Why is oxalic acid water-soluble

Oxalic acid (COOH) 

Colorless, prismatic crystals
Occurrence Plants (wood sorrel, sorrel, rhubarb), in the urine
molar mass 90.035 g / mol
(Dihydrate 126.065 g / mol)
AGW 1 mg / m3 E (TRGS 900)
density 1,900 g / cm3    
(Dihydrate 1.653 g / cm3)   
decomposition +189.5 ° C
Water solubility 
Conc. At 20 ° C 95.1 g / l
GHS 05
GHS 07
Hazard classes + category 

Acute toxicity oral / dermal 4
Serious eye damage 1
HP rates (see note)     
H 302, 312, 318 P 264, 280.1-3, 301 + 310, 305 + 351 + 338, 302 + 352
G 3
Print a labelGerman designation 
Synonyms (German)
English designation 
CAS 144-62-7Oxalic acid 
Ethanedioic acid, clover acid
Oxalic acid 
Ethanedioic acid
CAS 6153-56-6Oxalic acid dihydrate* 
Ethanedioic acid dihydrate
Oxalic acid dihydrates 
Ethanedioic acid dihydrate
*) Preferred for school
Effect on the human body

Oxalic acid and its salts irritate the skin and mucous membranes. When taken internally, vomiting and diarrhea as well as cramps occur. Severe kidney damage can also follow. The toxic effect is based on a disruption of the calcium balance. In oxaluria, there is too much calcium oxalate in the urine due to a metabolic disorder, and bladder and kidney stones form.



Oxalic acid dihydrate is available as a coarsely crystalline powder that appears white in crushed form. The colorless and odorless crystals crystallize according to the monoclinic system, while the potassium salt potassium oxalate monohydrate crystallizes according to the orthorhombic system. Oxalic acid dihydrate is readily soluble in water and in ethyl alcohol. Careful heating to 100 ° C gives anhydrous oxalic acid. If heated too quickly or under the action of hot concentrated sulfuric acid, oxalic acid breaks down into oxygen, carbon monoxide and water.


The wood sorrel Oxalis corniculata contains oxalic acid and its salts
The oxalic acid was obtained from J.C. Wiegleb (1732–1800) discovered in 1769 in wood sorrel. In 1776, Carl Wilhelm Scheele, together with Torbern Olof Bergmann (1735–1784), succeeded for the first time in the production of oxalic acid through the action of nitric acid on sugar. Friedrich Wöhler carried out the first synthesis from inorganic substances in 1824 from dicyan. However, Wöhler's method is no longer of any practical importance. Two methods are used today. In the first, the double sugar sucrose is oxidized with concentrated nitric acid in the presence of a vanadium (V) oxide catalyst:
C.12H22O11 + 9 O2  6 HOOC-COOH + 5 H2O
In the second process, sodium formate, a salt of formic acid, is heated to 360 ° C in the presence of sodium hydroxide, whereby the oxalic acid salt sodium oxalate is formed with evolution of hydrogen:
2 H-COONa NaOOC-COONa + H2   
A reaction with calcium hydroxide gives poorly soluble calcium oxalate, from which oxalic acid can be obtained with sulfuric acid. The processes can also be reversed so that formic acid can be produced from oxalic acid.
Oxalic acid is used in analysis as a basic titer substance for setting standard permanganate solutions. It serves as a pickling agent in dyeing and is used to bleach straw and stearin. It can be used to clean metals and remove rust: it forms soluble double salts with iron ions. Since oxalic acid also dissolves Prussian blue, it is suitable for making ink killers. The chemical industry needs oxalic acid as an intermediate for the synthesis of oxalates and dyes.