Oxalic Acid Dihydrate: Properties, Applications, and Safe Use

Oxalic acid dihydrate is the crystalline, hydrated form of oxalic acid (C₂H₂O₄·2H₂O)—a strong organic acid naturally found in many plants. Known for its powerful chelating and cleaning abilities, oxalic acid dihydrate plays a crucial role in laboratories, industrial processes, and even household products.

What is Oxalic Acid Dihydrate?

• Chemical formula: C₂H₂O₄·2H₂O

• Molar mass: 126.07 g/mol

• Common names: Ethanedioic acid dihydrate, oxalate

• CAS Number: 6153-56-6

• Appearance: White crystalline solid

Oxalic acid dihydrate is the most common commercial form of oxalic acid, containing two molecules of water per molecule of acid. It is soluble in water and alcohol, with strong acidic and reducing properties.

Physical and Chemical Properties

Property	Value
Appearance	White crystals or powder
Odor	Odorless
Solubility	~10 g/100 mL (in water at 20 °C)
Melting Point	~101–102 °C (decomposes)
pKa values	~1.23 (first) and ~4.19 (second)
Acidity	Strong organic acid
Stability	Stable when stored dry and cool

 

Where is Oxalic Acid Found Naturally?

Oxalic acid occurs in many vegetables and plants, especially in:

• Spinach

• Rhubarb

• Beets

• Swiss chard

• Tea and cocoa

In nature, it often forms calcium oxalate crystals, which are insoluble salts responsible for the sharp taste or texture in some greens and may contribute to kidney stones in excessive intake.

Key Applications of Oxalic Acid Dihydrate

1. Cleaning and Rust Removal

Oxalic acid is a powerful metal cleaner and rust remover, commonly used in:

• Bathroom and toilet cleaners

• Wood bleach and stain removers

• Rust removal from iron, steel, and concrete

It works by chelation, binding metal ions (like Fe³⁺ and Ca²⁺) to dissolve and remove stains.

2. Laboratory Reagent

Used as:

• A primary standard in permanganate titrations

• A reducing agent in chemical syntheses

• A reagent in the preparation of metal oxalates

3. Textile and Leather Industry

• Bleaching and stripping of fabrics

• Mordant in dyeing processes

• Leather tanning and finishing

4. Wood and Surface Treatments

Used in:

• Wood bleaching

• Marble and stone polishing

• Restoration of old wood and metal surfaces

5. Pharmaceuticals and Research

Oxalic acid is used in drug synthesis and as an intermediate in the production of various chemicals and rare earth element purifications.

Handling and Safety

Despite its usefulness, oxalic acid dihydrate is toxic and corrosive.

Health Hazards:

• Ingestion can lead to nausea, vomiting, kidney damage, and hypocalcemia

• Inhalation of dust may irritate respiratory tract

• Skin contact can cause irritation or dermatitis

• May form insoluble calcium oxalate, which is harmful if accumulated in the body

Safety Guidelines:

• Use gloves, eye protection, and mask when handling

• Work in a well-ventilated area or fume hood

• Store in a cool, dry place, away from bases and oxidizing agents

Environmental Considerations

Oxalic acid is biodegradable and breaks down into harmless byproducts (CO₂ and water) under normal environmental conditions. However, improper disposal in large quantities may affect water acidity and aquatic life.

Conclusion

Oxalic acid dihydrate is a highly versatile compound known for its strong acidity, chelating ability, and reducing properties. It plays a critical role in cleaning, chemical manufacturing, and laboratory processes. While it's widely useful, careful handling and awareness of its toxicity are essential for safe use.

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Keywords: oxalic acid dihydrate, C₂H₂O₄·2H₂O, rust remover, cleaning acid, chelating agent, wood bleach, oxalate, primary standard acid

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