Acidum Formicum.—Formic Acid.
FORMULA: H2CO2=HCHO2. MOLECULAR WEIGHT: 45.89.
Source and History.—This acid was first obtained by the distillation of ants, by Samuel Fisher (Lavoisier). A. S. Marggraff examined it in 1749, and Ardwissan and Oehrn, of Leipsic, in 1777, fairly establishing its identity as a definite compound. However, in 1802, Fourcroy and Vauquelin endeavored to prove that it was a mixture of acetic and malic acids; but their opinions were refuted by Suersen and Gehlen. Berzelius (1817) first attempted to determine its composition (Annals of Philosophy, ix, p. 107), by burning formiate of lead and chlorate of potassium in a glass tube, thus deciding that it contained oxygen, hydrogen, and carbon. Formic acid is found in certain caterpillars, and doubtless, the "bombic acid" Lavoisier mentions as being obtained from silk-worm larva that are changing to chrysalides, was impure formic acid.
The "spirit of magnanimity," of Hoffman, was made by extracting red ants with spirits of wine. Later, Mr. F. Will has shown that the fluid in the hairs of a species of caterpillar, which causes inflammation of the skin when handled, and the poisoning by the sting of some insects, is due to the formic acid present. It has also been demonstrated that the stinging hairs of the nettle, Urtica urens and Urtica dioica, contain this acid. Formic acid is also found in Pine-tree leaves, and in the blood, bile, urine, perspiration, and muscular tissues of man.
Preparation.—Formic acid derived its name from the fact that it was obtained by distillation of red ants (Formica rufa, Linné). Doebereiner discovered, in the early part of the present century, that it could be prepared artificially by the distillation of tartaric acid, water, sulphuric acid, and black oxide of manganese. At present, it is known that many organic bodies yield this acid, when distilled with black oxide of manganese and sulphuric acid, or with chlorinated lime, or with bichromate of potassium; but decidedly the best process is that based upon the principle discovered by Berthelot (Comptes Rendus, March 1856), that oxalic acid and glycerin will produce it, when heated together. 4 parts each of oxalic acid and glycerin are to be mixed with 1 part of water, and a heat of 100° C. (212° F.) applied 12 or 15 hours, or until the acid is decomposed. The residuum is now to be mixed in a still with its bulk of water, and distilled. It is then again mixed with the same quantity of water, and distilled, the operation being repeated, if the formic acid remains in amount to justify. In this process monoformate of glycerin is formed as an intermediary product, which, upon distillation with water, splits into glycerin and formic acid, as follows:
The distillates are to be mixed, neutralized with carbonate of lead, then evaporated to dryness, and distilled with sulphuric acid if desired pure; generally, however, the diluted acid is preferable. Another process was suggested by the Professors Rogers, in 1847, in which sugar was added to a solution of bichromate of potassium, and distilled, while sulphuric acid was slowly added. This requires great care, however, as, according to our experience, if the acid is allowed to enter too rapidly, the reaction becomes very violent, and may occasion disastrous effects. Besides, if distillation is conducted beyond a certain point, sulphurous acid contaminates the distillate.
Description.—Formic acid is represented by the formula HCHO2, thus a molecule contains the elements of water, H2O, and carbonic oxide, CO. It is a colorless, strongly acid liquid, when concentrated capable of removing the cuticle. It does not char when heated, even with sulphuric acid, but in the latter case splits up into water and carbonic oxide. It dissolves freely in water, glycerin and alcohol, and forms salts soluble in water. When ignited, the vapor burns with a blue flame. Its odor is pungently acid, and its taste a sharp, biting sour. The salts of gold, platinum, mercury and silver are reduced when heated with formic acid, and the metals deposited. Anhydrous formic acid crystallizes below 0° C. (32° F.), and, according to Attfield, boils at 105° C. (221° F.). The experiments of Jardin show that the molecule of formic acid is incapable of furnishing carbon assimilable to even the simplest cellular organisms, and solutions of formiates remain perfectly clear many months.
Action, Medical Uses, and Dosage.—Formic acid applied to the skin acts as an irritant, and produces a violent, burning pain, and may vesicate. If kept in contact with the tissues, it will even cause ulceration. Gastro-intestinal inflammation and bloody urine were the results of its administration to rabbits. In man it acts as an excitant to the circulation, and increases the renal secretions, sometimes causing the passage of clouded, offensive urine. Moderate doses induce hurried respiration and rise of temperature, while larger doses cause rapid breathing, and lower the temperature. If administered at all, it should be largely diluted.
Formic acid is rarely employed in medicine; it was, at one time, used externally as a local irritant, in sluggish capillary circulation, in certain painful affections, and in enfeebled or paralytic conditions of the limbs. The German Pharmacopoeia has a "spirit of ants" [Spiritus Formicarum=Formic acid (2), water (26), alcohol (70)], used in chronic rheumatism, etc., in doses of from 10 drops to a fluid drachm; also as an external rubefacient. It may also be made by macerating, for 2 days, 1 part of fresh red ants in 1 1/2 parts, each, of alcohol and water, and then distilling off 2 parts. It forms a clear, acid fluid, yielding feathery crystals when mixed with 1/20 part of subacetate of lead solution. Combined with ammonium (formiate of ammonium), the salt formed has been tried in chronic paralytic affections, and even in epilepsy, but without success; the dose is 4, 5, or 6 grains.