2-Hydroxyphosphonoacetic acid (HPAA, CAS 23783-26-8) is the phosphonate that carries a carboxylic acid on the same carbon as the phosphorus. It is made by adding phosphorous acid across the aldehyde of glyoxylic acid, and it is the one scale inhibitor where the raw material specification has a direct, calculable effect on whether the finished product works: the oxalic acid limit in the glyoxylic acid becomes the oxalate load in the inhibitor.
1. What HPAA Is
| Item | Value |
|---|---|
| Chemical name | 2-Hydroxyphosphonoacetic acid; hydroxyphosphonoacetic acid |
| CAS number | 23783-26-8 |
| Molecular formula | C2H5O6P |
| Molecular weight | 156.03 |
| Phosphorus content | 19.85% (30.974 / 156.03) |
| Functional groups | One phosphonic acid, one carboxylic acid, one alpha-hydroxy centre |
| Commercial form | Aqueous solution, typically 40-50% active |
HPAA is a threshold inhibitor: at a few milligrams per litre it prevents calcium carbonate from nucleating and growing, even though the water is far past saturation. Two different acid functions are what make it useful – the phosphonate anchors to the crystal surface, the carboxylate adds solubility and dispersancy. It is also relatively tolerant of free chlorine compared with amine-based phosphonates, which matters in circuits that are continuously chlorinated.
2. Synthesis and Stoichiometry
Phosphorous acid is added across the aldehyde of glyoxylic acid:
OHC-COOH + H3PO3 -> HOOC-CH(OH)-PO3H2
The addition is atom-economical – no small molecule is lost – so the raw material requirement follows directly from molecular weights:
| Raw material | CAS | MW | kg per tonne of HPAA (100% yield) |
|---|---|---|---|
| Glyoxylic acid, anhydrous basis | 298-12-4 | 74.04 | 474 |
| Phosphorous acid | 13598-36-2 | 82.00 | 526 |
In practice the acid is bought as 50% solution, so a tonne of HPAA consumes about 0.95 t of 50% glyoxylic acid solution. A plant making 2,000 t/a of HPAA therefore runs through roughly 1,900 t/a of 50% solution – the reason glyoxylic acid grade selection is a routine agenda item in this segment.
3. The Oxalate Budget: Why the Grade Is a Performance Parameter
Glyoxylic acid made by nitric acid oxidation always carries oxalic acid, and nothing in the phosphonation step destroys it. Whatever oxalate arrives in the drum leaves in the inhibitor. Work the arithmetic and the number stops being decorative.
| Glyoxylic acid grade | Oxalic acid, % max | Oxalate per t of HPAA (kg) | CaCO3-equivalent calcium it can precipitate (kg) |
|---|---|---|---|
| Technical | 1.0 | 4.74 | 5.27 |
| Chelating | 0.5 | 2.37 | 2.64 |
| High purity | 0.25 | 1.19 | 1.32 |
| Premium II | 0.2 | 0.95 | 1.05 |
Conversion used: 0.474 t of anhydrous glyoxylic acid per tonne of HPAA (section 2), and 1 kg of oxalic acid binds 40.08 / 90.03 = 0.445 kg of calcium, which is 1.11 kg expressed as calcium carbonate.
Now put it in water. At a typical 10 mg/L (ppm) product dose, one tonne of HPAA treats about 100,000 cubic metres of circulating water. The oxalate that came with the technical-grade raw material is therefore 4.74 kg spread over 100,000 m3, or 0.047 mg/L. In a hard water of 500 mg/L calcium, calcium oxalate (Ksp about 2.3 x 10^-9) is already saturated at roughly 0.017 mg/L of oxalate. The technical-grade route puts the system about three times past oxalate saturation, before any oxalate already present in the make-up water is counted. The 0.2% premium route lands at about 0.0095 mg/L, below saturation.
Assumptions, stated plainly: no oxalate is destroyed or removed in the phosphonation step; the product is dosed at 10 ppm into a system with 500 mg/L calcium at 25 degC; no allowance is made for the inhibiting effect of HPAA itself on calcium oxalate, which is real but partial. This is an order-of-magnitude check, not a scaling prediction – but the conclusion survives a factor of two either way, and it is the reason chelating grade exists as a named grade rather than as a marketing phrase.
4. HPAA Against the Other Phosphonates
| Inhibitor | CAS | Formula / MW | P content | Character |
|---|---|---|---|---|
| HPAA | 23783-26-8 | C2H5O6P / 156.03 | 19.85% | Phosphonocarboxylic acid; good calcium tolerance and dispersancy, chlorine-tolerant |
| HEDP | 2809-21-4 | C2H8O7P2 / 206.03 | 30.07% | Diphosphonate; the workhorse for calcium carbonate and corrosion control at high temperature |
| ATMP | 6419-19-8 | C3H12NO9P3 / 299.05 | 31.07% | Nitrogen-containing triphosphonate; strongest calcium carbonate threshold effect but the most sensitive to free chlorine |
| PBTCA | 37971-36-1 | C7H11O9P / 270.13 | 11.47% | Phosphonocarboxylic acid with three carboxyls; excellent dispersancy, lowest phosphorus |
Two numbers in that table drive most of the commercial choice. Phosphorus content determines how much of your permitted phosphorus discharge each kilogram of inhibitor consumes – 11.5% for PBTCA versus 31.1% for ATMP is a factor of 2.7 in phosphorus per unit of active. Chlorine tolerance determines whether you can chlorinate continuously: an amine-nitrogen backbone (ATMP) is progressively oxidised by free chlorine, while C-P bonded acids without an amine nitrogen (HEDP, HPAA, PBTCA) survive it far better.
HPAA’s position is in the middle: less phosphorus than HEDP or ATMP, more dispersancy than HEDP because of the carboxylate, and cheaper than PBTCA. It is the usual choice for cooling water with moderate to high hardness and for oilfield water flooding where barium and strontium sulphate also need suppressing.
5. Where HPAA Is Used
- Cooling water. Calcium carbonate and calcium sulphate control in open recirculating systems, typically formulated with a polymer dispersant and a corrosion inhibitor rather than used alone.
- Boiler and desalination. Threshold inhibition at low dose where a stoichiometric chelant would be uneconomic or would attack the metal.
- Oilfield water flooding. Sulphate and carbonate scale control in injection and production water, where the water is often at high temperature and high total dissolved solids.
- Detergent and cleaning formulations. A builder that provides threshold inhibition without the phosphorus load of a phosphate.
6. Buying Checklist for HPAA Producers
- Specify the glyoxylic acid grade against the oxalate ceiling you can tolerate, not against the assay. Chelating grade or Premium II, not technical.
- Ask for oxalic acid on every lot. It is a reaction-control number and it drifts with the oxidation step.
- Check the nitric acid figure if your reactor is stainless. Residual nitrate in the raw material adds to the acid load and to the corrosion risk.
- Agree the solution strength you are buying – 50% versus 40% changes your reactor volume, your water balance and your freight per tonne of active.
- Verify the phosphorous acid co-supply. HPAA production consumes more phosphorous acid than glyoxylic acid by weight (526 versus 474 kg per tonne), so the phosphorous acid market sets your cost more than the glyoxylic acid market does.
7. FAQ
Does HPAA replace HEDP?
Partly. HPAA offers better dispersancy per unit of phosphorus and better calcium tolerance; HEDP is more cost-effective where calcium carbonate threshold inhibition at high temperature is the only requirement. Most formulations use both.
Why is the phosphorus content of a scale inhibitor a commercial issue?
Because discharge permits are written in terms of total phosphorus, not product. Lower-phosphorus inhibitors let a plant hit the same permit with less treatment or treat more water under the same permit.
Can HPAA be used in drinking water systems?
That depends on the jurisdiction and on the grade of the product, not on the molecule alone. Potable-water additives are regulated separately from industrial water-treatment chemicals; confirm with the authority in the destination market.
What is the usual active strength supplied?
HPAA is normally traded as an aqueous solution in the 40-50% active range. Confirm whether the assay is expressed as HPAA or as the sodium salt, because the two differ by about 7% in molecular weight.
References and Notes
- Molecular weights and phosphorus contents calculated from the formulae shown; HPAA identity cross-checked against the product record for CAS 23783-26-8.
- Oxalate solubility arithmetic uses Ksp(CaC2O4) = 2.3 x 10^-9 at 25 degC and a 500 mg/L calcium background; state your own water analysis before applying it.
- Assumptions behind the 10 ppm dose scenario are listed in section 3.
Disclaimer: technical information for industrial buyers. Not a specification, not a dosing recommendation, and not a substitute for the supplier’s COA, SDS or site-specific water-treatment design.
Next Step
PolyblueChem supplies glyoxylic acid in 50% and 40% solution across five grades, including chelating grade for phosphonate production – see the glyoxylic acid product page – and HPAA (CAS 23783-26-8) for the finished inhibitor. For the impurity background behind the oxalate limit, read glyoxylic acid vs glyoxal vs oxalic acid; for the full grade and manufacturing picture, read grades, manufacture and industrial use.