Product name: Formic acid
CAS number: 64-18-6
Molecular formula: CH2O2
EINECS number: 200-579-1

Chemical properties

Colorless fuming flammable liquid with a strong pungent odor. Soluble in water, ethanol and ether, slightly soluble in benzene.

Formic acid is an important chemical raw material

Formic acid is an important chemical raw material. It was first discovered by distilling red ants, hence the name formic acid. Formic acid is widely found in nature, such as in the secretions of red ants, bees, caterpillars, etc., in the leaves and roots of plants, and in fruits. It is the simplest carboxylic acid. Compared with other fatty carboxylic acids, it has a special structure and is more acidic. It is a colorless, transparent, pungent, fuming liquid with a molecular formula of HCOOH. Relative molecular mass 46.03. Relative density 1.2196. Melting point 8.4℃. Freezing point 7℃. Boiling point 100.7℃, 50℃ (15.999×103Pa). Refractive index 1.3714. Flash point 68.9℃. Viscosity 1.784mPa·s. Ignition point 410℃. Miscible with water, ethanol, ether, etc. It forms a high-boiling binary azeotrope with water, containing 77.5% of this product, with an azeotropic point of 107.3℃, which is higher than the boiling points of pure water and pure formic acid. The carbonyl group of this product is directly connected to hydrogen, so that formic acid has both the characteristics of acid and some properties of aldehyde. It can form salts and esters; it can react with amines to form amides; it can add to unsaturated hydrocarbons to form esters. Its reducing property reflects the properties of aldehydes, for example, it can reduce the ammonia solution of silver nitrate (Toulon's reagent) to metallic silver, fade potassium permanganate, reduce sulfur dioxide to thiosulfate ions (SO2→S2O32-), and reduce mercuric nitrate to mercury. Formic acid reacts with concentrated sulfuric acid to dehydrate and generate carbon monoxide. It can also be decarboxylated when heated to 160℃ to generate carbon dioxide and hydrogen; it can also produce decomposition reactions under the catalysis of copper-chromium, platinum, palladium, etc. to generate carbon dioxide and hydrogen. This product is very irritating and corrosive. Inhaling its vapor can irritate the respiratory organs and cause inflammation; contact with the skin can cause blistering of the skin. Chronic poisoning can cause hematuria. The oral LD50 of rats is 1210mg/kg. The maximum allowable concentration in the workplace is 5×10-6. Keep away from fire and heat sources during storage, and isolate from oxidants, alkalis, strong acids, H pore-forming agents and cyanides. Handle with care during loading and unloading, and do not turn it upside down. Prevent water condensation and damage to the packaging. Rinse the leaked materials with water, but wear protective tools when handling. If it comes into contact with the skin, rinse immediately with plenty of water, and apply ointment if necessary. In case of inhalation poisoning, go to the hospital immediately.

The simplest carboxylic acid

Formic acid is the simplest carboxylic acid. It is also called formic acid because it was first obtained by distilling red ants. Formic acid exists in the secretions of ants, bees and caterpillars. The redness, swelling and itching of the skin after being stung by a bee are due to the irritation of formic acid released from the bee sting. Formic acid is also found in nettles, scorpion grass, pine needles and some fruits (such as green grapes) as well as human muscles, blood and excrement. Formic acid is the only acid in fatty acids that has hydrogen atoms attached to the carboxyl group. The electron repulsion force of hydrogen atoms is much smaller than that of hydrocarbon groups, making the electron density of the carboxyl carbon atom lower than that of other carboxylic acids. Due to the conjugation effect, the electrons on the carboxyl oxygen atom are more inclined to carbon, so the acidity of formic acid is stronger than other carboxylic acids in the same series. The structure of formic acid can be regarded as a combination of aldehyde and hydroxyl groups, so the formic acid molecule contains aldehyde groups and has some properties of aldehydes, such as being able to reduce silver ammonia solution to form a silver mirror. It can also fade potassium permanganate solution. These reactions can be used for qualitative identification of formic acid. Formic acid has the properties of general carboxylic acids and can undergo condensation reactions similar to aldehydes. Formic acid has reducing properties because of the aldehyde group in its molecule. It is used as an acidic reducing agent in the textile and printing and dyeing industries to bleach straw hats, leather, etc., and can remove ink spots and rust on clothes. It can coagulate rubber slurry into raw rubber and is used as a coagulant of lactic acid in the leather industry. It is a rubber coagulant. Formic acid is also a raw material for the preparation of oxalic acid. Heating sodium formate to 360°C can dehydrogenate to form oxalate. Formic acid is also a printing and dyeing mordant, a metal surface treatment agent, and a disinfectant preservative.

Purification of formic acid

Formic acid is used as a reducing agent to determine multiple elements such as arsenic and bismuth, and can be used as an organic and inorganic solvent. Method for purifying formic acid:
(1) Atmospheric distillation: Place 500 ml of analytical pure formic acid in a 1000 ml distillation bottle, heat it in a glycerol bath, and control the bath temperature below 110°C. Use atmospheric distillation to discard 50 ml of the front distillate and collect 350-400 ml of the middle distillate.
(2) Mixed distillation
1) Add propyl formate for mixed distillation. The distillate is divided into two phases. Then the propyl formate phase is distilled to obtain pure formic acid. The other phase is a water phase containing 1% formic acid.
2) During distillation, tertiary amines such as trimethylamine, quinoline, and dimethylaniline are added to the formic acid solution for mixed distillation. The water phase is distilled out first and discarded. Continue distillation, and the added amine and formic acid can be separated.

Formic acid Properties

Application in the leather industry

(1) Formic acid is the most acidic of the monocarboxylic acids. It has a loosening effect on collagen fibers but is milder and more uniform than inorganic acids. It is used for pickling in fur processing, and has the characteristics of less damage to fur, gentle effect, fine and uniform fiber dispersion, and soft and plump finished products.
(2) In chrome tanning, formic acid can play a masking role, and its masking effect is stronger than acetic acid.
(3) Formic acid is a strong reducing agent and is easily oxidized and decomposed.
(4) Formic acid has a dyeing and fixing effect on acid dyes, and can make the color of leather rich and bright.
(5) As a fur soaking auxiliary, it has the effect of lowering the pH value of the soaking liquid (5.0-5.5), promoting the water filling and expansion of fur, and inhibiting bacterial growth. Organic acid auxiliary agents such as formic acid and acetic acid are also commonly used for fur that is loose or difficult to soak in water.
(6) It can be used as a pickling material alone, or mixed with sulfuric acid for pickling.
(7) Pickling material in the rapid vegetable tanning process, which accelerates the penetration of tannin extract.
(8) Preparation of sodium formate masking agent. The optimal ratio of sodium formate for chrome tanning masking is: 1 mol sodium formate to 1 mol basic chromium sulfate.
(9) One of the neutralizing agents. In the production of furniture leather, adding 1% to 2% sodium formate during neutralization is conducive to deeper neutralization and maintaining smooth grain surface.
(10) Adjusting the pH value of dye bath and fixing color.
(11) Adding 1 to 2 g/L formic acid to fix color in the last brushing of leather.
(12) Reducing water hardness: Adding 3.2 mL of 50% formic acid (relative density 1.121) to 100 kg of water can reduce the hardness by one degree.
(13) Supplement in the production of regenerated leather, which has a coagulation-promoting effect and the dosage should be below 5% (concentration).
(14) For rinsing white leather, its whitening effect is better than that of hypochlorite and potassium permanganate, but inferior to that of oxalic acid.

Uses

Reducing agent, determination of arsenic, bismuth, aluminum, copper, gold, indium, iron, lead, manganese, mercury, molybdenum, silver and zinc, etc., testing of cerium, rhenium and tungsten, testing of aromatic primary and secondary amines, determination of molecular weight and solvent for crystallization, determination of methoxyl groups, used as a fixative in microscopic analysis, and manufacture of formate.

Production method

(1) Sodium formate method. Use 20%-30% sodium hydroxide solution to absorb refined CO gas at 160-200℃ and 1.4-1.8Mpa to generate sodium formate solution. Then mix the sodium formate solution with an equal amount of formic acid solution and react with dilute sulfuric acid to generate formic acid and sodium formate. Distill to obtain an azeotrope of formic acid and water (containing about 75% formic acid), which is then refined to obtain the finished product.
(2) Methyl formate method. Under the catalysis of sodium methoxide, methanol and CO react at 80℃ and 4Mpa to generate methyl formate. Under the catalysis of acid, methyl formate is hydrolyzed into formic acid and methanol at 90-140℃ and 0.5-1.8Mpa. Formic acid can be obtained after separation, and methanol can be recycled.
(3) Formamide method. The methanol-liquid ammonia solution absorbs CO at 70°C and 32.5Mpa to generate formamide, which is then separated and reacted with an equal amount of 68%-74% sulfuric acid to generate formic acid and ammonium sulfate. The formic acid is evaporated and refined to obtain the finished product.

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