by exactly apportioning tho quantity of the sulphuric acid to that of the chalk used, always remembering that it requires a quantity of liquid sulphuric acid, containing exactly 40 parts of dry acid, to decompose 50 parts of carbonate of lime. Commercial sulphuric acid is usually of the specific gravity of 1.815; it will therefore take exactly 49 pounds of this acid for 50 pounds of chalk. In practice it is found that a very slight excess of sulphuric acid is better than leaving any citrate of lime undecomposed. The first crop of crystals is called " brown citric acid," and is much used by the calico printers. Sometimes a little nitric acid is added to the solution of the colored crystals, for the purpose of whitening them, but in this way a minute quantity of oxalic acid is formed. Good lemon juice yields fully 5 per cent, of lemon acid, or 2 gallons yield about 1 pound of crystals. If the imported citrate of lime be used, a given quantity must be heated to redness, and then weighed, when the percentage of lime present will be ascertained; every 28 pounds of which will require 49 pounds of sulphuric acid of 1.845 (or a corresponding quantity containing exactly 40 parts of dry acid) for its complete decomposition.