Chelants and Sequestrants — Which One to Use
Direct answer
Use a true chelant (EDTA, citrate, gluconate) at full dose when every metal ion must stay in solution. Use a threshold inhibitor (SHMP, phosphonate) at a few ppm when you only need to stop scale forming.
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 much 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, 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
What Allzone supplies
Sodium hexametaphosphate, EDTA and its sodium salts, citric acid and sodium citrate, sodium gluconate and phosphonate scale inhibitors are all quoted as industrial chemicals from the Industrial Chemicals catalogue. Grade, assay basis and packaging are confirmed against the supplier documentation on file before quotation.
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