Industrial grade Oxalic acid dihydrate 6153-56-6

I. What is Oxalic Acid Dihydrate?

Oxalic acid dihydrate, also known as oxalic acid dihydrate, is the most common crystalline form of oxalic acid in commercial circulation. Its chemical formula is:

H2C2O4·2H2O

Oxalic acid is the simplest dicarboxylic acid, and its molecular structure can be viewed as two carboxyl groups directly linked together—HOOC-COOH. This simple molecular skeleton gives oxalic acid a unique dual chemical identity: it is both a moderately strong organic acid (pKa₁ ≈ 1.23, significantly stronger than acetic acid) and a strong reducing agent (the carbon in the oxalate ion is in a +3 oxidation state and can be oxidized to a +4 oxidation state CO₂). This dual function of "acidity + reducing power" is the chemical basis for oxalic acid in almost every industrial application.

Oxalic acid dihydrate is a colorless, transparent crystal or a white crystalline powder, odorless, and readily soluble in water and ethanol. In the industrial market, it is typically classified into different grades based on purity. Industrial-grade oxalic acid generally has a purity between 99.0% and 99.6%, and is an important basic organic chemical raw material in China with stable demand growth (an average annual compound growth rate of approximately 4.2% in recent years).

II. Main Uses of Industrial-Grade Oxalic Acid Dihydrate

1. Rare Earth Metallurgy – The Core Precipitator for Global Rare Earth Separation and Purification

This is the most important application area for industrial oxalic acid in terms of strategic significance and scale. China is the world's largest producer and exporter of rare earth elements. Rare earth elements (17 elements in total, including 15 lanthanides, scandium, and yttrium) are the "industrial vitamins" of modern high-tech industries—from smartphones and electric vehicles to precision-guided weapons and wind turbines, almost all cutting-edge technologies rely on rare earth elements.

In rare earth hydrometallurgy, ore is leached with sulfuric acid or hydrochloric acid to obtain a mixed leach solution (PLS) containing multiple rare earth ions. The selective separation and purification of individual rare earth elements from this mixed leach solution is the most technically challenging step in the entire process. Oxalic acid plays an irreplaceable role in this step—as a rare earth oxalate precipitant.

The chemical principle behind oxalic acid precipitation of rare earth elements:

Rare earth ions (RE³⁺) react with oxalic acid to form rare earth oxalate precipitates with extremely low solubility.

2RE³⁺ + 3H₂C₂O₄ → RE₂(C₂O₄)₃↓ + 6H⁺

Rare earth oxalates have extremely low solubility (typically on the order of 10⁻²⁵ to 10⁻³¹), meaning that even with low concentrations of rare earth ions in the leachate, oxalic acid can efficiently and selectively precipitate them from complex solutions containing large amounts of impurity ions such as aluminum, iron, calcium, and magnesium. The precipitate is washed, filtered, and then calcined at high temperature to decompose it, yielding high-purity rare earth oxides.

RE₂(C₂O₄)₃ → RE₂O₃ + 3CO + 3CO₂

The core value of oxalic acid precipitation in rare earth metallurgy:

- High selectivity: Rare earth oxalate precipitation results in less co-precipitation of associated ions such as aluminum, iron, alkali metals, and alkaline earth metals.

- Complete precipitation and simple operation: Mature and reliable industrial conditions, with precipitation efficiency exceeding 95%.

- The calcination product is a rare earth oxide, which directly enters the next process. The oxalate ions are completely decomposed into gas and escape without introducing impurities.

- Applicable to the separation of rare earth elements across the entire spectrum, from light rare earths (lanthanum, cerium, praseodymium, neodymium) to heavy rare earths (terbium, dysprosium, holmium).

It can be said that without a large-scale industrial supply of oxalic acid, the efficient separation and purification of rare earth elements globally would face serious cost and efficiency challenges. Oxalic acid is one of the underlying chemical foundations of China's rare earth industry's global competitive advantage.

2. Metal Surface Treatment – From Rust Removal of Steel to Surface Cleaning of Precision Parts

Metal surface treatment is the most widespread and industry-wide application of oxalic acid in industrial use. The value of oxalic acid in this field is based on its unique selective dissolution ability of iron oxides (rust).

Steel pickling and rust removal:

During hot rolling, storage, and transportation, a layer of oxide scale (mill scale) composed of ferrous oxide, magnetite, and ferric oxide forms on the surface of steel. This oxide scale must be removed before further processing (cold rolling, electroplating, painting, etc.), a process called pickling.

Although sulfuric acid and hydrochloric acid are the main acids used in steel pickling, oxalic acid has irreplaceable advantages in certain scenarios:

- Oxalic acid rust removal is based on a complexation dissolution mechanism (rust → soluble oxalic acid iron complex ions), and its corrosion rate on the metal matrix is much lower than that of strong inorganic acids.

- Surface cleaning can be completed without severely eroding the dimensional tolerances of precision parts.

- Suitable for processing precision mechanical parts, bearings, molds, and medical devices, as well as other metal products requiring extremely high dimensional accuracy and surface finish.

Stainless steel surface passivation and cleaning:

After stainless steel processing (welding, cutting, forming), oxalic acid solution is used to remove surface oxidation (tempering color) and weld marks, restoring the original silvery-white luster of the stainless steel. The weak acidity of oxalic acid allows it to be used as a "gentle polishing" chemical tool in this process.

Surface treatment of aluminum and its alloys:

Oxalic acid is used as an electrolyte component in the anodizing process of aluminum profiles. The oxide film generated by oxalic acid anodizing is dense and has a special yellowish-brown to bronze decorative effect, which has unique aesthetic value in the processing of architectural decorative aluminum panels and high-end electronic product casings.

3. Textile Printing and Dyeing – The Triple Role of Reducing Agent, Bleaching Agent, and Dyeing Auxiliary Agent

The textile industry is a traditional major consumer of industrial oxalic acid. Oxalic acid has functional components throughout the entire textile industry chain—from pretreatment to dyeing to finishing.

Bleaching and stain removal of cotton fabrics:

Cotton fibers inevitably carry natural pigments, cottonseed hull residues, and field contaminants during growth and harvest. Oxalic acid is used as an auxiliary bleaching agent in the pretreatment bleaching process of cotton fabrics, utilizing its reducing properties to remove rust stains and stains caused by metal ions. Compared to the oxidative bleaching of hydrogen peroxide, the reducing bleaching of oxalic acid is more precise in removing specific types of metallic-tannin stains.

Printing and dyeing reducing agents and color developing auxiliaries:

In the dyeing and printing processes of some dyes (especially ice dyeing of insoluble azo dyes and leuco dyeing of vat dyes), oxalic acid is used as a reducing agent and a protective agent. It helps maintain the reduction potential of the dye bath, protects vat dyes from premature oxidation by air, and ensures that the dyes fully penetrate the fibers and develop color evenly.

Acidity adjustment as an alternative to acetic acid:

Oxalic acid is a stronger acid than acetic acid. In dyeing and printing processes that require strong acidic conditions (such as the application and fixing of some acid dyes), oxalic acid can serve as a substitute or supplement for acetic acid, providing pH adjustment. For large-scale continuous dyeing and printing processes, the unit acidity cost of oxalic acid is lower than that of acetic acid.

4. Leather Processing – From Raw Hide Bleaching to Finished Leather Quality Improvement

In the leather industry, oxalic acid is required in both the pre-tanning and post-tanning stages of raw hides.

Raw skin bleaching and iron spot removal:

During slaughter and initial processing, raw hides often develop rust spots and a dull color due to contact with iron tools, blood, and environmental pollutants. Oxalic acid is used as a bleaching agent and iron spot remover in the pickling or pre-tanning stages, converting insoluble iron-protein complexes into soluble ferric oxalate salts, thus restoring the clean base color of the hide.

Tanning and pH adjustment:

In multiple processes such as chrome tanning and vegetable tanning, oxalic acid is used to precisely adjust the pH value of the working solution. Precise pH control directly affects the penetration rate and fixation effect of chrome tanning agents in leather fibers, and has a substantial impact on the softness, fullness, and physical strength of the finished leather.

Pretreatment before staining:

Before dyeing leather, oxalic acid solution treatment helps remove residual metal ions and surface impurities from the leather surface, allowing the dye to penetrate and fix more evenly, thus improving the dyeing uniformity and color fastness of the finished leather.

5. Electroplating and Surface Engineering – Complexing Agents and Bath Maintenance

In the electroplating industry, oxalic acid plays two very different roles:

Complexing agent for acidic tin plating:

In acidic tin plating, oxalic acid forms a stable tin oxalate complex with stannous ions (Sn²⁺), shifting the deposition potential of tin negatively and preventing the disproportionation reaction and rapid hydrolysis precipitation of divalent tin in acidic solutions. Tin plating using the oxalic acid complex system produces a fine-grained coating with good adhesion, and is widely used in tin plating of electronic component leads, circuit boards, and food packaging materials.

Maintenance of zinc and nickel plating baths:

Oxalic acid is used as an iron ion masking agent in plating bath maintenance. During electroplating, iron ions introduced by the workpiece accumulate in the plating bath, leading to increased roughness and brittleness of the coating once a certain concentration is reached. Oxalic acid reduces the activity of free iron ions by forming stable ferric oxalate complexes with them, thus delaying the negative impact of iron ions on the plating bath performance and extending the service life of the plating bath.

6. Catalyst Manufacturing – Precursors for Iron-Based Catalysts and Fine Chemical Catalytic Systems

The value of oxalic acid in the preparation of industrial catalysts comes from its unique "clean precursor" property—oxalate ions are completely decomposed into CO and CO₂ gases during calcination and escape without leaving any anion residue (unlike sulfate and chloride precursors).

Main application areas:

- Preparation of iron-based catalysts via Fischer-Tropsch Synthesis: Highly active, high specific surface area iron-based catalysts are prepared by calcining ferric oxalate.

- Catalysts for the synthesis of low-carbon alcohols

- Ammonia decomposition catalyst

- Pretreatment of the support for volatile organic compound (VOCs) catalytic combustion catalysts

7. Other industrial applications

Rubber and plastics industry:

Oxalic acid is used as an additive in the coagulation of natural rubber and in the production of some synthetic resins.

Construction and Concrete Industry:

Oxalic acid solution is used to clean efflorescence and rust stains on concrete surfaces and is commonly used in building exterior renovation and decorative concrete projects.

Fine chemical synthesis:

Oxalate esters (such as dimethyl oxalate (DMO) and diethyl oxalate) are important intermediates in the coal-to-ethylene glycol route and are also basic raw materials for the synthesis of fragrances, flavorings, and pharmaceuticals.

Oxalates (sodium oxalate, potassium oxalate, ammonium oxalate, etc.) have specific uses in analytical reagents, printing and dyeing, and electroplating.

III . Summary

Oxalic acid dihydrate is the simplest dicarboxylic acid, oxalic acid, and its standard industrial crystalline form has the chemical formula C₂H₂O₄·2H₂O. In daily life, we are indirectly connected to industrial oxalic acid through industrial end products such as rare-earth permanent magnets in mobile phones, drive motors in new energy vehicles, textiles and leather products we wear, antibiotics in pharmacies, and clean metal materials in buildings.

Industrial-grade oxalic acid has applications spanning multiple pillar industries of the national economy: rare earth metallurgy is the application area with the highest strategic value for oxalic acid—as an irreplaceable precipitant for the separation and purification of rare earths globally, oxalic acid is a cornerstone of the global competitiveness of China's rare earth industry; in the pharmaceutical industry, oxalic acid participates in the chemical synthesis of many basic drugs such as tetracycline and phenobarbital; in the field of metal surface treatment, oxalic acid's selective complexation rust removal mechanism is utilized, covering the entire metal treatment chain from steel pickling to stainless steel cleaning; in textile printing and dyeing and leather processing, oxalic acid plays multiple roles as a reducing agent, bleaching agent, and dyeing auxiliary agent; in the electroplating industry, oxalic acid, as a complexing agent and iron ion masking agent, ensures the long-term stability of the plating solution. From rare earth ores to mobile phone screens, from steel workshops to pharmacy counters, industrial oxalic acid provides invisible and continuous chemical support to modern manufacturing in a massive scale.