Formic Acid

Formic acid is a fuming liquid. Its chemical formula is HCOOH. It is also known as methanoic acid (IUPAC name). It is the first member in a series of homologous carboxylic acids. It is found naturally in the venom of ants and bees and is an important intermediate in chemical synthesis. The itching or burning sensation that we feel when bit by wasps, honey bees, etc., is caused by the entry of formic acid into our body. Further, it is also present in small amounts in sweat, urine, and meat extract.

Formic Acid Structure

Formic acid has a simple structure wherein it has a single carbon atom that's why it is named methanoic acid. In its molecule, a carbon atom is bonded to hydrogen by a single bond and bonded to oxygen by a double bond and bonded to another oxygen atom by a single bond, this oxygen in turn is bonded with a hydrogen atom. In short, a single carboxylic acid group is bonded to a single hydrogen atom.

Physical Properties of Formic Acid

• It appears as a thick transparent liquid.
• Its melting point is very low, which is 8.4 degrees Celsius.
• It has a boiling point of 100 degrees Celsius, only a little higher than water.
• It has a density of 1.22g/cm3, which is not very dense for a liquid.
• Because formic acid is the first member of the homologous series, its molecular weight is also not very high. It has a molecular weight of 46.03 g/mol.
• It smells pungent and irritating.
• It is soluble in water and ethanol in all proportions as it forms hydrogen bonds with water and ethanol molecules.
• In a gaseous state, it does not follow the ideal gas law due to the formation of hydrogen bonds.
• In the vapour phase, it is made of hydrogen-bonded dimmer rather than discrete molecules.
• In solid-state, it is made of a network of hydrogen-bonded molecules.

Chemical Properties of Formic Acid

• It turns blue litmus red.
• It is a hydrogen bond donor as it donates the hydrogen atom of the hydrogen bond.
• Its conjugate base is formate.
• It has only covalent bonds all of which are formed by carbon atoms.
• It can reduce mercuric chloride into mercurous chloride to form a white precipitate; the reaction is as follows;

HCOOH + 2HgCl₂ → Hg₂Cl₂ + 2HCl + CO₂

• It reacts with phosphoric pentachloride to form formyl chloride, phosphoryl chloride, and hydrogen chloride. The reaction takes place as follows;

HCOOH + PCl₅ → HCOCl + POCl₃ + HCl

Preparation of Formic Acid

Industrial preparation:

i) By the reaction of sodium formate and sulphuric acid:

Sodium formate is produced from carbon monoxide and sodium hydroxide, the reaction takes place as follows;

CO + NaOH + H₂O → HCOONa (sodium formate) + H₂SO₄ → HCOOH (formic acid)

ii) By the hydrolysis of methyl formate

Methyl formate is formed when methanol combines with carbon monoxide in the presence of a strong base, as shown below;

CH₃OH + CO → HCO₂CH₃ (Methyl formate)

The methyl formate undergo hydrolysis to form formic acid as shown below;

HCO₂CH₃ + H₂O → HCOOH + CH₃OH

iii) By the reaction of formamide with sulphuric acid

In some cases, methyl formate is treated with ammonia to form formamide, which is later hydrolysed with sulphuric acid to form formic acid as shown below:

HCO₂CH₃ + NH₃ →HC(O)NH₂ + CH₃OH

2HC(O)NH₂ + 2H₂O + H₂SO₄ → 2HCO₂H + (NH₄)₂SO₄

Laboratory Method

In this method, it is prepared from glycerol and oxalic acid. The steps are described below:

i) Add 40 gm of oxalic acid crystals and 50 ml of anhydrous glycerol in a distillation flask.

ii) Heat the flask slowly at 100 degrees Celsius to 110 degrees Celsius using a sand boiler.

iii) The aqueous solution of formic acid is distilled and then collected in the receptor. The glycerol is left behind in the distillation flask. So, formic acid can be prepared again by adding oxalic acid to the distillation flask and heating it to the required temperature.

The reactions involved in the above method are as follows;

i) Glycerol and oxalic acid react to form glycerol mono oxalate.

ii) When glycerol mono oxalate is heated to 100 degrees Celsius, one carbon dioxide molecule separates and it is converted into additive glycerol monoformate.

iii) Glycerol monoformate reacts with water produced in the first step of this reaction, and oxalic acid crystals to form Formic acid and glycerol.

Uses of Formic Acid

• It is used in combination with citric acid to remove deposits of iron oxide.
• It acts as a reducing agent to reduce potassium dichromate and sodium.
• It is used by beekeepers as a miticide against the Varroa mite.
• It is also used in the textile and leather industry.
• Some formate esters used as artificial flavourings
• It is the main ingredient in some households like limescale remover and toilet bowl cleaner.
• In laboratories, it is used as a solvent modifier, e.g. during the HPLC separation of proteins and peptides.
• It also serves as a source of hydride ion and hydrogen.
• It is also a source of carbon monoxide and formyl group in the laboratory.
• It is also used as a preservative and antibacterial agent for animal feed.
• It is used as a volatile pH scale modifier in capillary electrophoresis.
• It is used for producing formic acid fuel cells and hydrogen fuel cells.
• It is also an effective pesticide, which protects crops from a large variety of pests.
• Further, owing to its anti-bacterial properties widely used in the agriculture industry.

Health Hazards

Dilute formic acid is not toxic, it acts as a food additive. But, in concentrated form, it is highly corrosive and harmful. For example, if someone happens to inhale its fumes, it may cause irritation of the mucous membrane and may also cause burns and blisters on the skin. Further, exposure to formic acid for a long time may damage the kidney.