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Overview
Chemical Identity
Oxalic acid, systematically named ethanedioic acid, is the simplest dicarboxylic acid with the chemical formula HOโC(=O)โC(=O)โOH, often represented as (COOH)2. It is a white crystalline solid that readily dissolves in water, forming a colorless solution. Its name originates from its isolation from plants of the genus Oxalis, commonly known as wood-sorrels.
Acidity and Reactivity
Compared to acetic acid, oxalic acid is a significantly stronger acid. Its two dissociation constants (pKa values) are approximately 1.27 and 4.27, indicating its capacity to donate protons in aqueous solutions. It also functions as a potent reducing agent and its conjugate bases, hydrogenoxalate and oxalate, act as effective chelating agents for various metal cations.
Natural Occurrence
Oxalic acid is naturally present in numerous plants, including spinach, rhubarb, and various brassicas. It is also produced by certain fungi and bacteria. While beneficial in small quantities, excessive ingestion or prolonged skin contact can pose significant health risks.
Historical Context
Early Investigations
The preparation of oxalic acid salts from plants dates back to at least 1745. By 1773, Herman Boerhaave had isolated a salt from wood sorrel. Later, in 1776, Carl Wilhelm Scheele and Torbern Olof Bergman produced oxalic acid by reacting sugar with nitric acid, initially calling it "sugar acid." Scheele confirmed its identity with the naturally occurring acid in 1784. The name "oxalic acid" was formally introduced by Lavoisier and colleagues in 1787.
First Synthesis
A significant milestone occurred in 1824 when Friedrich Wรถhler synthesized oxalic acid by reacting cyanogen with ammonia. This achievement is often cited as one of the earliest instances of synthesizing a naturally occurring compound, marking a pivotal moment in the development of organic chemistry.
Production Methods
Industrial Synthesis
The primary industrial method involves the oxidation of carbohydrates, such as glucose, using nitric acid, often catalyzed by vanadium pentoxide. Alternative processes utilize oxygen and precursors like ethylene glycol. Historically, oxalic acid was obtained from sawdust treated with caustic solutions, followed by acidification. Modern methods also include the oxidative carbonylation of alcohols to form diesters, which are subsequently hydrolyzed.
Laboratory Preparation
In a laboratory setting, oxalic acid can be synthesized by oxidizing sucrose with nitric acid, employing vanadium pentoxide as a catalyst. The anhydrous form can be obtained from the dihydrate through heating or azeotropic distillation.
Molecular Structure
Anhydrous Form
Anhydrous oxalic acid exists in two crystalline polymorphs. These forms differ in their hydrogen-bonding patterns, resulting in either chain-like or sheet-like arrangements of the molecules. Its hydrophilic nature and acidity make it useful in esterification reactions.
Dihydrate Form
Oxalic acid commonly occurs as the dihydrate, H2C2O4ยท2H2O. The crystal structure of the dihydrate has been determined through X-ray and neutron diffraction studies, revealing specific lattice parameters and inter-atomic distances that define its molecular arrangement.
Chemical Reactions
Acid-Base Properties
With pKa values of approximately 1.27 and 4.27, oxalic acid is a relatively strong organic acid. It readily forms salts and esters, such as dimethyl oxalate, and can be converted into its acid chloride, oxalyl chloride.
Metal Binding
Oxalic acid and its oxalate anion are effective chelating agents, forming stable complexes with various metal ions. This property is utilized in applications like rust removal, where it forms a water-soluble complex with ferric iron (ferrioxalate ion). It also plays a role in lanthanide chemistry, facilitating their separation and conversion into oxides.
Decomposition
Upon heating, oxalic acid decomposes. Vapor phase decomposition occurs between 125โ175 ยฐC, yielding carbon dioxide and formic acid. Photolysis with UV light also leads to the formation of carbon monoxide and water. It reacts with strong oxidizers like permanganate in an autocatalytic process.
Natural Occurrence
Plant and Fungal Sources
Oxalic acid is prevalent in many plants, notably in the leaves of spinach, rhubarb, and sorrel. It is also found in significant amounts in amaranth, parsley, and quinoa. Certain fungi, like Aspergillus niger, are known producers, contributing to nutrient availability in soil through metal cation solubility and the formation of calcium oxalate crystals.
Dietary Content
The concentration of oxalic acid varies considerably across different food items. Leafy greens and certain vegetables tend to have higher levels. For instance, spinach can contain up to 0.97%, while amaranth and parsley show concentrations around 1.09% and 1.70% respectively. Understanding these levels is important for dietary considerations.
Biochemical Significance
Enzyme Inhibition
Oxalate acts as a competitive inhibitor of the lactate dehydrogenase (LDH) enzyme. LDH is crucial for converting pyruvate to lactic acid, a process vital for anaerobic energy metabolism in cells, including cancer cells (Warburg effect). Inhibiting LDH can suppress tumor formation and growth, making it a subject of research in cancer therapy.
Plant-Fungal Interactions
In plant-pathogen interactions, oxalic acid plays a dual role. Low concentrations can enhance a plant's resistance to fungal infections, while higher concentrations, often secreted by pathogenic fungi like Sclerotinia sclerotiorum, can induce plant cell death and facilitate infection by degrading cell walls.
Gut Microbiome
The bacterium Oxalobacter formigenes resides in the gut and plays a significant role in degrading oxalate, aiding animals, including humans, in managing dietary oxalate intake and preventing its systemic absorption.
Industrial Applications
Cleaning and Bleaching
Oxalic acid is widely used as a cleaning agent, particularly effective for removing rust (iron stains) due to its ability to form stable, water-soluble ferrioxalate complexes. It is a key ingredient in many household cleaners and is employed as a bleaching agent for materials like pulpwood, cork, straw, and feathers.
Dyeing and Ceramics
Approximately 25% of industrially produced oxalic acid serves as a mordant in dyeing processes, helping to fix dyes to fabrics. In the ceramics industry, dilute solutions are used to remove iron impurities from clays, yielding lighter-colored final products.
Niche Uses
Beekeepers utilize oxalic acid as a miticide to control parasitic Varroa destructor mites in honeybee colonies. It also finds application in the semiconductor industry for polishing copper layers and is explored for potential roles in carbon capture technologies.
Toxicity and Hazards
Acute Toxicity
Oxalic acid is toxic upon ingestion and skin contact. The lowest published lethal dose (LDLo) is reported as 600 mg/kg orally in dogs. A lethal oral dose is estimated to be between 15 to 30 grams for humans. Its toxicity primarily stems from the precipitation of insoluble calcium oxalate in the kidneys, leading to acute kidney failure.
Hazard Classification
Globally Harmonized System (GHS) classification indicates oxalic acid is harmful if swallowed or in contact with skin (H302+H312), causes serious eye damage (H318), and is harmful to aquatic life (H402). It carries the signal word "Danger" and requires appropriate precautionary statements regarding handling, storage, and disposal.
Cellular Effects
Beyond kidney toxicity, oxalate is known to induce mitochondrial dysfunction. Furthermore, ethylene glycol, a common industrial chemical, is metabolized in the body to oxalic acid, which is responsible for its severe nephrotoxicity.
Kidney Stones
Calcium Oxalate Formation
A significant proportion of kidney stones, approximately 76%, are composed of calcium oxalate. The precipitation of calcium oxalate crystals in the renal system, often exacerbated by high dietary intake of oxalic acid or impaired oxalate metabolism, is a primary cause of nephrolithiasis.
Notes
Data Standards
Unless otherwise specified, chemical and physical data presented are based on standard conditions (25 ยฐC, 100 kPa). Reference values for properties like solubility, melting point, and pKa may exhibit slight variations across different sources due to experimental conditions and methodologies.
Related Compounds
Chemical Relatives
Oxalic acid is chemically related to several other compounds, including oxalyl chloride, various oxalate salts such as disodium oxalate and calcium oxalate, and phenyl oxalate esters. Understanding these related substances provides a broader context within organic and inorganic chemistry.
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References
References
- Oxalic acid from PubChem
- "Rock Currier รขยย Cleaning Quartz". mindat.org
- "CDC รขยย Immediately Dangerous to Life or Health Concentrations (IDLH): Oxalic acid รขยย NIOSH Publications and Products". cdc.gov
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Important Notice
This page was generated by an Artificial Intelligence and is intended for informational and educational purposes only. The content is derived from publicly available data and may not be exhaustive or entirely up-to-date. While efforts have been made to ensure accuracy, users should consult primary sources and qualified experts for critical applications.
This is not chemical safety advice. The information provided herein is not a substitute for professional chemical handling, safety consultation, or regulatory guidance. Always refer to official Safety Data Sheets (SDS), technical documentation, and relevant safety protocols when working with or handling chemical substances like oxalic acid. Never disregard professional advice or delay in seeking it because of information found on this website.
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