Chelants and Sequestrants — What They Do and Which One to Use
Direct answer
A sequestrant holds a metal ion in solution so it cannot precipitate, stain, foul a membrane or deactivate a surfactant. A threshold inhibitor does something different and cheaper: at a fraction of stoichiometric dose it distorts crystal growth so scale never forms a coherent deposit. Confusing the two is the most common formulating error in industrial water and cleaning chemistry. If you need to hold every calcium ion — a plating bath, a high-purity rinse — you need a true chelant such as EDTA at full stoichiometric dose. If you only need to stop scale forming on a heat exchanger, you dose SHMP or a phosphonate at a few parts per million and let threshold inhibition do the work at a tenth of the cost.
The main options
- Sodium hexametaphosphate (SHMP). The classic low-cost threshold inhibitor and dispersant for calcium carbonate and calcium sulfate. Effective at 2–10 ppm in cooling and process water. Its weakness is hydrolysis: in hot or acidic water it reverts to orthophosphate, which then precipitates calcium phosphate — the scale you were trying to avoid. Keep it below roughly 60 °C and near-neutral pH, and prepare fresh solutions.
- EDTA and its sodium salts. A true chelant that binds calcium, magnesium, iron and copper in a 1:1 molar complex across a wide pH range. Powerful, predictable, and expensive because you must dose stoichiometrically. Discharge limits apply in many jurisdictions because it is poorly biodegradable and can remobilise metals in effluent.
- Citric acid and sodium citrate. Moderate chelating strength, readily biodegradable, and effective on iron in the acidic range. The default for membrane cleaning, food-plant CIP acid steps and boiler passivation where a residue must be benign.
- Sodium gluconate. Strong on iron and aluminium in alkaline conditions, where citrate is weak. Standard in caustic bottle-wash and alkaline degreasers, and as a concrete set retarder.
- Phosphonates (ATMP, HEDP, PBTC). Threshold inhibitors that are far more hydrolysis-resistant than SHMP and tolerate chlorine and heat. The choice for demanding cooling and reverse-osmosis duties where SHMP breaks down.
Choosing in one page
| Requirement | Use | Not this |
|---|---|---|
| Stop carbonate scale in a cooling loop | SHMP or phosphonate at threshold dose | EDTA (cost) |
| Hold hardness completely in a bath or rinse | EDTA at stoichiometric dose | SHMP (insufficient capacity) |
| Iron removal in acidic cleaning | Citric acid | Gluconate (weak below pH 7) |
| Iron and aluminium in caustic cleaner | Sodium gluconate | Citrate (weak above pH 10) |
| Hot water above 60 °C, or chlorinated water | Phosphonate | SHMP (hydrolyses) |
| Effluent with strict chelant limits | Citrate or gluconate | EDTA (biodegradability) |
Dose calculation, briefly
For true chelation, calculate on moles: each mole of chelant sequesters approximately one mole of divalent metal. Convert water hardness expressed as mg/L CaCO₃ to moles, add the iron and manganese present, and apply a modest safety factor. For threshold inhibition, do not calculate stoichiometrically — dose to a field-proven ppm and verify with a scaling index and heat-exchanger inspection. Overdosing a threshold inhibitor buys nothing and, with SHMP, can create phosphate scale of its own.
What to state on the RFQ
- Product and grade: technical, food grade, or NSF-listed for potable duty
- Assay basis — SHMP is quoted as % P₂O₅; EDTA as acid or a specific sodium salt
- Physical form: powder, granular, or a stated-strength solution
- Chloride, iron and heavy-metal limits if the chemical enters a sensitive process
- Packaging and pallet configuration
- SDS, TDS, certificate of analysis and any regulatory listing you must hold