Sodium Metabisulphite vs Sodium Bisulphite: Differences, Grades and Uses
Two closely related sulphites, often confused. Here is how they differ and which one your process needs
Walk into any chemical market in India and ask for "bisulphite" and you may be handed either of two products: sodium metabisulphite (SMBS) or sodium bisulphite. They are chemical cousins and do much the same job, but they are not interchangeable on paper. Their sulphur dioxide (SO₂) content, physical form, grades and standards are different, and that matters for dosing, costing and compliance.
The chemistry in one minute
- Sodium metabisulphite: Na₂S₂O₅, CAS 7681-57-4, molar mass about 190.1 g/mol. A white to off-white crystalline powder.
- Sodium bisulphite (sodium hydrogen sulphite): NaHSO₃, CAS 7631-90-5, molar mass about 104.06 g/mol. It exists mainly in solution; what the trade sells as "solid sodium bisulphite" is largely metabisulphite.
The link between them is simple. When SMBS dissolves in water it forms bisulphite:
Na₂S₂O₅ + H₂O → 2 NaHSO₃
So in most water-based processes, dissolving SMBS gives you a bisulphite solution. The working species is the same; what differs is how much active SO₂ you buy per kilogram.
SO₂ content: the number that decides dosing and cost
Both products are valued for the sulphur dioxide they release. On a pure basis:
| Property | Sodium metabisulphite (SMBS) | Sodium bisulphite |
|---|---|---|
| Formula | Na₂S₂O₅ | NaHSO₃ |
| Theoretical SO₂ content | about 67.4% | about 61.6% |
| Usual commercial form | Powder, 25 or 50 kg bags | Mostly as an aqueous solution |
| Food additive number | E223 | E222 |
| Indian Standard (technical) | IS 248, sodium bisulphite, technical (covers the metabisulphite) | |
Because SMBS carries more SO₂ per kilogram and is a stable dry powder, it is the form most industries buy and dissolve on site. Always compare offers on SO₂ content, not on product name alone.
Grades and Indian Standards
Technical grade: IS 248
IS 248 covers technical-grade sodium bisulphite in its commercial solid form, which is chiefly sodium metabisulphite. The 1987 edition, still widely quoted on certificates, sets for example:
- SO₂ content: minimum 60%
- pH of a 5% solution: 4.5 to 5.5
- Iron: maximum 0.02%
- Matter insoluble in water: maximum 0.1%
BIS issued a fifth revision of IS 248 in 2023; ask your supplier which edition the batch is tested against.
Photographic grade: IS 11752
Photographic-grade sodium metabisulphite (IS 11752) has much tighter limits, for example a pH of 4.0 to 4.6 and iron of maximum 0.005%. A batch that meets the technical grade does not automatically meet the photographic grade, so a certificate claiming IS 11752 should show those tighter results.
Food grade
For food and beverage use, buy a food-grade product with the right documents. SMBS is the additive E223 and sodium bisulphite is E222; both are sulphites, which must be declared on food labels because some people are sensitive to them.
Where each one is used
Water treatment
- Dechlorination: removing residual chlorine before reverse-osmosis membranes, and from treated effluent before discharge.
- Oxygen scavenging: removing dissolved oxygen from boiler feed water and oilfield injection water to limit corrosion.
Food and beverage
Preservative and antioxidant in products such as wine, dried fruit and some processed vegetables, within the limits set by food regulations.
Textiles, paper and photography
As an antichlor after bleaching, as a reducing agent in dyeing and printing, in pulp and paper processing and as a preservative in photographic developers.
Mining and chemical processing
As a reducing agent in chemical synthesis and as an SO₂ source in cyanide-destruction processes at gold mines.
How to choose and order
- State the SO₂ content you need, for example "SMBS, SO₂ 64% min" rather than just "SMBS".
- Name the grade and standard: technical (IS 248), photographic (IS 11752) or food grade.
- Check the limits that matter to you: iron for photographic work, heavy metals for food, insolubles for RO dosing systems.
- Choose the form: powder in 25 or 50 kg bags for most plants; a ready solution only if your dosing set-up needs it.
- Ask for the COA of the actual batch, with SO₂ content, pH and iron reported.
Storage and handling
- Keep bags tightly closed in a cool, dry place. Moisture and air slowly oxidise the product to sulphate, so SO₂ content falls over time.
- Keep away from acids: contact releases sulphur dioxide gas, which is irritating to breathe.
- Use dust masks, gloves and goggles when handling the powder, and follow the Safety Data Sheet.
Frequently asked questions
Is sodium metabisulphite the same as sodium bisulphite?
No. They are different compounds (Na₂S₂O₅ and NaHSO₃), but SMBS turns into bisulphite when it dissolves in water, so in solution they behave very similarly. SMBS contains more SO₂ per kilogram.
What is the SO₂ content of sodium metabisulphite?
Pure SMBS contains about 67.4% SO₂. Technical grades under IS 248 must contain at least 60%, and good commercial batches typically report well above that.
Which one should I use for dechlorination before RO membranes?
Most plants dose a solution made from SMBS powder. Choose a grade with low insolubles and iron, and follow your membrane supplier's dosing guidance.
Why does my SMBS lose strength in storage?
Exposure to air and moisture oxidises it to sodium sulphate. Store it sealed and dry, and use older stock first.
Source it from Lithionyx Chemicals
We supply Sodium Metabisulphite to manufacturers across India, with export on order. Every batch ships with the manufacturer's Certificate of Analysis and Safety Data Sheet, and we confirm grade, packing and dispatch before we quote. See the Sodium Metabisulphite product page, use the Request a Quote button, or write to hello@lithionyx.com. Our team replies within one business day.
