Sodium metabisulfite (CAS 7681-57-4) is the standard chemical for dechlorinating water — it rapidly neutralizes residual chlorine and chloramine, protecting reverse-osmosis membranes, ion-exchange resins and downstream processes. This guide covers how it works, the dosing, and where it is used in water treatment.
1. Why Dechlorination Matters
Municipal water is disinfected with chlorine or chloramine. If this residual oxidant reaches sensitive equipment, it causes damage:
- RO membranes: chlorine degrades polyamide membranes rapidly (permanent loss of rejection).
- Ion-exchange resins: oxidants degrade the resin beads.
- Process water: chlorine can interfere with product quality and downstream chemistry.
Sodium metabisulfite is the preferred dechlorination agent because it reacts quickly, is easy to dose (solid), and leaves harmless sulfate by-products.
2. The Chemistry
Sodium metabisulfite dissolves to release bisulfite, which reduces free chlorine and chloramine to chloride:
Na2S2O5 + H2O → 2 NaHSO3
NaHSO3 + HOCl → NaCl + H2SO4
The bisulfite is consumed stoichiometrically; a slight excess is used to guarantee complete removal.
3. Typical Dosing
| Parameter | Typical value |
|---|---|
| Dose ratio | About 1.5-3 mg/L metabisulfite per mg/L free chlorine (varies with chloramine) |
| Contact time | Seconds to minutes (fast reaction) |
| Injection point | Upstream of the RO membrane / resin, after filtration |
| Monitoring | ORP (oxidation-reduction potential) to confirm removal |
Dose is confirmed by measuring the chlorine/ORP at the injection point and adjusting the metering pump.
4. Applications
- RO pretreatment: the standard dechlorination step before reverse osmosis.
- Ion exchange: protecting softening and demineralization resins.
- Municipal and industrial water: final dechlorination before discharge or reuse.
- Aquaculture and process water: removing chlorine before use.
5. FAQ
How much sodium metabisulfite do I need to dechlorinate?
Roughly 1.5-3 mg/L per mg/L of free chlorine; verify with ORP/chlorine measurement at the dosing point.
Does it remove chloramine too?
Yes, sodium metabisulfite reduces both free chlorine and chloramine, though the dose requirement for chloramine is typically higher.
Is sodium metabisulfite safe for RO membranes?
Yes — it is the standard membrane-safe dechlorination chemical when dosed correctly and monitored.
Can I use ascorbic acid instead?
Yes, ascorbic acid (vitamin C) is an alternative dechlorination agent, but it is more expensive; metabisulfite is the economical standard.
6. References and Further Reading
- RO membrane manufacturer guides on pretreatment dechlorination
- Industry literature on water dechlorination chemistry
This article is published for industrial and technical reference by PolyblueChem. Doses are starting points — confirm by measurement.
PolyblueChem supplies sodium metabisulfite (CAS 7681-57-4) with COA and MSDS. See our sodium metabisulfite guide. Contact us for specifications and quotations.
Grades and Specifications
Sodium metabisulfite is supplied in food and technical grades; for dechlorination the technical/food grade both work, and the active ingredient is the same (sodium metabisulfite, Na2S2O5).
| Parameter | Typical value |
|---|---|
| Formula / MW | Na2S2O5 / 190.11 g/mol |
| CAS number | 7681-57-4 |
| Assay (as Na2S2O5) | Typically 97–98% or higher |
| SO2 equivalent | Approx. 65% (the active reducing power) |
| Heavy metals | Per grade (tighter for food grade) |
| Appearance | White to slightly yellow crystalline powder |
Dechlorination: SMBS Compared with Alternatives
| Reductant | Notes |
|---|---|
| Sodium metabisulfite (SMBS) | Low cost, easy to dose as a solution, widely used for RO membranes and water treatment |
| Sodium sulfite | Similar chemistry, slightly different handling and solubility |
| Sodium thiosulfate | Common for dechlorination; usually more expensive per unit of chlorine removed |
| Hydrogen peroxide | An oxidant, not a reductant; used elsewhere in water treatment |
| Activated carbon | Physical removal; needs replacement/regeneration, not a dosing approach |
Dosing, Monitoring and Control
Dosing is set by the free-chlorine (and chloramine) load, not by feed water volume alone. Practical guidance:
- Stoichiometry. Roughly 1.3–1.5 kg of sodium metabisulfite removes 1 kg of free chlorine; practical dosing is usually a little higher to keep a small residual margin.
- Control. The dose is monitored by measuring residual free chlorine (or by ORP) downstream of the injection point, and adjusted automatically where the load varies.
- Mixing. Inject into a well-mixed point and allow adequate contact time before the membranes.
- Overdose. Excess bisulfite adds sulfate and can consume dissolved oxygen; keep the residual small and controlled.