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Views: 0 Author: Site Editor Publish Time: 2026-08-04 Origin: Site
Oxalic acid dihydrate (chemical formula C₂H₂O₄·2H₂O, CAS 6153-56-6) is the hydrated crystalline form of oxalic acid — the simplest dicarboxylic acid in organic chemistry. It occurs naturally in many plants (rhubarb, spinach, sorrel) but is manufactured at industrial scale primarily for its powerful reducing, chelating, and acidic properties.
In commercial settings, oxalic acid is almost always handled as the dihydrate because the anhydrous form (CAS 144-62-7) is highly hygroscopic and absorbs moisture from air to revert to the dihydrate. The dihydrate appears as colorless, odorless crystals or a white crystalline powder with excellent water solubility.
Form | Formula | Appearance | Key Trait |
Dihydrate (commercial) | C₂H₂O₄·2H₂O | Colorless crystals / white powder | Stable, non-hygroscopic |
Anhydrous | C₂H₂O₄ | White powder | Hygroscopic; absorbs moisture from air |
Oxalic acid dihydrate is a relatively strong organic acid — roughly 3,000 times stronger than acetic acid — which makes it especially effective at dissolving metal oxides, mineral deposits, and rust stains. This combination of acidity and chelating ability is what underpins its value across multiple industries.
Understanding oxalic acid dihydrate's properties helps explain why it is so versatile:
Property | Value |
Chemical Formula | C₂H₂O₄·2H₂O |
CAS Number | 6153-56-6 |
Molecular Weight | 126.07 g/mol |
Appearance | Colorless crystals / white crystalline powder |
Odor | Odorless |
Melting Point | 104–106°C |
Sublimation Point | 150–160°C |
Density | 1.65 g/cm³ |
Bulk Density | ~813 kg/m³ |
Water Solubility | ~14.3 g/100 mL at 25°C |
pH (0.1 M solution) | ~1.3 |
pKa₁ / pKa₂ | 1.25 / 4.14 |
Anhydrous Equivalent | C₂H₂O₄, MW 90.03, CAS 144-62-7 |
Key functional characteristics:
Reducing agent:
● Oxalic acid readily donates electrons, making it effective at reducing metal ions (e.g., Fe³⁺ → Fe²⁺) and acting as a primary standard for permanganate titrations in analytical chemistry
Chelating agent:
● The two adjacent carboxyl groups form stable five-membered chelate rings with metal cations — particularly iron, calcium, and rare earth elements — forming insoluble or soluble oxalate complexes depending on the metal
Acid strength:
● As a diprotic acid with pKa₁ = 1.25, it is strong enough to dissolve rust, scale, and mineral deposits while remaining significantly less aggressive than mineral acids like sulfuric or hydrochloric acid
Metal surface treatment is one of the largest and fastest-growing application segments for oxalic acid dihydrate. Its unique ability to dissolve rust (iron oxide) without aggressively attacking the underlying metal makes it the preferred choice for precision cleaning across multiple industries.
How rust removal works with oxalic acid:
1. Oxalic acid dissolves in water and dissociates, releasing oxalate ions (C₂O₄²⁻) and hydrogen ions
2. The oxalate ions chelate iron(III) from rust (Fe₂O₃), forming the soluble ferrioxalate complex [Fe(C₂O₄)₃]³⁻
3. Simultaneously, oxalic acid reduces Fe³⁺ to the more soluble Fe²⁺ form
4. The dissolved iron-oxalate complex is rinsed away, leaving a clean metal surface
This dual-action mechanism — chelation plus reduction — is what distinguishes oxalic acid from simple acid pickling, which relies solely on proton attack and can cause hydrogen embrittlement in sensitive steels.
Key metal cleaning applications:
Industry | Application | Benefit |
Automotive restoration | Removing rust from classic car parts and body panels | Selective rust removal without pitting base metal |
Industrial maintenance | Cleaning heat exchangers, boilers, and cooling systems | Dissolves iron oxide scale without corroding steel |
Manufacturing | Pre-painting surface preparation | Produces a clean, etch-ready surface for coatings |
Marine | Removing rust stains from stainless steel fittings and hulls | Non-abrasive; safe for polished surfaces |
Tool & die maintenance | Cleaning precision tools and molds | Removes corrosion without dimensional change |
Oxalic acid is the most widely used wood bleach for restoring grayed, weathered, or iron-stained wood. Unlike chlorine-based bleaches that remove the wood's natural color, oxalic acid specifically targets iron tannate stains (the blue-black discoloration that appears when iron fasteners, nails, or tools contact tannin-rich woods like oak and mahogany) and water stains.
Typical wood bleaching process:
1. Dissolve oxalic acid crystals in warm water at 80–120 g per liter
2. Apply to the wood surface with a brush, sponge, or sprayer
3. Allow to work for 15–30 minutes — iron stains begin to disappear visibly
4. Rinse thoroughly with clean water to neutralize and remove all acid residue
5. Allow wood to dry completely before sanding or finishing
Woods most responsive to oxalic acid bleaching:
● Oak, mahogany, walnut, teak (high tannin content — strong response to iron stain removal)
● Cedar, redwood (weathering gray reversal)
● Maple, birch (water stain and general brightening)
● Any exterior wood exposed to iron fasteners, nails, or metal furniture
The textile industry is historically the largest-volume consumer of oxalic acid. Its applications span the entire textile finishing chain:
Bleaching assistant:
● Oxalic acid is used as a reducing bleach and bleaching aid, particularly for removing iron stains picked up during wet processing in iron-containing water systems. Iron contamination is a persistent problem in textile mills — even trace levels cause yellowing and dullness in finished fabrics.
Dyeing mordant:
● Oxalic acid acts as a mordant in certain dyeing processes, helping fix dyes to fibers and improving wash fastness
Stripping agent:
● Used to remove unwanted dye from fabrics that need re-dyeing or correction
Scouring:
● Assists in the removal of metallic impurities from natural fibers like cotton and wool
Oxalic acid is widely used for cleaning and restoring natural stone, marble, tile, and concrete surfaces because it effectively dissolves iron stains, rust marks, and mineral deposits — without etching or dulling calcium carbonate-based materials the way stronger mineral acids would.
Common stone cleaning applications:
● Removing iron/rust stains from marble floors, countertops, and monuments
● Cleaning efflorescence (white mineral deposits) from brick and concrete
● Brightening grout lines and tile surfaces
● Restoring terrazzo flooring
Oxalic acid dihydrate is an important building block in industrial organic synthesis. It is used in the production of:
Oxalate salts:
● Ferric oxalate (photography, blueprint paper), ammonium oxalate (analytical chemistry), potassium oxalate, and numerous metal oxalates used as catalysts and intermediates
Formic acid:
● Via thermal decarboxylation
Oxamide:
● A slow-release nitrogen fertilizer produced from oxalic acid and ammonia
Oxalic acid esters:
● Used as solvents and intermediates in pharmaceutical and agrochemical synthesis
Aluminum anodizing:
● Oxalic acid anodizing produces hard, wear-resistant, gold-colored oxide coatings used in architectural and decorative applications
In water treatment, oxalic acid is used for:
Boiler cleaning:
● Removing iron oxide scale and hardness deposits from boiler tubes and heat exchange surfaces
Cooling system descaling:
● Dissolving calcium and iron deposits in cooling towers and closed-loop systems
Metal precipitation:
● Removing dissolved iron and manganese from process water
In the pulp and paper industry, oxalic acid serves as a bleaching agent and is used to remove iron and manganese from pulp, which would otherwise cause discoloration in the finished paper product.
Oxalic acid dihydrate is widely used by beekeepers worldwide for controlling Varroa destructor mites — the single most damaging parasite of honey bees (Apis mellifera). It is one of the few naturally occurring organic acids approved for use in organic apiculture across many jurisdictions.
Primary application methods:
Method | Description | Typical Timing |
Dribble / Trickle | 3.5% oxalic acid in 1:1 sugar-water solution, dribbled between frames | Broodless period (late autumn / winter) |
Sublimation / Vaporization | Oxalic acid crystals heated to ~157°C; vapor fills the sealed hive | Any broodless period; increasingly popular |
Spray | 3% oxalic acid solution sprayed directly on bees in each frame | Package bee treatment |
How it works: Oxalic acid vapor or solution contacts the mites on adult bees. The exact mode of action is still under study, but it damages the mite's mouthparts and footpads, causing detachment and death. Because oxalic acid does not penetrate capped brood cells, treatment is most effective when the colony is naturally broodless or during a forced brood break.
Typical efficacy: 90–97% mite kill in broodless colonies with a single treatment. Repeated treatments may be needed when brood is present.
Different applications demand different purity levels and impurity profiles:
Grade | Typical Purity | Key Applications | Key Quality Parameters |
Industrial / Technical Grade | ≥ 99.0% | Metal cleaning, textile processing, wood bleaching, stone cleaning | Standard heavy metal limits; consistent crystal size |
Refined / High-Purity Grade | ≥ 99.6% | Rare earth processing, chemical synthesis, aluminum anodizing | Low iron (≤ 5 ppm), low chloride/ sulfate residues |
ACS / Analytical Reagent Grade | ≥ 99.5% (per ACS spec) | Laboratory use, primary standard, titrations | Very low heavy metals; residue after ignition ≤ 0.01% |
Feed Grade (limited use) | ≥ 99.0% | Beekeeping (varroa treatment) | Controlled impurity profile per local registration |
Oxalic acid dihydrate (C₂H₂O₄·2H₂O, MW 126.07) contains two molecules of water of crystallization, while anhydrous oxalic acid (C₂H₂O₄, MW 90.03) does not. The dihydrate is the commercially dominant form because it is stable and non-hygroscopic in storage. The anhydrous form is highly hygroscopic — it absorbs moisture from the air and converts back to the dihydrate. For most industrial applications, the dihydrate is preferred; 126.07 g of dihydrate is equivalent to 90.03 g of anhydrous oxalic acid. When calculating formulations, always check whether your recipe or specification is based on the dihydrate or anhydrous basis.
Oxalic acid is often preferred over mineral acids (phosphoric, hydrochloric) for precision rust removal because it selectively attacks iron oxide without aggressively corroding the underlying steel. Phosphoric acid converts rust to a passive iron phosphate layer (good for paint prep), while hydrochloric acid removes rust quickly but can pit the base metal. Oxalic acid occupies a middle ground — effective rust dissolution with lower risk of metal damage, making it ideal for restoration work on valuable or thin-gauge metal parts.
Oxalic acid is safe for calcium carbonate-based stones (marble, limestone, travertine) and most granites because it does not etch calcite the way stronger acids do. However, it should not be used on certain colored stones — particularly green marbles (serpentine) or onyx — without testing a small, inconspicuous area first. Some colored marbles contain iron as part of their natural mineralogy and may lighten unevenly with oxalic acid treatment.
There are two common types of wood bleach, and it is important not to confuse them. Oxalic acid is a two-part bleach (often sold as a crystalline powder to be mixed with water) that removes iron stains and weathering gray but does not change the wood's natural color. The other type is a two-part peroxide bleach (sodium hydroxide + hydrogen peroxide) that actually lightens the wood's natural color. If your goal is to remove black iron stains or restore weathered gray wood, oxalic acid is the correct choice. If you want to lighten the wood itself, use peroxide bleach.
Both are organic acids with chelating properties, but they serve different niches. Citric acid is milder (pKa₁ = 3.13 vs. oxalic acid's 1.25), more food-safe, and better for light scale removal in food-processing equipment. Oxalic acid is a stronger acid and a more powerful chelator for iron, making it more effective for heavy rust removal, industrial boiler descaling, and metal surface preparation. Citric acid is often preferred where worker exposure risk and environmental discharge are primary concerns; oxalic acid is preferred where cleaning power and speed are paramount.
Rare earth elements are chemically similar and notoriously difficult to separate from each other. Oxalic acid solves a critical problem in the final purification step: it selectively precipitates rare earth ions as insoluble oxalates from acidic solution, while many contaminant metals remain dissolved. The resulting rare earth oxalate precipitate is filtered, washed, and calcined to produce high-purity rare earth oxides. This step is essential to achieving the 99.9%+ purity required for magnet, phosphor, and catalyst applications. As global demand for neodymium magnets (EV motors, wind turbines) and other rare earth products grows, so does the demand for high-purity oxalic acid in the rare earth supply chain.
Always wear appropriate PPE: chemical-resistant gloves (nitrile), safety goggles, and protective clothing. Work in a well-ventilated area or use local exhaust ventilation when handling bulk powder to avoid dust inhalation. Never eat, drink, or smoke while handling the product. Keep oxalic acid away from strong oxidizers (fire/explosion risk), strong bases, and silver compounds. In case of skin or eye contact, flush with plenty of water. Store in a locked, labeled container away from food, feed, and incompatible materials. Always read the supplier's SDS before first use.
Oxalic acid dihydrate is a versatile, cost-effective organic acid with a diverse range of industrial and agricultural applications — from heavy rust removal and wood restoration to textile bleaching, rare earth processing, and beekeeping. Its unique combination of strong acidity, powerful reducing action, and selective metal chelation makes it irreplaceable in many industrial processes.
