Sodium Metabisulfite vs Bisulfite vs Sulfite: Choosing a Reducing Agent

Sodium metabisulfite, sodium bisulfite and sodium sulfite are all sulfur-based reducing agents, and buyers often treat them as interchangeable. They are not: they differ in active sulfur content, form, solution pH and stability, which changes the dose and the choice. This page compares the three so you can select correctly.

1. The three products

Product	CAS	Formula	SO₂ equivalent	Usual form
Sodium metabisulfite (SMB)	7681-57-4	Na₂S₂O₅	About 65% (theoretical 67.4%)	Solid (powder/granular)
Sodium bisulfite	7631-90-5	NaHSO₃	About 61% (as solution, 38-40% active)	Usually solution
Sodium sulfite	7757-82-6	Na₂SO₃	About 50%	Solid

The key relationship: when sodium metabisulfite dissolves in water it forms sodium bisulfite (one molecule of SMB gives two bisulfite equivalents). That is why SMB is often described as the solid, shippable form of bisulfite.

2. Head-to-head comparison

Property	Sodium metabisulfite	Sodium bisulfite	Sodium sulfite
Form	Solid – easy and safe to ship and store	Solution (or solid, less common)	Solid
Solution pH	Acidic (about 4 – 5)	Acidic (about 4 – 5)	Alkaline (about 9 – 10)
Reducing power	Strong (acidic, ready to act)	Strong (acidic)	Weaker – needs acid to activate
Dechlorination	Standard choice	Used where a solution is preferred	Not usual
Food use	E223 (as such)	E222	E221
Oxygen scavenging	Common (boiler water)	Used in some systems	Less common
Handling	Corrosive solid; dust control	Liquid; pumps and dosing systems	Solid; alkaline, irritant

3. How to choose

Shipping and storage matter? Choose metabisulfite – it is the solid, low-hazard-to-freight form of bisulfite.
Plant already has liquid dosing? Sodium bisulfite solution is convenient, but you pay to ship water.
Need the strongest, fastest reduction? Metabisulfite or bisulfite (both acidic and ready to react) over sulfite.
Alkaline system, or need an alkaline sulfite? Sodium sulfite, accepting that it must be activated by acid to reduce strongly.
Food applications? Check which E number and monograph applies (E221 / E222 / E223).
Confirm the dose on the chemistry – the SO₂ equivalent differs between the three, so the same reducing job needs different weights.

4. Why metabisulfite dominates

For most industrial applications – dechlorination, cyanide destruction, Cr(VI) reduction, oxygen scavenging – metabisulfite is the default because it is a solid that is easy and economical to ship, has a high SO₂ equivalent, and gives an acidic (immediately active) solution. Bisulfite solutions are used where a plant prefers liquid handling; sulfite is used where its alkaline chemistry or specific reactivity is wanted.

5. Dosage notes

Job	Typical relationship (starting point)
Dechlorination	About 1.5 – 3 mg/L metabisulfite per mg/L chlorine
Cr(VI) reduction	Stoichiometric requirement plus an excess (see the Cr(VI) guide)
Oxygen scavenging	Set on measured dissolved oxygen, with a small excess

Because the three products differ in SO₂ equivalent, always state which product a dose refers to – a dose written for metabisulfite is not the same weight of bisulfite or sulfite.

Frequently Asked Questions

Is metabisulfite the same as bisulfite?

No, but they are closely related: dissolving metabisulfite in water forms bisulfite. Metabisulfite is the solid form (about 65% SO₂ equivalent); bisulfite is usually supplied as a 38-40% solution.

Can sodium sulfite replace metabisulfite?

In some uses, but sulfite has a lower SO₂ equivalent, is alkaline, and needs acid to act as a strong reducing agent. For dechlorination and Cr(VI) reduction, metabisulfite or bisulfite is preferred.

Which one is best for dechlorination?

Metabisulfite is the standard industrial choice; bisulfite solution is used where liquid dosing is preferred.

Do they have different food additive numbers?

Yes – E223 (metabisulfite), E222 (bisulfite) and E221 (sulfite). Check which applies to your market and application.