citric acid(redirected from Acid of lemon)
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2-hydroxy-1,2,3-propanetricarboxylic acid,HO2CCH2C(OH)(CO2H)CH2CO2H, an organic carboxylic acid containing three carboxyl groupscarboxyl group
, in chemistry, functional group that consists of a carbon atom joined to an oxygen atom by a double bond and to a hydroxyl group, OH, by a single bond. Carboxylic acids are compounds whose molecules contain a carboxyl group that is joined to a hydrogen atom, an
..... Click the link for more information. ; it is a solid at room temperature, melts at 153°C;, and decomposes at higher temperatures. It is responsible for the tart taste of various fruits in which it occurs, e.g., lemons, limes, oranges, pineapples, and gooseberries. It can be extracted from the juice of citrus fruits by adding calcium oxide (lime) to form calcium citrate, an insoluble precipitate that can be collected by filtration; the citric acid can be recovered from its calcium salt by adding sulfuric acid. It is obtained also by fermentation of glucose with the aid of the mold Aspergillus niger and can be obtained synthetically from acetone or glycerol. Citric acid is used in soft drinks and in laxatives and cathartics. Its salts, the citrates, have many uses, e.g., ferric ammonium citrate is used in making blueprintblueprint,
white-on-blue photographic print, commonly of a working drawing used during building or manufacturing. The plan is first drawn to scale on a special paper or tracing cloth through which light can penetrate.
..... Click the link for more information. paper. Sour salt, used in cooking, is citric acid.
a tribasic organic hydroxy acid widely distributed in living nature. Citric acid is readily soluble in water and alcohol and poorly soluble in most organic solvents. Its melting point is 153°C; its structural formula is
At 175°C one molecule of water is split off, converting citric acid into aconitic acid. Citric acid forms monosubstituted, disub-stituted, and trisubstituted salts known as citrates. Sodium salts of citric acid are readily soluble in water, while tribasic calcium salts are poorly soluble in water, a fact used in extracting citric acid from solutions.
Citric acid is an essential link in the system of biochemical reactions of cell respiration. Along with other components of the tricarboxylic acid cycle, small quantities of citric acid are found in the mitochondria of all cells. Another function of citric acid involves maintenance of the acid-base balance and the ion composition in an organism. Unlike animals, a number of plants and microorganisms are capable of accumulating large quantities of citric acid (5–6 percent in lemon juice, 3–14 percent in makhorka [Nicotiana rustica] leaves, up to 10 percent in the culture fluid of the fungus Aspergillus niger).
Citric acid is used in the food-processing industry, in medicine (for blood preservation), and in photography; it is also used in various dyeing processes. In industry it is prepared from makhorka (after nicotine extraction) or, more often, by the fermentation of sugar or molasses with the aid of Aspergillus niger or other fungi.
E. P. SEREBRIAKOV