ACIDUM CITRICUM - CITRIC ACID, H3C6H1O7, H1O=200.

This acid occurs in many plants and acidulous fruits, especially in those belonging to Aurantiaceae or orange family, and is commonly obtained from lemon or lime-juice (cf. vol. i., p. 96).

Preparation

The lemon-juice is first boiled, in order to coagulate the albuminous substances, and then saturated, while hot, with chalk, so that citrate of calcium is formed; this is washed with hot water (it would dissolve in cold), and then treated with sulphuric acid, which forms sulphate of calcium (to be removed by filtration), and the solution concentrated at a moderate temperature, to allow citric acid to crystallize out. (1) 2H3C6H1O7+3CaCO3=Ca32C6H1O7 + 3H1O + 3CO2. (2) Ca32C6H1O7 +3H1SO4=3CaSO4+2H3C6H1O7.

Good lemon-juice yields from 2 1/2 to 10 per cent, of citric acid, the quantity varying because it decreases in proportion to the time that the lemons are kept, and it may quite disappear, separating into glucose, carbonic anhydride, and some acetic acid.

Characters

Occurs in colorless crystals of which the right rhombic prism is the primary form; they are permanent in dry, but become moist in a damp atmosphere; sp. gr. 1.6; taste strongly acid, and almost caustic, but in weak solutions only agreeably acid and refreshing; very soluble in water and in glycerin, less so in spirit, and not at all in ether. Seventeen grains of citric acid (equivalent to about 1/2 oz. lemon-juice) saturate 13 of magnesian carbonate, 15 of the ammoniacal, 20 of the soda, 25 of the potash salt; somewhat less than these quantities are commonly used. A small proportion of citric acid prevents some chemical decompositions, e.g., in the syrupus ferri iodidi.

Absorption And Elimination

Citric acid is readily absorbed by the stomach and intestines, and is excreted in part by the kidneys in the urine in combination with bases. The greater part is oxidized in the system, forming carbonic acid and water. Buchheim and Pietrowski found none in the urine after 30 to 60-gramme doses. According to Bence Jones and Eylandt, the acidity of urine is increased by citric, as it is by tartaric acid, and a deposit of free uric acid may be induced by it.

Physiological Action

Concentrated solutions do not irritate the skin, hence Mitscherlich and others conclude citric to be less irritant than tartaric acid, but abraded surfaces and mucous membranes become irritated by it; and although it is not known to have caused death in man, Husemann considers it, from its effects on animals, to be more poisonous than tartaric acid. Eight to fifteen grammes destroy large rabbits within one hour, with cramp, opisthotonos, dyspnoea, weakened heart-action, and general prostration. Bobrik observed tremor and cramp, difficulty of breathing, slowing of pulse, and lowering of temperature (in rabbits). On dissection the blood was found fluid, and the gastro-intestinal mucous membrane inflamed and ecchymosed.

Synergists

Tartaric and other vegetable acids.

Incompatibles

Alkaline carbonates, acetates, and sulphurets; tartrate of potash.

Therapeutical Action (External)