This is an excellent question that gets to the heart of why HEDP is such an effective and widely used chemical.
The short answer is: HEDP is highly resistant to hydrolysis, especially when compared to other common scale inhibitors like phosphonates and polyphosphates. Its stability under a wide range of conditions is one of its key advantages.
Here’s a detailed breakdown of its resistance:
The Core Reason: Strong P-C-P Bonds
The exceptional stability of HEDP comes from its molecular structure. The phosphorus atoms in HEDP are directly bonded to carbon atoms, forming P-C-P bonds. These carbon-phosphorus bonds are very strong and require a significant amount of energy to break.
-
HEDP’s Key Bond: P-C-P (Highly stable, resistant to chemical and thermal attack)
-
Weak Link in Others: Polyphosphates (e.g., Sodium Tripolyphosphate) have P-O-P bonds, which are much more vulnerable to breaking down (hydrolyzing) in hot or acidic water.
Resistance Under Different Conditions
1. Acidic Conditions (Low pH)
HEDP exhibits good stability in acidic solutions. While extremely strong acids and high temperatures can eventually cause some breakdown, it is far more stable than polyphosphates. For all practical purposes in water treatment (e.g., in acid cleaning formulations), it remains effective.
2. Neutral to Alkaline Conditions (High pH)
This is where HEDP truly excels. It has outstanding stability in alkaline environments.
-
It can withstand high pH levels (even above 14) without significant degradation.
-
This makes it the inhibitor of choice for applications in high-pH cooling waters, boiler water systems, and textile bleaching where caustic soda (NaOH) is used.
3. Thermal Stability (High Temperature)
HEDP has excellent thermal stability.
-
It remains stable in aqueous solutions at temperatures exceeding 100°C (212°F).
-
Under high pressure in boiler systems, it can withstand temperatures up to 200°C (392°F) without significant hydrolysis.
-
This is a critical advantage over polyphosphates, which rapidly hydrolyze at high temperatures, losing their effectiveness and forming orthophosphate scale.
4. Resistance to Chlorine (Oxidizing Biocides)
It’s important to distinguish hydrolysis from oxidation. While HEDP is highly resistant to hydrolysis, it is susceptible to degradation by strong oxidizing agents like chlorine (hypochlorite). However, its resistance is generally better than that of other common phosphonates like ATMP or EDTMPA.
Comparison with Other Common Inhibitors
To put HEDP’s resistance in context, here is a comparison:
| Chemical | Key Bond | Hydrolytic Stability | Key Weakness |
|---|---|---|---|
| HEDP | P-C-P | Excellent | Oxidizing agents (e.g., Chlorine) |
| ATMP / EDTMPA | P-C-N | Very Good | C-N bond is more vulnerable to oxidizers than HEDP’s C-C bond |
| Polyphosphates | P-O-P | Poor | Hydrolyzes rapidly in hot/acidic water to orthophosphate |
| Phosphinates (e.g., PBTC) | P-C-C | Excellent | Very good stability, even against chlorine |
Note on PBTC: While HEDP is highly resistant to hydrolysis, another phosphonate called PBTC (Phosphonobutane-1,2,4-Tricarboxylic Acid) is often noted for having even better resistance to oxidizing biocides like chlorine, while also being very hydrolytically stable.
Summary and Practical Implications
HEDP is highly resistant to hydrolysis across a wide pH and temperature range. This stability translates into several key benefits in industrial applications:
-
Long-Lasting Effect: It does not break down quickly in the system, providing sustained scale inhibition over time.
-
Application Flexibility: It can be used in high-temperature systems (boilers, heat exchangers) and across a broad pH range without losing efficacy.
-
No Harmful By-Products: Unlike polyphosphates, it does not hydrolyze to form orthophosphate (PO₄³⁻), which is itself a scaling ion and can contribute to eutrophication in wastewater.
In conclusion, you can confidently use HEDP in most water systems with the knowledge that it will remain structurally intact and functionally effective under normal operating conditions. Its resistance to hydrolysis is a cornerstone of its performance.