It occurs as potassium hydrogen oxalate in the wood sorrel, rhubarb, and tomatoes. The insoluble calcium oxalate is found in some stony deposits in kidneys and bladder in the human body.
Preparation of Oxalic Acid.
(1) Manufacture. Oxalic acid is made industrially by heating sodium formate to 400°C.
The sodium oxalate thus formed is dissolved in water and calcium hydroxide added to precipitate calcium oxalate. The solution is filtered and the filtrate treated with the calculated quantity of dilute sulphuric acid to liberate the oxalic acid.
Calcium sulphate precipitates, and oxalic acid is crystallised from the filtrate as the hydrate, COOH)2.2H2O
Laboratory Method of Oxalic Acid. In the laboratory, oxalic acid is made by oxidation of sucrose or molasses with concentrated nitric acid in the presence of vanadium pentoxide as a catalyst.
The -CHOH.CHOH- units present in sucrose molecule are split out and oxidised to oxalic acid.
Properties of Oxalic Acid.
Physical Properties of Oxalic Acid. When crystallised from water, colourless prismatic crystals of oxalic acid dihydrate, (COOH)2. 2H2O, are obtained. The dihydrate melts at 101.5°C, while the anhydrous acid melts at 189.5°C. The hydrate acid becomes anhydrous when carefully heated to 150°C. Oxalic acid is an active poison, depressing the central nervous system and causing malfunction of kidneys.
Chemical Properties of Oxalic Acid. The oxalic acid molecule is made of two carboxyl groups in the direct union. It gives all the usual reactions of COOH group twice. Also, the acid gives some peculiar reactions which involve the cleavage of the weakened linkage between the two highly oxidised carbon atoms.
(1) Formation of Mono-and Di-derivatives. Oxalic acid is a much stronger acid than acetic acid and readily forms two series of salts, esters, acid halides and amides.
(2) The action of Heat. When heated at 150C, it decarboxylates to give formic acid.
(3) Action with Glycerol. Oxalic acid reacts with glycerol to form formic acid or allyl alcohol, depending upon experimental conditions.
(4) Action with H2SO4. When heated with concentrated sulphuric acid, it is decomposed to give carbon dioxide, carbon monoxide and water.
(5) Oxidation. It is readily oxidised, for example with acidified potassium permanganate.
Oxalic Acid Uses.
Uses of oxalic acid.
- For removing ink stains and for bleaching straw for hats, since it reduces brown ferric compounds to soluble and almost colourless ferrous salts.
- As a mordant in dyeing and calico printing.
- In the manufacture of inks and metal polishes.
- For preparing allyl alcohol and formic acid in the laboratory.
- In redox Titrations.