Can EDTMPS replace HEDP?

Yes, EDTMPS can often replace HEDP, but it is not a simple 1:1 substitute. The replacement requires careful consideration of the specific application, water chemistry, and performance requirements.

Below is a detailed comparison and analysis to guide the decision.

Core Difference at a Glance

Feature HEDP (Hydroxyethylidene Diphosphonic Acid) EDTMPS (Ethylenediamine Tetra(methylene phosphonic acid) Sodium Salt)

Chemical Structure 1 organic C-P-C core, 2 phosphonate groups Ethylenediamine backbone, 4 phosphonate groups, supplied as sodium salt

Molecular Weight Low (~206 g/mol as acid) High (~612 g/mol as sodium salt)

Effective Phosphorus Content High (~30% P) Lower (~10% P, due to sodium salt and larger mass)

Primary Function Excellent CaCO₃ scale inhibition, corrosion inhibition (zinc stabilization) Superior chelation, dispersancy, inhibition of Ca₃(PO₄)₂ & iron oxide deposition

Calcium Tolerance Good, but can precipitate at very high [Ca²⁺] Exceptional, high resistance to precipitation with Ca²⁺

Thermal Stability Good (up to ~120°C) Slightly lower (generally stable up to ~100-110°C)

pH Range Effective over a wide range (optimal ~7-9.5) Effective over a wide range, often used in alkaline conditions

Form (Commercial) Usually acidic liquid or solid acid Typically supplied as neutral or alkaline aqueous solution (Na salt)

When and Why EDTMPS Can Be a Better Replacement

Superior Dispersancy & Chelation:

Scenario: Systems with high suspended solids, iron (Fe³⁺), or manganese. EDTMPS is a powerful dispersant and chelant due to its 6-8 donor atoms (N and O). It excels at keeping particulates and oxidized metals in suspension.

HEDP Limitation: HEDP is primarily a threshold inhibitor and crystal modifier; its dispersancy is weaker.

High Calcium/High pH Systems:

Scenario: Cooling water with high calcium hardness and high cycles of concentration (high pH). EDTMPS has a much higher calcium tolerance and is less likely to form calcium phosphonate sludge.

HEDP Limitation: HEDP can precipitate as a calcium salt at high pH and high [Ca²⁺], losing efficacy and potentially fouling systems.

Need for Phosphate-Based Programs:

Scenario: Programs using orthophosphate or phosphonate as the primary corrosion inhibitor. EDTMPS's h3 chelation helps stabilize soluble phosphate and prevent calcium phosphate scale.

HEDP Limitation: Less effective at controlling calcium phosphate scaling.

Convenience in Dosing:

Scenario: EDTMPS (Na salt) is often supplied as a ready-to-use, neutral liquid, easier and safer to handle than acidic HEDP.

When HEDP Might Still Be Preferred

Superior CaCO₃ Inhibition at Lower Dosage:

HEDP is often more cost-effective on an active basis for preventing calcium carbonate scale due to its high phosphorus content and efficient threshold effect.

Zinc Stabilization:

HEDP is the benchmark for stabilizing zinc ions in corrosion inhibitor formulations. While EDTMPS also stabilizes zinc, HEDP is typically more effective and commonly specified.

High-Temperature Applications:

For systems consistently above 100-110°C, HEDP generally offers slightly better thermal stability.

Where Low Organic Content is Needed:

HEDP has a lower molecular weight and can be preferred in situations where minimizing total organic carbon (TOC) load is a concern.

Replacement Guidelines & Formulation Adjustments

If replacing HEDP with EDTMPS, consider these steps:

Dosage Adjustment: Do not substitute based on equal product volume or weight.

Start with a dosage of EDTMPS that provides an equivalent molar amount of phosphonate groups. Since EDTMPS has 4 groups vs. HEDP's 2, you may theoretically need ~1/2 the molar amount of EDTMPS.

In practice, begin with a 20-30% lower active mass dosage of EDTMPS than your previous HEDP active dosage and conduct performance tests.

Monitor and Rebalance:

Corrosion Protection: If the formula relied on HEDP for zinc stabilization, closely monitor corrosion rates after switching. You may need to adjust the zinc or other corrosion inhibitor levels.

Scale Control: Monitor scaling tendencies, especially for CaCO₃. The required dosage for equivalent inhibition may differ.

Dispersancy: You may see improved control of suspended solids and iron.

Synergistic Blends: Often, the best performance comes from using both together. HEDP provides excellent threshold inhibition, while EDTMPS provides superior chelation and dispersion. A blend like HEDP + EDTMPS + Polymer is a classic, robust cooling water formula.

Conclusion

EDTMPS is a viable and often superior replacement for HEDP in systems where:

Dispersancy and metal ion control are critical.

High calcium and high pH conditions exist.

Phosphate-based corrosion programs are used.

HEDP may remain the preferred choice when:

The primary goal is cost-effective CaCO₃ scale inhibition at low dosage.

Superior zinc stabilization is required.

Operating at very high temperatures.

Final Recommendation: Before fully switching, conduct bench-top tests (e.g., static jar tests for scale inhibition) and preferably a side-stream pilot test to determine the optimal dosage of EDTMPS for your specific system water and to ensure all performance criteria (scale, corrosion, dispersion) are met. Water treatment is system-specific, and performance validation is key.