The limiting carboxylic dibasic acid with the gross formula C2H2O4 is called, according to the systematic nomenclature, ethanedioic acid. This substance is also known under another, more common name - oxalic acid. It was first obtained by the German chemist F. Weller from tizyan (oxalic acid dinitrile) in 1824. Colorless acid crystals dissolve in water to form colorless solutions. The molar mass is 90.04 g / mol. In appearance it resembles monoclinic colorless crystals. At a temperature of 20 ° C, 8 g of oxalic acid are dissolved in 100 g of water. It is well soluble in acetone, ethyl alcohol and sulfuric ether. Density 1.36 g / cm³. It melts at a temperature of 189.5 ° C, sublimes at 125 ° C, decomposes at 100-130 ° C.
Oxalic acid has all the chemical properties characteristic of carboxylic acids. Its formula: NOOS — UNO. Despite the fact that it belongs to carboxylic acids, it is considered a strong organic acid (3,000 times stronger than acetic acid): C2O4H2 → C2O4H- + H + (pK = 1.27) and further: C2O4H- → C2O42 - + H + (pK = 4.27). The esters and salts of this acid are called oxalates. The oxalate ion C2O42− is a reducing agent. When oxalic acid reacts with a solution of potassium permanganate (KMnO4), the latter is reduced and the solution decolours. It is characterized by a reversible and slowly proceeding reaction of interaction with alcohols (esterification), as a result of which esters are formed: HOOC – COOH + 2HOR ↔ 2H2O + ROOC – COOR.
In industry, oxalic acid is obtained by oxidizing chemical compounds. For example, in the presence of a vanadium catalyst (V2O5), a mixture of nitric (HNO3) and sulfuric (H2SO4) acids oxidizes alcohols, carbohydrates and glycols. A method for oxidizing ethylene and acetylene with nitric acid (HNO3) in the presence of palladium salts of Pd (NO3) 2 or PdCl2 is also used. Oxalic acid is obtained from propylene, which is oxidized with liquid nitrogen dioxide (NO2). There is a good prospect for a method for producing acid by the interaction of sodium hydroxide (NaOH) with carbon monoxide (CO) through an intermediate stage of the formation of sodium formate: NaOH + CO → HCOONa. Then sodium oxalate is formed and hydrogen is released: HCOONa + NaOH → NaOOC — COONa + H2 ↑. From oxalate of sodium in an acidic medium, oxalic acid is obtained: NaOOC — COONa + 2H + → HOOC — COOH + 2Na +.
The main applications of oxalic acid are cleaning or bleaching. Oxalic acid can effectively remove rust, which is why many cleaning products contain this chemical compound. About a quarter of the oxalic acid produced is used as a mordant for dyeing in leather and textile industries. It is also used as a reagent (GOST 22180-76) in analytical chemistry. Ethanedioic acid dihydrate (NOOS — COOH • H2O) according to TU 2431-002-77057039-2006 with a mass fraction of the main substance ≥ 99.3% is used in organic synthesis production processes, for cleaning of rust and scale of metals, for bleaching sections in microscopy. Beekeepers use a solution of oxalic acid with a mass fraction of 3.2% in sugar syrup to combat parasitic ticks. At the completion of marble structures, surfaces are treated with it to densify and give them shine.
Oxalic acid and oxalates are present in many plants, including black tea, found in animals. The main harm to humans is associated with renal failure, which occurs due to the interaction of oxalic acid with calcium, resulting in the precipitation of solid calcium oxalate (CaC2O4) - the main component of kidney stones. Acid provokes pain in the joints due to the deposition of such compounds in them. Oxalic acid can form in the body as a result of the metabolism of ethylene glycol from the environment (for example, an anti-icing agent for treating runways and aircraft at airports, as well as from other anthropogenic sources). Potential problems with oxalates in the human body can be divided into two. The first - an important macrocell, calcium binds with oxalic acid and its deficiency is formed in the cells of tissues and organs. The second is the formation of kidney stones. The largest amounts of oxalic acid are found in spinach, leaves and stems of rhubarb, sorrel, beets, parsley, and green onions.