Table of Content
Introduction to Formic Acid
Formic acid is a weak organic acid. Its molecular formula is HCOOH. According to IUPAC, it is termed as methanoic acid. Laboratory preparation of formic acid involves the reaction of oxalic acid crystals with glycerol.
- Formation of glycerol monoformate: Firstly, glycerol is treated with oxalic acid crystals at about 120oC or 393 K to form glycerol monoxalate. Glycerol monoxalate decomposes itself to give glycerol monoformate.
- Hydrolysis of glycerol monoformate: The glycerol monoxalate thus formed reacts with fresh water [obtained from oxalic acid crystals, (COOH)2.2H2O] to produce formic acid.
In this reaction, glycerol is recovered back which can be used over again and again. Hence glycerol acts as catalyst.
The mixture of about 40 g oxalic acid crystal and 50 mL anhydrous glycerol is taken in a distillation flask which is placed over a sand bath. The flask is provided with the thermometer and the delivery tube is surrounded with a condenser. Upon heating the mixture at about 120oC, the reaction initiates with the evolution of CO2. When the evolution of CO2 stops, then the flask is cooled and fresh oxalic acid crystals are again added in the flask. The flask is again heated upto 120oC to form aqueous formic acid. Finally it is collected in reciever.
Preparation of anhydrous formic acid
Formic acid is soluble in water and forms aqueous formic acid during its preparation. Hence we must dehydrate it to get anhydrous formic acid in lab as follows:
The aqueous formic acid is first neutralized by lead carbonate (PbCO3) or litharge(PbO) to get lead formate. The solution of lead formate thus formed is filtered and concentrated to get lead formate crystals.
The crystals of lead formate are filled in the inner tube of condenser. One end of the condenser is fitted with hydrogen sulphide gas generator and the other end is fitted with a buchner flask guarded with a calcium chloride tube as shown in the above figure.
H2S gas is passed through he condenser while steam is passed through its outer jacket. The anhydrous formic acid collects into the buchner flask.
- We cannot subject aqueous f
ormic acid to fractional distillation to remove waterbecause boilingpoint of formic acid is nearly equal to that of water. (Boiling point of water :100oC; Boilingpoint of formic acid :100.5oC)
- Dehydrating agents like Conc. H2SO4, KOH, P2O5
etccannot be used as formic acid reacts with them.
General properties of formic acid
- Molecular mass: 42.03 g/mol
- State: Liquid due to intermolecular H-bonding
- Odor: Sharp pungent smell
- Solubility: Soluble in water and most of the polar organic solvents
- Melting point: 281 K
- Boiling point: 374 K
- Molecular shape: Planar
- pH: Acidity (3.77)