Formic Acid : Preparation, Reaction, uses

Formic Acid

Formic acid is also known as methanoic acid. Formic acid is a pungent corrosive liquid, m.p. 8.4^oC, b.p. 100.5^oC, miscible in all proportions with water, ethanol and ether. It forms salts which, except for the lead and silver salts, are readily soluble in water. Formic acid is a stronger acid than any of its homologues.

Preparation of Formic acid

Formic acid is prepared industrially by heating sodium hydroxide with carbon monoxide at 210^oC, and at a pressure of 6-10 atmospheres:

NaOH   +   CI    ⟶     HCO₂Na

An aqueous solution of formic acid is obtained by distilling the sodium salt with dilute sulphuric acid:

2HCO₂Na   +   H₂SO₄     ⟶     2HCO₂H   +   Na₂SO₄

Anhydrous formic acid is obtained from the aqueous solution (70-77 per cent) by the addition of butyl formate followed by distillation. The first fraction is an azeotrope of ester and water, and then the excess of ester is removed from the formic acid by fractionation.

The most convenient laboratory preparation of formic acid is to heat glycerol with oxalic acid at 100-110^oC. Glyceryl monoxalate is produced, and decomposes into glyceryl monoformate (monoformin) and carbon dioxide. W hen the evolution of carbon dioxide ceases, more oxalic acid is added, whereupon formic acid is produced:

The distillate contains formic acid and water. The aqueous formic acid solution cannot be fractionated to give anhydrous formic acid because the boiling point of the acid is 100.5^oC. the procedure adopted is to neutralize the aqueous acid solution with lead carbonate, and concentrated the solution until lead formate crystallizes out. The precipitate is then recrystallised, dried, and heated at 100^oC in a current of hydrogen sulphide:

(HCO₂)₂Pb   +   H₂S     ⟶   2HCO₂H   +   PbS

The anhydrous formic acid which distils over contains a small amount of hydrogen sulphide, and may be free from the latter by adding some dry lead formate and redistilling. This procedure for obtaining the anhydrous acid from its aqueous solution can only be used for volatile acids.

Formic acid is dehydrated to carbon monoxide by concentrated sulphuric acid and when heated under pressure at 160^oC, is decomposed into carbon dioxide and hydrogen.

CO   +   H₂O     (H₂SO₄) ⟵      HCO₂H     (Heat) ⟶     CO₂   +   H₂

The same decomposition takes place at room temperature in the presence of catalyst such as iridium, rhodium, etc.

When metallic formats are heated with an alkali, hydrogen is evolved.

HCO₂Na   +   NaOH     ⟶    H₂   +   Na₂CO₃

When calcium or zinc formate is strongly heated, formaldehyde is produced:

(HCO₂)₂Ca     ⟶     HCHO   +   CaCO₃

When sodium or potassium formate is rapidly heated to 360^oC, hydrogen is evolved and the oxalate is formed:

2HCO₂Na     ⟶     (COONa)₂   +   H₂

Formic acid forms esters, but since it is a relatively strong acid it is not necessary to use a catalyst; refluxing 90 per cent formic acid with the alcohol is usually sufficient.

Formic acid differs from the rest of the members of the monocarboxylic acid series in being a powerful reducing agent; it reduces ammoniacal silver nitrate and the salts of many of the heavy metals, e.g., it converts mercuric chloride into mercurous chloride.