What Is Oxalic Acid?
Oxalic acid, known as ethanedioic acid in IUPAC nomenclature, is a dicarboxylic acid comprising two carbon atoms each bonded to a carboxyl group. This acid is commonly found in plants in its salt form and can be synthesized through various methods, including the reaction of sodium hydroxide with carbon monoxide to form sodium formate, which is then converted to calcium salts and reacted with sulfuric acid. Additionally, oxalic acid can be produced by oxidizing ethylene glycol or glyoxal with potassium dichromate.
Uses of Oxalic Acid
Oxalic acid has wide-ranging applications, serving as a raw material for dyes, a bleaching agent for wheat straw and cotton, and as a precursor for various chemicals.
1. Food Industry
In the food industry, it is utilized in producing glucose syrup and refining vegetable oils.
2. Medical Field
In pharmaceuticals, it aids in manufacturing persistent sulfa drugs, cerium oxalate, amino acid preparations, and alpha-keto acids.
3. Metal Processing Field
The metal processing industry employs oxalic acid for chemical polishing and pickling, vehicle and ship cleaning, radiator cleaning (for its rust removal and descaling properties), as a lubricant in cold-drawing stainless steel, in anodizing processes, and in refining rare earth metals. Furthermore, it is used as a standard for acid-alkali and redox titrations, thanks to the stability of its pure dihydrate crystals.
Properties of Oxalic Acid
Oxalic acid dissolves readily in cold water and ethanol, and to a lesser extent in hot water, but not in organic solvents like ether. Although often classified as a weak acid, it exhibits strong acidity in solution, demonstrating significant ionization.
Upon heating, oxalic acid anhydride decomposes, releasing carbon monoxide, carbon dioxide, and formic acid. Under certain conditions, such as exposure to sulfuric acid, the resultant formic acid further decomposes into water and carbon monoxide. Oxalic acid is hygroscopic, forming a dihydrate in moist air, which can be reverted to an anhydrous form by heating or using a desiccator.
Structure of Oxalic Acid
Oxalic acid, the simplest dicarboxylic acid, is denoted chemically as HOOC-COOH. It dissociates in water to form hydrogen oxalate ions H(COO)2− in the first step and oxalate ions (COO)22− in the second step, with the structure of these ions being planar and featuring resonant carbon-oxygen bonds.
Other Information on Oxalic Acid
1. Compounds Containing Oxalic Acid Ion
Oxalic acid forms salts known as oxalates, which are ionic crystals, and hydrogenoxalates, which are acidic salts. While oxalates of alkali metals, aluminum, ammonium, and iron (III) are water-soluble, many oxalates of alkaline earth metals are not. Iron (III) oxalates decompose gradually in water, forming iron (II) oxalate, whereas silver oxalates decompose explosively upon heating.
2. Oxalic Acid in Nature
Abundantly present in plants, oxalic acid salts like sodium hydrogen oxalate occur in families such as Taxaceae, Catabaceae, and Acacaceae. Conversely, families like Araceae contain insoluble oxalate salts such as calcium oxalate, whose needle-like crystals can cause skin irritation upon contact.