Oxalic acid is an organic compound with the chemical formula C2H2O4. It is a dicarboxylic acid, and is a reducing agent. Oxalic acid exists in two forms depending on the hydrogen bonding: a chain-like structure or a sheet-like structure. The anhydrous form of oxalic acid is both acidic as well as hydrophilic. Oxalic acid is a colorless liquid and it is crystalline in structure. It is also soluble in water.
It occurs in nature in wood-sorrels, spinach (broccoli, cabbage, and Brussels sprouts), Rhubarb leaves and parsley. Some bacteria produce oxalic acid through the oxidation of carbohydrates. Plants that belong to a genus called fenestraria produce crystalline oxalic acid to help in transmitting light for photosynthesis.
Oxalic acid is manufactured by the oxidation of carbohydrates or glucose using nitric acid or air, in the presence of a catalyst (vanadium pentoxide) for commercial use. The raw materials used for the production can be molasses/cellulosic wastes and nitric-sulphuric acid mixture. The process starts with a batch process, where the nitric acid is added to the molasses or cellulosic wastes. This mother liquor is then fed to a reactor and sulphuric acid is added to increase the temperature of the reactor. There are cooling coils present, through which a coolant is circulated to reduce the temperature. Then nitric acid is added into the reactor, followed by the gradual addition of sugar.
The rate of adding sugar is increased gradually, to avoid rapid increase in the temperature. The oxalic acid that is formed, is centrifuged and then purified by recrystallization using water. The recrystallized product is further centrifuged and dried. The reaction can be written as below:
C6H12O6 + 6 HNO3 → 3 H2C2O4 + 6 NO + 2 H2O
As oxalic acid is an iron complexing agent, it is used in the removal of rust during metal treatment, used as a commercial rust remover to remove rust from tubs, sinks, plumbing fixtures, etc.
Oxalic acid is used as a mordant for dyeing and printing applications. It can substitute acetic acid as coloring mordant for fast pigment dyestuff.
Leather and Textile Industry
Oxalic Acid is used as a bleaching agent in the textile industry. It is used as a mordant in the dyeing process. It aids by forming a coordination complex with dyes and setting the dyes on fabrics and cellulosic. Oxalic acid is also employed in the leather tanning process.
It is used as a precipitating agent for Rare-earth metal.
Paper and Pulp Industry
It is used as a bleaching agent to bleach the pulp during paper making.
Oxalic acid is used by pharmaceutical industry to manufacture antibodies such as tetracycline, phenobarbital, oxytetracycline and aureomycin. It is also used in the manufacturing of borneol, which is an important ingredient for Chinese medicines, precursor to perfume, antiseptics, as well as cosmetics and personal care products.
It is used in waste water treatment to remove calcium from water, in marble polishing as a grinding agent and to sterilize home brewing equipment.