Acetic Acid

Acetic acid, CH₃-C-OH, methane-carboxylic acid, the second member of the homologous series of monobasic saturated carboxylic acids. It occurs in both animal and plant organisms, partly in free form, partly in the form of salts and complex esters. Free acetic acid is usually found in human feces, and is often found in gastric contents (vomit) in disorders of gastric digestion as a result of fermentation of milk, bread, etc., especially frequently in children. Acetic acid is apparently a normal intermediate product of metabolism and has been found in trace amounts in various organs (spleen, muscles), in the blood in leukemia, in sweat, bile, and urine. It is also formed during the putrefaction of proteins, during fermentation, dry distillation and oxidation of carbohydrates, during the hydrolysis of mucins and other substances containing glucosamine residues, during autolysis of the liver. It is contained in sour wine, in cheese, sometimes in rancid oil. There are assumptions that acetic acid is an intermediate product in the transformation of fatty acids in the body. For practical production of acetic acid: 1) Dry distillation of wood. The resulting pyroligneous water contains up to 10% acetic acid. The distillate is neutralized with lime or soda and after evaporation, acetic acid is obtained in the form of calcium or sodium salts (acetate powder, wood powder). Distillation with concentrated sulfuric (or less often phosphoric) acid gives acetic acid: CH₃.COONa+H₂SO₄ → CH₃COOH+NaHSO₄. In case of insufficient purification, acetic acid obtained by this method contains charred substances, sometimes also formic acid and other reducing substances; during distillation with H₂SO₄, SO₃ is easily formed, which also occurs as a harmful impurity in such acetic acid. 2) Acetic fermentation of alcoholic liquids. Under the influence of enzymes produced by Mycoderma aceti, alcohol is oxidized to acetic acid. The overall reaction is expressed by the scheme: CH₃CH₂OH+O₂ → CH₃COOH+H₂O. In reality, the course of the reaction is more complex; as intermediate products, acetaldehyde (acetaldehyde, CH₃CHO) is obtained, which polymerizes into acetoin (acetylmethylcarbinol), 2CH₃CHO → CH₃CH(OH)COCH₃; the latter, as a result of isomerization into acetic ethyl ester and hydrolysis, gives acetic acid and a restored alcohol molecule: CH₃CH(OH)COCH₃ → CH₃CH₂OCOCH₃ → CH₃CH₂OH+HOOCCH₃. By the presence of acetone, which gives characteristic Voges-Proskauer reactions (eosin-red coloring when oxygen or air is blown through alkaline and containing pepton solutions of acetone) and Le Moigne's reaction (precipitate of dimethylglyoxime-nickel when oxidizing solutions containing acetone, e.g. with bromine, and adding hydroxylamine, sodium acetate and nickel chloride), as well as by the presence of acetic ethyl ester, which has a characteristic odor, one can distinguish fermentation vinegar from vinegar obtained by dilution of synthetic or from dry distillation of wood acetic acid (Jungkunz und Pritzker, Gorovits-Vlasova and Obergard). To obtain acetic acid, 10% alcohol is allowed to slowly flow through beech shavings infected with Mycoderma aceti. Air is blown through these shavings from below, providing the oxygen necessary for oxidation. From the resulting liquid, fractional distillation yields 70-80% acetic acid, sold under the name of acetic essence. 3) Catalytic method of obtaining acetic acid by oxidation of acetaldehyde or acetylene: 2CH₃CH+2H₂O+O₂ → 2CH₃COOH. Anhydrous acetic acid is known under the name of glacial acetic acid (Acid, aceticum glaciale). Sp. gr. 1.0553 (at 15°), melting point +16.5°, boiling point +118.1°. A colorless liquid, with a sour, pungent odor and taste (taste only in highly diluted solutions!), strongly acidic to litmus reaction. Burns with a blue flame. Mixes with water, alcohol and ether. When mixing glacial acetic acid with water, the specific gravity of the solution first increases and then decreases again. Glacial acetic acid burns the skin, causing painful blisters. In commerce, acetic acid containing up to 4% water, sp. gr. up to 1.064, is found under the name Acidum aceticum (concentr.). It is used as a caustic and antiseptic agent: against warts, calluses, rarely for infected wounds, cuts, bites. - Acidum aceticum dilutum, sp. gr. about 1.041, contains 30% acetic acid. Of the derivatives of acetic acid, trichloroacetic acid, CCl₃.COOH, is important for medicine, used as a reagent for the detection and precipitation of proteins and as a caustic agent for removing papillomas, condylomas, telangiectasias, lupus nodules, chancres. A 1% solution is used as a local caustic and antiseptic in lesions of the pharynx, nose, in gonorrhea, etc. Of the salts of acetic acid, lead acetate, Plumbum aceticum, Pb(CH₃COO)₂·3H₂O, Saecharum Saturni, lead sugar, is used in solutions for compresses; internally, up to 0.2 g (!) per dose, in hemoptysis; in combination with precipitated sulfur (prohibited in the USSR) as a harmless hair dye ('regenerator'). A solution of Pb(CH₃COO)₂ can colloidally dissolve a significant amount of lead oxide PbO, forming 'lead acetate'-Liq. Plumbi subacetici. Such a preparation with 18% Pb content is included in the Pharmacopoeia and is used in dilutions (1:50) for compresses (lead compress-Aq. Plumbi). A reagent for CO₂ and CO₃^2- (distinction of simple or freshly distilled water from fresh water), precipitant of proteins, tannins, etc. The previously accepted presence of 'basic lead acetate' (CH₃COO·PbOH) has been refuted: the preparation contains a colloidal solution of Pb(OH)₂ along with lead acetate (Obergard). Similarly, in Burow's solution (Liq. Burowi, Liq. Aluminii acetici) there is Al(OH)₃ in a colloidal solution stabilized by acetic acid. Burow's solution is used for compresses. Iron acetate in the form of 'Klaproth's drops' was used as an iron preparation - without effect (Starkenstein). Salts of iron and aluminum are used in the textile industry as mordants, copper salts are used for preparing paints. Esters of acetic acid are very common in production: methyl acetate, CH₃COOCH₃, amyl acetate, C₅H₁₁COOCH₃, etc. Inhalation of their vapors leads to occupational diseases. Acetic acid serves as the starting material for the preparation of complex pharmaceutical preparations, such as antifebrin, aspirin, phenacetin, etc. For the detection of acetic acid, the wine-red coloring of neutral solutions of its salts by ferric chloride, the odor of acetic ethyl ester formed when heating salts of acetic acid with a mixture of equal volumes of concentrated sulfuric acid and alcohol, the odor of cacodyl appearing when heating dry acetate salts with arsenious anhydride can serve, although other fatty acids give similar reactions. Acetic acid and its solutions (vinegar, 4-6% aqueous solution of acetic acid) are widely used as a growth-inhibiting agent for microorganisms in the food industry (marinades, canned foods). A. Kuzin, I. Obergard. Detection of acetic acid in forensic-chemical cases. Acetic acid is almost never the object of forensic-chemical research of internal organs of the corpse. The picture of the autopsy, the circumstances of the case decide the question of poisoning. On the other hand, the presence of acetic acid in gastric contents is a frequent phenomenon, therefore its detection cannot indicate former poisoning. For the detection of acetic acid in other objects, the following can serve: 1) the reaction of formation of acetic ethyl ester when acting with ethyl alcohol and excess concentrated sulfuric acid; 2) the formation of a complex salt of red-brown color when mixing a neutralized (with soda) solution of acid with ferric chloride.