A practical guide to hydroxyethylidene diphosphonic acid, why its oxidant stability matters, and where it earns its premium over ATMP, from a US-based distributor.
HEDP (etidronic acid, C2H8O7P2, CAS 2809-21-4) is an organophosphonate scale and corrosion inhibitor and chelant, supplied typically as a 60% aqueous solution. It controls scale at threshold dosages, protects metal surfaces, and holds up better than most phosphonates against chlorine, which is why chlorinated cooling systems and copper-bearing circuits specify it. Stratus Chemical supplies solution and solid grades across the United States.
| Property | Value |
|---|---|
| Chemical name | 1-Hydroxyethylidene-1,1-diphosphonic acid |
| Synonyms | HEDP, HEDPA, etidronic acid, hydroxyethylidene diphosphonic acid, 1-hydroxyethylidenediphosphonic acid |
| CAS number | 2809-21-4 |
| Molecular formula | C2H8O7P2 |
| Molecular weight | 206.03 g/mol |
| EC / EINECS | 220-552-8 |
| Appearance | Clear colorless to pale yellow aqueous solution |
| Solubility | Soluble in water |
| Grades supplied | Technical, typically 60% solution; solid grades available |
| Packaging | Drums, totes |
| Documentation | COA & SDS provided on request |
The figures below are representative of 60% solution grade HEDP. They are indicative rather than guaranteed: specifications vary by source and by lot, and the certificate of analysis for your material governs.
| Parameter | Typical value |
|---|---|
| Appearance | Clear colorless to pale yellow aqueous solution |
| Physical state | Aqueous solution |
| Active content, as acid | 60% |
| Specific gravity at 25 °C | 1.44 |
| pH, 1% solution at 25 °C | Less than 2 |
| Freezing point | −25 °C |
| Chloride as Cl− | Less than 0.01% |
| Iron (Fe) | Less than 10 ppm |
The freezing point is worth noting for outdoor storage in northern climates. Solid grades run a substantially higher active content and are chosen where freight cost or cold-weather handling favors a powder.
HEDP is an organophosphonate carrying two phosphonic acid groups on a single carbon that also bears a hydroxyl group. As with all phosphonates, the important structural feature is the carbon-phosphorus bond, which resists the hydrolysis that eventually converts a polyphosphate back to orthophosphate. A phosphonate keeps working where a polyphosphate reverts and can then precipitate as calcium phosphate.
What distinguishes HEDP from the other common phosphonates is its balance. It is a competent threshold scale inhibitor, a strong chelant, and a better corrosion inhibitor than most of its peers, and it tolerates oxidizing conditions that degrade alternatives. It is rarely the cheapest option and rarely the best at any single function, but it is the one that copes with the widest range of conditions.
This is the practical reason HEDP gets specified over cheaper phosphonates, and it is worth understanding before comparing prices.
Most cooling systems run a chlorine or bromine residual for microbiological control. Phosphonates vary considerably in how well they survive that. When a phosphonate is attacked by an oxidant, it does not simply lose potency: the carbon-phosphorus bond breaks and releases orthophosphate into the system. That orthophosphate then reacts with the calcium already present and deposits as calcium phosphate scale.
The failure mode is deceptive. Scale appears in a system that is being dosed with a scale inhibitor, so the natural response is to increase the dose, which releases more orthophosphate and makes the deposit worse. HEDP resists this considerably better than ATMP, which is why chlorinated systems commonly specify it despite the higher price.
HEDP also performs better on copper and copper alloys, forming a protective surface film. In systems with copper condenser tubes or brass components, that matters as much as the scale control.
The primary market. HEDP controls calcium carbonate and calcium sulfate scale at threshold dosages while contributing corrosion protection to the metal surfaces in the circuit, allowing towers to run at higher cycles of concentration and cutting blowdown. In low and medium pressure boilers it keeps hardness off tube surfaces where deposits cause overheating.
HEDP acts as a builder and chelant, sequestering the calcium and magnesium that would otherwise consume surfactant, and holding iron in solution so it does not redeposit as staining on the cleaned surface. It is stable in the strongly alkaline and strongly acidic formulations where many chelants break down.
In peroxide bleaching, trace iron and copper catalyze runaway decomposition of the peroxide, which wastes bleach and produces radical attack that weakens or holes the fabric. HEDP sequesters those metals so the bleach decomposes in a controlled way. It is also used as a dye-fixing aid.
HEDP complexes metal ions strongly enough to serve as a chelant in plating baths formulated to avoid cyanide, which remains one of the more significant occupational and environmental hazards in metal finishing.
Water injection and produced water systems scale as pressure and temperature change through the system. HEDP is dosed alongside other phosphonates and polymers to keep injection wells and downhole equipment clear.
| Inhibitor | Where it wins | Trade-off |
|---|---|---|
| HEDP | Best all-round balance: scale threshold inhibition, strong chelation, corrosion protection, good oxidant tolerance | More expensive than ATMP and not the strongest carbonate inhibitor |
| ATMP | Stronger calcium carbonate threshold inhibition at lower cost | Degrades faster under chlorine, releasing orthophosphate that can deposit as calcium phosphate |
| PBTC | The most chlorine-stable of the common phosphonates, high calcium tolerance | Highest cost, and often more stability than the duty actually requires |
| Polyacrylate and polymer dispersants | Excellent at dispersing suspended solids and iron oxide, phosphorus-free | Weaker as a pure threshold inhibitor; usually blended with a phosphonate rather than replacing it |
How to choose: work from the oxidant program and the metallurgy rather than from the price list. A continuous free chlorine residual, or copper alloys anywhere in the circuit, points to HEDP or PBTC. A moderately treated system whose only real problem is carbonate scale points to ATMP, which will deliver more inhibition per dollar. Where the water is difficult on both counts, most commercial programs blend a phosphonate with a polymer dispersant rather than relying on either alone.
One constraint to settle early: HEDP is a phosphorus-containing product, and phosphorus discharge limits are tightening in many jurisdictions. If the site discharges to a sensitive watershed, confirm the permit position before building a program around any phosphonate.
HEDP solution is a strong acid. At 1% dilution the pH is still below 2, so the concentrate is corrosive to skin and eyes and aggressive toward mild steel, galvanized surfaces, and aluminum. Handle it as an acid rather than as a water treatment additive.
Store in closed original packaging in a cool, well-ventilated area away from alkalis and strong oxidizing agents. Use plastic, lined, or suitable stainless equipment for storage, transfer, and dosing. When diluting, add product to water rather than water to product.
The solution freezes around −25 °C, so outdoor storage in cold climates needs either heated space or a plan for thawing and remixing. Wear the eye and skin protection specified in the Safety Data Sheet, and confirm classification and handling requirements against the SDS for the specific grade.
HEDP is used as a scale and corrosion inhibitor and chelating agent, principally in recirculating cooling water, low and medium pressure boilers, and oilfield water. It also serves as a builder and chelant in detergents and industrial cleaners, as a peroxide stabilizer in textile bleaching, and as a complexing agent in non-cyanide electroplating. Stratus Chemical supplies HEDP as a 60% solution and in solid form across the United States.
The CAS number for HEDP is 2809-21-4. Its molecular formula is C2H8O7P2 and its molecular weight is 206.03 g/mol. Its EC number is 220-552-8. It is also known as etidronic acid, HEDPA, 1-hydroxyethylidene-1,1-diphosphonic acid, and hydroxyethylidene diphosphonic acid.
They overlap heavily and are frequently blended together. ATMP is generally the stronger calcium carbonate threshold inhibitor and is usually cheaper. HEDP performs better as a corrosion inhibitor, particularly on copper and copper alloys, and holds up considerably better against chlorine and other oxidizing biocides. If the system carries a continuous oxidant residual or contains copper, HEDP is normally the safer specification. If the duty is carbonate scale control in a lightly treated system, ATMP often gives more inhibition per dollar.
A representative 60% solution grade runs active content at 60% as acid, specific gravity of 1.44 at 25 °C, pH below 2 in a 1% solution, chloride below 0.01%, and iron below 10 ppm, supplied as a clear colorless to pale yellow aqueous solution with a freezing point around −25 °C. Solid grades run a substantially higher active content. Specifications vary by source and lot, so confirm against the certificate of analysis for the material you are quoted.
Trace iron, copper, and manganese catalyze the rapid decomposition of hydrogen peroxide, which wastes bleach and, worse, produces localized radical attack that weakens or holes the fabric. HEDP sequesters those metal ions so the peroxide decomposes at a controlled rate instead, protecting both the bleach bath and the fiber. Its stability at the high pH and temperature of a bleaching liquor is what makes it suitable where a weaker chelant would fail.
HEDP solution is a strong acid, with a pH below 2 even at 1% dilution, and is corrosive to skin, eyes, and many metals. Store in closed original packaging in a cool, well-ventilated area away from alkalis and strong oxidizers, and use plastic, lined, or suitable stainless equipment for storage and dosing rather than mild steel. Wear the eye and skin protection specified in the Safety Data Sheet and add product to water rather than water to product when diluting.
Stratus Chemical supplies HEDP in drums and totes, as a technical grade 60% solution and in solid form, with a certificate of analysis and Safety Data Sheet provided on request. Tell us the active content, form, and volume and we will confirm pricing and availability within one business day.
Comparing phosphonates? See ATMP, or sodium gluconate for a phosphorus-free chelant. Also treating water? See the full water treatment range, or browse all products and request a quote.
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