Scale inhibitor 1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP)

1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP) is an organophosphonic acid scale and corrosion inhibitor widely used in water treatment systems. It is also known as Etidronic Acid, HEDPA, or Dequest 2010.

Key Properties

Molecular Formula: C₂H₈O₇P₂

Molecular Weight: ~206

Appearance: Colorless or pale yellow transparent liquid (or white crystalline powder in solid form)

Thermal Stability: Remains effective up to 250°C

pH Resistance: Stable under high pH conditions, not easily hydrolyzed

Main Applications

1. Scale Inhibition

HEDP effectively prevents the formation of calcium carbonate, calcium sulfate, and other mineral scales by chelating metal ions (especially calcium) to form stable hexagonal ring complexes. It has a strong chelating ability with Fe, Cu, and Zn ions and can dissolve oxidized materials on metal surfaces.

2. Corrosion Inhibition

HEDP can form protective films on metal surfaces, making it effective for corrosion protection of steel, copper, and other equipment.

3. Circulating Cooling Water Systems

Used in power plants, chemical plants, metallurgy, fertilizer production, and air conditioning cooling systems.

4. Other Industrial Applications

Low-pressure boiler water treatment

Reverse osmosis system scale prevention

Non-cyanide electroplating as a chelating agent

Textile and dyeing auxiliaries (as stabilizer and dispersant)

Household and industrial cleaners

Recommended Dosage

Application Dosage Range

Scale Inhibitor 1 – 10 mg/L

Corrosion Inhibitor 10 – 50 mg/L

Cleaning Agent / Detergent 1000 – 2000 mg/L

Note: HEDP is typically used in combination with polycarboxylate scale inhibitors and dispersants for synergistic effects. The actual dosage should be determined based on specific water quality conditions and system requirements.

Formulations and Recent Advances

Conventional Use

In industrial circulating cooling water systems, HEDP is often formulated into composite scale inhibitors. For example, one formulation for high-hardness, high-alkalinity water contains HEDP combined with polyaspartic acid, polymaleic anhydride, and polyacrylic acid dispersants.

Controlled-Release Technology

Recent research has developed cement-based controlled-release beads (CRBs) that encapsulate HEDP for sustained, passive release in flowing water systems. This approach converts continuous chemical injection into intermittent, passive release, reducing operational intensity and chemical waste. The release behavior follows a two-stage pattern: an initial rapid release followed by a slower, sustained phase. Key factors affecting release include bead size, temperature, and flow velocity.

Microencapsulation

HEDP encapsulated in melamine-formaldehyde microcapsules has demonstrated approximately 86% scale inhibition efficiency by inducing lattice distortion in calcium carbonate crystals.

Safety and Storage

Safety: HEDP is acidic. Avoid contact with eyes and skin. If contact occurs, rinse immediately with plenty of water.

Storage: Store in a cool, dry, and well-ventilated indoor area, protected from moisture. Shelf life is typically 10–12 months.

Advantages

Excellent thermal stability up to 250°C

Superior resistance to acid, alkali, and chlorine oxidation compared to other organophosphonates

Low toxicity and environmentally friendly profile (does not bioaccumulate significantly)

Compatible with other water treatment chemicals