Ferric vs Ferrous Sulphate: Why It Matters in Water Treatment
Iron salts are among the most widely used coagulants in water and wastewater treatment, and buyers frequently have to choose between ferric sulphate (Fe2(SO4)3, CAS 10028-22-5) and ferrous sulphate (FeSO4). The two are related but behave differently in the plant, and the choice affects dosing, oxidation requirements and performance.
This article compares the two for water-treatment use. For the general chemistry and specifications of the ferric salt, see our overview of ferric sulphate: chemistry, production and applications.
The Key Difference: Oxidation State
The single most important difference is the oxidation state of the iron:
| Salt | Iron state | Formula | Consequence |
|---|---|---|---|
| Ferric sulphate | Fe3+ (iron(III)) | Fe2(SO4)3 | Already in the coagulating form |
| Ferrous sulphate | Fe2+ (iron(II)) | FeSO4 | Must be oxidised to Fe3+ to coagulate effectively |
Ferric iron is the species that forms the hydroxide flocs which remove turbidity, colour and phosphate. Ferrous iron must first be oxidised (commonly with chlorine or another oxidant) before it becomes an effective coagulant – an extra chemical step that ferric sulphate avoids.
Ferric vs Ferrous Sulphate: Practical Comparison
| Factor | Ferric sulphate | Ferrous sulphate |
|---|---|---|
| Need for oxidation | None – works as supplied | Usually needs oxidation first |
| Coagulation performance | Strong, direct | Weaker unless oxidised |
| Phosphate removal | Effective direct precipitation | Requires oxidation |
| Working pH | Acidic to near-neutral | Narrower; pH-sensitive |
| Chemical consumption | Single coagulant | Coagulant + oxidant |
| Typical use | Drinking water, industrial effluent | Where oxidation is already part of the process |
When to Choose Which
- Choose ferric sulphate when you want a single-step coagulant with strong performance and no separate oxidation – the usual choice for drinking-water and industrial-effluent coagulation and for phosphate removal.
- Ferrous sulphate is often chosen where an oxidation step is already present, or where it is used for other purposes (for example as a reducing agent or in chromate reduction) rather than purely as a coagulant.
- Blends and dual systems. Many plants combine an iron salt with an organic coagulant such as PolyDADMAC to reduce total dose and improve floc quality.
Dosing and Testing
| Point | Practice |
|---|---|
| Dose | Set by jar testing; depends on turbidity, colour, alkalinity and phosphate |
| pH adjustment | Iron salts depress pH, so an alkali may be needed to hold the optimum range |
| Monitoring | Turbidity, residual iron, phosphate and pH |
Because iron coagulants consume alkalinity and lower pH, the dose and any alkali addition are always established by jar testing on the actual water.
Buying the two: what differs at the loading dock
The chemistry difference drives a purchasing difference that is easy to miss. Ferrous sulphate is usually supplied as the heptahydrate solid or as a solution, and it oxidises in storage if it is not protected. Ferric sulphate is supplied as a solid or a solution too, but the figures on the certificate are not comparable unless the basis is stated.
| Item | Ferric sulphate, Fe2(SO4)3 | Ferrous sulphate, FeSO4.7H2O |
|---|---|---|
| Iron content basis | Solid grades typically around 18-23% Fe; solution around 10-12% Fe | Around 20% Fe, as Fe(II) |
| Basis trap | A “40% Fe2(SO4)3” solution is roughly 11% Fe – confirm which basis the COA uses | Usually quoted directly as Fe, which makes the two easy to mis-compare |
| Storage behaviour | Solid is hygroscopic and cakes; solution should be kept above its crystallisation point | Susceptible to oxidation and caking; colour change signals Fe(III) formation |
| Packaging | 25 kg sacks or 1000 kg bulk bags; IBC, drums or tanker for solution | Same range, but moisture protection matters more |
| Effect on dosage plant | No oxidation step needed, so the same dosing skid works | Needs aeration or an oxidant if the plant relies on Fe(II) converting in situ |
Regulatory and documentation
| Framework | What it governs | What to confirm |
|---|---|---|
| Drinking-water approval | Whether a coagulant may be used for potable supply, and at what maximum dose | The certification or approval required in your market, for the specific grade supplied |
| TSCA (US) / REACH (EU) | Inventory listing and registration | Status for the CAS number on enquiry; it is your registration that must match |
| Transport classification | Shipping class for the concentrate | The class for the strength you order – acidic concentrates and dilute solutions are not classified identically |
| Effluent limits | Sulphate and iron load in the discharge | Whether your permit constrains sulphate, which is the parameter that can rule ferric sulphate out |
| Documentation | Batch COA plus SDS | That the COA states Fe content and the basis, not only the product strength |
Frequently Asked Questions
What is the difference between ferric sulphate and ferrous sulphate?
Ferric sulphate contains iron in the Fe3+ state and ferrous sulphate in the Fe2+ state. Ferric sulphate coagulates directly, while ferrous sulphate usually needs to be oxidised to Fe3+ first.
Which is better for water treatment?
For coagulation and phosphate removal, ferric sulphate is generally preferred because it works without a separate oxidation step. The right choice depends on the process and the water.
Is ferric sulphate the same as ferric sulfate?
Yes – “sulphate” and “sulfate” are the same compound (Fe2(SO4)3, CAS 10028-22-5); only the spelling differs.
Does ferric sulphate lower pH?
Yes. Like other iron and aluminium coagulants, it consumes alkalinity and depresses pH, so the optimum dose and any alkali addition are established by jar testing.
How do I buy ferric sulphate?
PolyBlueChem supplies ferric sulphate (CAS 10028-22-5) in solution and solid grades. Contact our team for specifications, COA and a quotation.