Oxalic acid is a fascinating molecule that bridges the realms of nature and industry, being abundant in many plant species yet also synthesized on a large scale for an array of applications.
This article will delve into the many facets of oxalic acid, exploring its vast industrial applications and their underlying mechanisms, illuminating its chemical properties, and shedding light on the safety considerations and regulations governing its use. It aims to serve as a comprehensive guide for those who seek to harness the remarkable potential of this humble yet potent molecule.
What is Oxalic Acid?
Oxalic acid, or ethanedioic acid, is a colorless crystalline compound with the chemical formula (COOH)₂. Due to its two carboxylic acid functional groups, it belongs to the family of organic acids and is classified as a dicarboxylic acid. Oxalic acid is highly soluble in water and has a sour taste. It is naturally found in various plants, such as spinach, rhubarb, and sorrel, as well as in some vegetables, fruits, and nuts.
As a relatively strong organic acid, oxalic acid acts as a cleaning and bleaching agent, pharmaceutical intermediate, and a vital component in the leather, tanning, and rare earth industries. Its chelating ability, derived from its chemical structure, is particularly noteworthy as it enables it to bind with metal ions, thereby facilitating processes such as rust removal and the extraction of rare earth elements.
How is Oxalic Acid Produced?
Oxalic acid occurs naturally in various plants, vegetables, fruits, and other organic sources. Some common natural sources of oxalic acid include:
|Rhubarb leaves are known to contain high levels of oxalic acid. However, it is essential to note that the stalks of rhubarb are safe for consumption as they have lower oxalic acid content.
|Spinach is another vegetable that contains oxalic acid, though in moderate amounts. Cooking spinach can help reduce its oxalic acid content.
|Wood sorrel, also known as oxalis, is a plant that inspired the name “oxalic acid” due to its high compound concentration. The leaves and stems of sorrel contain significant amounts of oxalic acid.
|The greens or leaves of beets contain oxalic acid, although the beetroot has a lower content.
|Cocoa powder, derived from cocoa beans, contains oxalic acid in small quantities.
|Certain fruits, such as kiwi, star fruit, and berries (including cranberries and gooseberries) contain oxalic acid, but the levels are generally low.
Oxidation of carbohydrates
Oxalic acid is mainly manufactured by the oxidation of carbohydrates or glucose using nitric acid or air in the presence of vanadium pentoxide as a catalyst. Here is the chemical reaction:
C₆H₁₂O₆ + 4 HNO₃ → 2 C₂H₂O₄ + 4 NO₂ + 2 H₂O
In this reaction, glucose (C₆H₁₂O₆) reacts with nitric acid (HNO₃) to produce oxalic acid (C₂H₂O₄), nitrogen dioxide (NO₂), and water (H₂O). The vanadium pentoxide (V₂O₅) catalyst facilitates oxidation by promoting the desired reaction. This oxidation method is one of the primary industrial processes used to produce oxalic acid. The reaction conditions, such as temperature, pressure, and concentration, may vary depending on the specific setup and desired yield.
Oxidation of ethylene glycol
Ethylene glycol (HOCH₂CH₂OH) is heated to convert it into a vaporized form. This vaporization process allows for better contact between the reactant and the catalyst. The vaporized ethylene glycol is passed over a bed of small copper pellets. The copper catalyst, in the form of pellets, provides a large surface area for the reaction. The formula for the oxidation ratio is as follows:
2 HOCH₂CH₂OH + O₂ → 2 C₂H₂O₄ + 2 H₂O
Air, which contains oxygen (O₂), is blown through the bed of copper pellets. The oxygen from the air acts as an oxidizing agent in the reaction, facilitating the conversion of ethylene glycol into oxalic acid. The ethylene glycol undergoes oxidation in the copper catalyst and oxygen presence. The oxidation reaction converts the ethylene glycol molecules into oxalic acid (C₂H₂O₄). The oxalic acid produced in the reaction is collected by condensation. As the gaseous mixture containing oxalic acid cools down, the oxalic acid condenses into a liquid form. The liquid oxalic acid is then collected and further processed or purified.
Oxidative carbonylation of alcohols
Oxalic acid can also be produced through the oxidative carbonylation of alcohols to form diesters of oxalic acid, which are then hydrolyzed to yield oxalic acid. The reaction can be represented as follows:
4ROH + 4 CO + O₂ → 2 (CO₂R)₂ + 2 H₂O
In this reaction, alcohols (ROH) react with carbon monoxide (CO) and oxygen (O₂) to produce diesters of oxalic acid ((CO₂R)₂) and water (H₂O), where R represents an alkyl or aryl group. The diesters are then hydrolyzed, typically using an acid or base, to convert them into oxalic acid (C₂H₂O₄).
“Wood-based” process for oxalic acid production
Oxalic acid can be obtained by treating caustics, such as sodium or potassium hydroxide, with sawdust, followed by acidification of the resulting oxalate with mineral acids like sulfuric acid. Sawdust, a byproduct of wood processing, is treated with caustic solutions, typically sodium hydroxide (NaOH) or potassium hydroxide (KOH). The sawdust is a source of cellulose, a complex carbohydrate found in wood.
The caustic solution reacts with the cellulose in the sawdust, forming sodium or potassium oxalate. The sodium or potassium oxalate solution obtained from the previous step is acidified using a mineral acid, commonly sulfuric acid (H₂SO₄). The acidification process converts the oxalate ions into oxalic acid. The oxalic acid formed during acidification precipitates as crystals from the solution. The crystals are separated through filtration or other separation techniques.
Oxalic acid from Sodium Formate
Sodium formate is heated with sodium hydroxide to form sodium oxalate:
2HCOONa + 2 NaOH → Na₂C₂O₄ + 2 H₂O
The sodium oxalate obtained in the previous step is further reacted with a calcium salt solution (usually calcium chloride or calcium nitrate) to form calcium oxalate:
Na₂C₂O₄ + CaCl₂ (or Ca(NO₃)₂) → CaC₂O₄ + 2 NaCl (or 2 NaNO₃)
The calcium oxalate is then treated with sulfuric acid (H₂SO₄) to obtain free oxalic acid:
CaC₂O₄ + H₂SO₄ → C₂H₂O₄ + CaSO₄
The resulting oxalic acid solution is typically purified through filtration or other separation techniques to remove any remaining impurities or solids. The purified oxalic acid can be further processed to obtain the desired form or concentration.
Uses and Applications of Oxalic Acid in Industry
Cleaning and Household Products
Oxalic acid is used as a cleaning agent to remove rust and mineral stains. It is commonly found in rust removers, stain removers, and wood cleaning products. Oxalic acid can effectively remove rust from metals, clean mineral deposits, and brighten wood surfaces. Oxalic acid’s utility in rust removal agents is attributed to its ability to form a stable and water-soluble salt with ferric iron, known as ferrioxalate ion. When oxalic acid comes into contact with rust, it reacts with the rust’s ferric iron oxide (Fe₂O₃), converting it into a soluble compound.
Tooth Whitening Products
Oxalic acid is sometimes used as an ingredient in tooth-whitening products. Oxalic acid’s bleaching properties effectively remove stains and discoloration from teeth surfaces. In tooth whitening products, oxalic acid is typically used in low concentrations and combination with other ingredients. It helps break down and dissolve surface stains on the teeth, resulting in a brighter and whiter appearance.
Descaling Agent in Boilers
Oxalic acid is a descaling agent to remove calcium and magnesium deposits from boilers and other equipment. These deposits, called limescale or scale, can accumulate over time and reduce equipment efficiencies like boilers, heat exchangers, and pipes. Oxalic acid’s ability to dissolve and chelate calcium and magnesium ions effectively remove these mineral deposits. When oxalic acid comes into contact with limescale, it reacts with the calcium and magnesium ions, forming soluble complexes easily rinsed away.
Mordant in Dyeing
Oxalic acid can be used as a mordant in dyeing processes. A mordant is a substance that helps fix dyes onto fabrics, improving color fastness and enhancing the bonding between the dye and the fibers. It helps create a chemical bond between the dye molecules and the fibers, promoting better adhesion and color retention.
Oxalic acid is commonly used as a wood bleach to remove stains, discoloration, and dark spots from wood surfaces. It is particularly effective in lightening the color of tannin-based stains, water stains, and rust stains on wood. Oxalic acid is typically used in its crystalline form, which is then mixed with water to create a solution of the desired concentration for wood bleaching. The solution is applied to the wood surface, and the oxalic acid works by breaking down and removing the pigments responsible for the discoloration.
Oxalic acid is used in the electronic and semiconductor industries, particularly in the fabrication process of semiconductor devices. One of its applications is in copper layers’ electrochemical-mechanical planarization (ECMP).
ECMP is a crucial step in the manufacturing process of integrated circuits and semiconductor devices. It involves removing excess copper and planarizing the copper surface to ensure uniformity and smoothness. Oxalic acid is used as a complexing agent in the slurry during the ECMP process.
Paper & Pulp Industry
In the pulp and paper industry, oxalic acid is used in the bleaching process to brighten and whiten paper products. It helps to remove lignin and other impurities from the pulp, resulting in a higher-quality and more visually appealing paper. Oxalic acid is often combined with other bleaching agents and processes, such as chlorine dioxide or hydrogen peroxide, to achieve the desired bleaching effect.
Oxalic acid is a bleaching agent for fabrics and fibers in the textile industry. It helps to remove natural colorants, stains, and impurities from the textile materials, resulting in a lighter and brighter appearance. Oxalic acid is beneficial for bleaching natural fibers such as cotton, linen, and silk.
Properties of Oxalic Acid
|Molecular weight (g/mol)
|Boiling point (°C)
|Melting point (°C)
|Vapor Pressure (at 20°C) (mmHg)
|Highly soluble in water. Oxalic acid is also soluble in various organic solvents, such as ethanol, methanol, acetone, and ethyl acetate.
|Oxalic acid should be stored in a cool, dry place away from direct sunlight, heat sources, and moisture. Exposure to moisture can lead to the formation of oxalic acid dihydrate crystals, which may affect the purity and handling properties of the substance.
Oxalic Acid Derivatives
|This derivative is obtained by replacing one hydroxyl group (-OH) of oxalic acid with a chlorine atom. The reaction uses thionyl chloride (SOCl₂) as a reagent.
|By reacting oxalic acid with an alcohol in the presence of an acid catalyst, oxalate esters can be obtained. The general formula for an oxalate ester is R(OOC-COO)2, where R represents an alkyl or aryl group.
|This derivative is obtained by esterifying oxalic acid with methanol (CH₃OH) in an acid catalyst. It has the chemical formula (CH₃OOC-COOCH₃).
Another versatile organic acid with numerous industrial applications is Acetic Acid. Though utilized for different purposes than oxalic acid due to their distinct chemical properties, Acetic Acid also has a role in cleaning processes and is used for its acidic properties.
Safety & Regulatory Considerations
Despite its numerous applications, oxalic acid poses certain risks that must be carefully managed. There are also environmental considerations, as oxalic acid is listed as a hazardous substance under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and the Environmental Protection Agency (EPA) mandates reporting spills of 100 lbs (45.4 kg) or more.
Safety of Oxalic Acid
When working with oxalic acid, wearing appropriate personal protective equipment is essential. Handlers should wear gloves, safety goggles, face shields, and a lab coat or protective clothing to prevent skin contact and eye exposure. Ensure good ventilation in the area where oxalic acid is used or stored. Proper ventilation helps to prevent the buildup of potentially harmful fumes or vapors. If working with oxalic acid in a confined space, use an appropriate fume hood or ensure adequate airflow. In case of accidental exposure or ingestion, it is crucial to seek medical attention immediately. Rinse affected areas with plenty of water and remove contaminated clothing. If swallowed, do not induce vomiting unless directed by medical professionals.
Toxicity of Oxalic Acid
Oxalic acid is highly toxic if swallowed. Ingesting even a small amount of concentrated oxalic acid can cause severe damage to the digestive system, including the mouth, throat, esophagus, and stomach. Symptoms of ingestion may include burning pain, difficulty swallowing, nausea, vomiting, abdominal pain, and potentially life-threatening complications. Oxalic acid is a skin irritant, and prolonged or repeated contact with the skin can cause irritation, redness, and burns. Sensitization to oxalic acid may occur in some individuals, leading to allergic reactions. Inhalation of oxalic acid dust or fumes can irritate the respiratory system. Breathing in oxalic acid may cause respiratory tract irritation, coughing, chest pain, and shortness of breath. Prolonged or repeated exposure to high concentrations of oxalic acid fumes may lead to more severe respiratory effects.
Acceptable Limits or Maximum Usage
|1 mg/m3 over an 8-hour workday
|0.5 mg/m3 over an 8-hour workday
Fun Facts About Oxalic Acid
- The extraction of salts of oxalic acid from wood sorrel by Herman Boerhaave in 1745 marked an important milestone in understanding this compound. These salts, known as “salts of sorrel,” were found to possess acidic properties. However, it was not until 1773 that François Pierre Savary successfully isolated oxalic acid from these salts, thereby identifying it as a distinct compound. The synthesis of oxalic acid in a laboratory occurred in 1776, further advancing the understanding and ability to produce the compound artificially. This synthetic preparation involved the reaction of sugar or molasses with nitric acid, forming oxalic acid.
- Oxalic acid is a significant component of calcium oxalate kidney stones. Excessive consumption of oxalate-rich foods or metabolic disorders can lead to the formation of these stones in the urinary system.
- Oxalic acid can act as a defense mechanism against herbivores. It is often stored in specialized cells called idioblasts or crystal cells. When herbivores attempt to consume plant tissues containing oxalic acid, the sharp crystals can cause mechanical damage to their mouthparts or digestive systems, leading to irritation and discomfort. This deterrent effect helps to protect plants from being eaten or damaged by herbivorous insects, animals.