Oxalic acid, also known as oxalic acid, is the simplest dibasic acid with the molecular formula HOOCCOOH. There are only two carbon atoms in the molecule. It is widely found in nature, especially in plants, such as herbs, rhubarb, sorrel, spinach, etc., and often exists in the form of potassium salt. In the urine of humans or carnivores, oxalic acid occurs as a calcium salt or as oxaluric acid HOOCONHCONH2. In addition, calcium oxalate can also be found in kidney or bladder stones.
The melting point of anhydrous oxalic acid is 189.5°C; it begins to sublimate below 100°C and sublimates rapidly when it reaches 125°C; it is easily soluble in water and soluble in ether. Commercial oxalic acid contains two molecules of crystal water; it is colorless crystal; melting point is 101.5°C. It will lose crystal water when heated to 100°C; it is slightly soluble in ether. The two carboxyl groups in the oxalic acid molecule are directly connected and have some special properties. For example, oxalic acid has reducing properties and can reduce potassium permanganate to divalent manganese. This reaction is used in quantitative analysis as a method to determine the concentration of potassium permanganate. Oxalic acid can also be used as a bleaching agent in fiber, oil and tanning industries, also taking advantage of its reducing properties. Oxalic acid undergoes decarboxylation and dehydration when heated, eventually producing carbon dioxide, carbon monoxide and water. Oxalic acid can form water-soluble complexes with many metals.
The industrial method of producing oxalic acid is to use the interaction between carbon monoxide and sodium hydroxide to first generate sodium formate, which is then rapidly heated to 300°C to convert into oxalic acid. , and release oxygen. If carbohydrates such as sawdust are heated together with concentrated sodium hydroxide aqueous solution at 240-285°C, sodium oxalate can also be generated. Under the catalysis of vanadium, carbohydrates are oxidized by concentrated nitric acid, and the final product is also oxalic acid. The nitrogen oxides formed during the reaction can be converted into nitric acid and recycled.
In industry, oxalic acid can be used to remove rust accumulated in condensation systems. It can also be used as a cleaning agent for rust and ink stains and a polishing agent for metals. Antimony oxalate can be used as a mordant. Ferric ammonium oxalate is a blueprint-printing agent. Calcium oxalate is insoluble in water, and this property is used in quantitative analysis to determine the content of calcium or oxalic acid. Oxalic acid can also be used as a precipitant for rare earth elements to extract and purify rare earth elements.
[Related chemical reactions]
In nature, oxalic acid is usually found in the form of salts in many plant cell membranes. In the past, sawdust and strong alkali were fused together in industry at 240-250°C. Oxalate was first produced, and then oxalic acid was obtained through acidification. Later, the sodium formate dehydrogenation method was used to produce oxalic acid. Industrially, carbon monoxide (such as yellow phosphorus production tail gas) is absorbed by caustic soda to produce sodium formate. The latter is dehydrogenated at 380°C to obtain sodium oxalate, which is then treated with lime and sulfuric acid to produce oxalic acid. The nitrogen oxides formed during the reaction can be converted into nitric acid and recycled.
The reaction formula is:
[1]CO+NaOH─→HCOONa
[2]2HCOONa─→(COONa)2+H2
[3](COONa)2+Ca(OH)2─→(COO)2Ca+2NaOH
[4](COO)2Ca+H2SO4─→(COOH)2CaSO4
It has reducing properties and can quantitatively reduce heptavalent manganese to divalent manganese. The reaction equation is as follows: 5C2H2O4+2KMnO4+3H2SO4→ K2SO4+2MnSO4+8H2O+10CO2. This property is often used for quantitative analysis of permanganate.
Phosphorus oxychloride can be generated by reacting with phosphorus pentachloride. The reaction equation is as follows: C2H2O4+PCl5→POCl3+CO2+CO+2HCl
Can react with many metals to form oxalates. Except for alkali metal salts and ferrous iron salts, other oxalates are insoluble in water. Although some metal salts are poorly soluble in water, they can form water-soluble complexes.
Fe2(C2O4)3+3K2C2O4+6H2O→ 2K3[Fe(C2O4)3]·6H2O
When alkali metal and alkaline earth metal oxalates are heated, they lose carbon monoxide and form carbonates. The carbonates continue to be heated and decompose into oxides and carbon dioxide. Oxalates of nickel, cobalt, and silver are heated to form metals rather than metal oxides.
The structure of oxalic acid is that two carboxyl groups are directly connected. Therefore, among the saturated dibasic acids, it is the most acidic and can easily be oxidized into carbon dioxide and water. , this property of oxalic acid is often used to calibrate potassium permanganate solution in quantitative analysis.
Oxalic acid can react with certain metals to form complex ions that are soluble in water. For example: Fe2(C2O4)3+3K2C2O4+6H2O—→ 2K3〔Fe(C2O4)3〕·6H2O
Therefore, oxalic acid can be used to remove traces of rust or blue ink. Calcium oxalate is poorly soluble in water, so oxalic acid is often used to quantitatively measure calcium.
Oxalic acid is found mostly in the form of calcium and potassium salts in many plant cells. However, in industrial production of large quantities of oxalic acid, carbon monoxide and sodium hydroxide are first used to generate sodium formate, and then sodium formate is heated to obtain oxalic acid.
(1) Oxalic acid is easily oxidized to produce carbon dioxide and water, so it can be used as a reducing agent. Such as reducing hexavalent chromium in chromate to trivalent chromium.
(2) The two carboxyl groups of oxalic acid are directly connected, its acidity is stronger than other dicarboxylic acids, and the capping effect of its acid radical is stronger than that of other organic acids.
(3) Oxalic acid can form a stable chelate with chromium, which can easily squeeze out the collagen carboxyl groups from the complex between chromium and collagen carboxyl groups. This is the so-called “detanning” or “detanning” effect.
(4) It can form a water-soluble complex with iron to remove rust.
Mix sawdust and potassium hydroxide.Oxalic acid can also be produced by melting.
Oxalic acid is also called dicarboxylic acid. There are anhydrate and dihydrate. It is widely distributed in plants in nature, mostly in the form of oxalate. C.W. Scheele first prepared oxalic acid in 1776.
Anhydrous oxalic acid is precipitated from glacial acetic acid and is a colorless, transparent needle-like or prismatic crystal of the orthorhombic system. Relative molecular mass 90.04. There are α-body and β-body. Relative density (17℃) 1C.900 (α-body), 1.895 (β-body). Melting point: 189.5°C (α-body), 182.0°C (β-body). Hygroscopic. Can be sublimated. Decomposes at 157℃.
Oxalic acid dihydrate is precipitated from water into monoclinic colorless transparent flake and prismatic crystals. Relative molecular mass 126.07. Relative density 1.653 (19℃). Melting point: 101~102℃. Anhydrous material can be obtained by slowly heating at 100°C. At this time, there is a small amount of sublimation.
Hydrate and anhydrous are insoluble in benzene, chloroform and petroleum ether, but soluble in water, ethanol, ether and glycerol. Solubility: water 16 (20°C), 50 (100°C); ethanol 24 (15°C), 40 (26°C); glycerin 18 (20°C); ether 1.37 (15°C). The pH of 1.3% aqueous solution is 1.3. It is the strongest acid among dicarboxylic acids. In addition to the properties of general carboxylic acids, it also has reducing properties and can quantitatively reduce heptavalent manganese to divalent manganese. This property is often used for quantitative analysis of permanganate.
5C2H2O4+2KMnO4+3H2SO4→ K2SO4+2MnSO4+8H2O+10CO2Polytetrafluoroethylene wax powder
Trivalent iron can also be reduced to divalent iron. Since divalent iron is easily soluble in water, rust on clothing can be removed.
Reacts with phosphorus pentachloride to generate phosphorus oxychloride. C2H2O4+PCl5→POCl3+CO2+CO+2HCl
It can react with many metals to form oxalates. Except for alkali metal salts and ferrous iron salts, the other oxalates are insoluble in water. Although some metal salts are poorly soluble in water, they can form water-soluble complexes.
Fe2(C2O4)3+3K2C2O4+6H2O→ 2K3[Fe(C2O4)3]·6H2O
Alkali metal and alkaline earth metal oxalates lose carbon monoxide when heated to form carbonates. The carbonates continue to be heated and are decomposed into oxides and emulsifying thickeners. carbon dioxide. Oxalates of nickel, cobalt, and silver are heated to form metals rather than metal oxides.
The decomposition products of oxalic acid when heated are carbon dioxide, carbon monoxide and water.
Oxalic acid and oxalates are toxic. Mouse oral LD502000～0mg/kg.
【Purpose】
Bleach is used to bleach wood and wheat straw to make the wood and wheat straw shiny and uniform in color. It is also used to bleach food. However, after bleaching, the remaining oxalic acid must be washed away;
Metal treatment agent. Use this product solution to wash the surface of ferrous metal or non-ferrous metal to form a smooth oxalate film, which can protect the workpiece and extend its life;
Used as dyeing auxiliaries, decontamination and bleaching; aluminum salts and antimony salts of oxalic acid can be used as mordants; in the production of anti-wrinkle and anti-shrink fibers
This product is used as a catalyst; anodizing agent. Aluminum anodization can produce a thicker coating, which is hard and corrosion-resistant; a photographic agent; a tanning agent; a chelating agent to extract rare metals;
Organic intermediates for preparing oxalamide, oxalate, oxalate ester, etc.; pharmaceutical raw materials.
【Preparation method】
[1]: Oxalic acid anhydrous [2]: Oxalic acid dihydrate
(1) React carbon monoxide with sodium hydroxide at 2MPa and 150°C to generate sodium formate. After concentration, it is heated to ℃ and dehydrogenated to generate sodium oxalate, which can then be acidified with sulfuric acid [1].
(2) It can be produced by using ethylene glycol as raw material, performing liquid-phase catalytic oxidation with air at 120°C, and then recrystallizing and purifying [1]. Or use mixed acid (30% ~ 40% sulfuric acid + 20% ~ 25% nitric acid) and vanadium pentoxide for catalytic oxidation. At this time, it can be carried out at a lower temperature (50 ~ 70°C).
(3) Using carbohydrates as raw materials, such as corn starch, cassava starch, potato starch, corn cobs, molasses, etc., it can be produced by oxidizing it with nitric acid at 70-80°C [2]. C6H12O6+12HNO3→3[2]+3H2O +3NO+9NO2
(4) Use propylene as raw material, oxidize it with nitric acid (or mixed acid) to generate α-nitropropionic acid, and then oxidize it with air [1].
(5) Use wood chips as raw materials, fuse soda lime at 250-280°C to generate oxalate, leaching with hot water, concentrating, crystallizing, and finally recrystallizing, it can be produced [2].
(6) It can also be prepared by fermentation of corn starch.
(7) It can be produced by using ethylene and glyoxal as raw materials and oxidizing it with nitric acid [1].
(8) It can be produced by using acetylene as raw material and palladium as catalyst [1].
(9) Using carbon monoxide, oxygen and water as raw materials, it can be prepared by reacting in ethanol medium under the action of palladium catalyst at 1.0~1.5MPa and 115℃ [1].
(10) Use corn cobs as raw materials to produce furfural (see this article for the preparation method), and then use concentrated nitric acid to oxidize furfural at 75-85°C [2].
