The short answer
Potassium metabisulfite (also spelt potassium metabisulphite, and often shortened to KMS) is K2S2O5, a white crystalline powder that releases sulphur dioxide (SO2) when it dissolves. That SO2 makes it a preservative, an antioxidant and a reducing agent. As a food additive it is E224, and it is the standard sulphite used by winemakers, cider makers and mead makers.
It does the same job as sodium metabisulfite, but it adds potassium instead of sodium to the product. Its strength is measured by the SO2 it delivers, and that number is the key to dosing it correctly.
Potassium metabisulfite formula and identifiers
| Item | Value |
|---|---|
| Molecular formula | K2S2O5 |
| Molar mass | 222.32 g/mol |
| CAS number | 16731-55-8 |
| Food additive number | E224 (INS 224) |
| Synonyms | Potassium metabisulphite, potassium pyrosulfite, potassium disulfite, KMS, KMBS; “kalium metabisulfite” (kalium is Latin for potassium) |
| Theoretical SO2 content | 57.6% |
The molar mass comes from two potassium at 39.098, two sulphur at 32.06 and five oxygen at 16.00: 78.20 + 64.12 + 80.00 = 222.32 g/mol. One mole of K2S2O5 can release two moles of SO2 (2 × 64.07 = 128.13 g), and 128.13 ÷ 222.32 gives the 57.6% theoretical SO2 content. Real lots deliver a little less, and stock that has been stored open loses strength as it oxidises to sulphate.
How it works: SO2, pH and free sulphite
In water, potassium metabisulfite turns into potassium bisulphite: K2S2O5 + H2O → 2 KHSO3. The bisulphite is in balance with dissolved SO2, and the more acidic the liquid, the larger the share present as “molecular” SO2, the form that actually inhibits yeast and bacteria.
- Antimicrobial: it suppresses wild yeasts, acetic acid bacteria and other spoilage organisms.
- Antioxidant: it binds oxygen and the products of oxidation, protecting colour and aroma.
- Anti-browning: it inhibits the enzymes that brown cut fruit and fresh juice.
Because the effect depends on pH, the same dose protects a low-pH white wine much better than a higher-pH red. Part of the added SO2 also binds to sugars and aldehydes and stops working; winemakers therefore track “free” SO2 separately from “total” SO2.
Potassium metabisulfite in wine, cider and mead
Sulphite is used at several stages of making wine, cider and mead:
- At crushing or on fresh must or juice, to hold back wild yeast and bacteria and to limit browning before the chosen yeast takes over.
- After fermentation and at racking, to protect the wine from oxidation and spoilage while it ages.
- Before bottling, to bring free SO2 to the target level. For sweet wines it is normally used together with potassium sorbate, which stops renewed fermentation; sulphite alone does not reliably do that.
- Cleaning: an acidified sulphite solution is a common rinse for bottles and equipment. It releases SO2 gas, so use it with good ventilation.
How much potassium metabisulfite to add
Dose by the SO2 you want, not by habit. Using the 57.6% figure:
Grams of potassium metabisulfite = target SO2 (mg/L) × volume (litres) ÷ 576
| Batch size | To add 30 mg/L SO2 | To add 50 mg/L SO2 |
|---|---|---|
| 1 US gallon (3.785 L) | 0.20 g | 0.33 g |
| 5 US gallons (18.9 L) | 0.99 g | 1.64 g |
| 100 litres | 5.2 g | 8.7 g |
| 1,000 litres | 52 g | 87 g |
These figures are theoretical maximums. The right addition depends on the wine’s pH, how much SO2 is already present and how much becomes bound, so measure free SO2 and adjust. Dissolve the powder in a little water or wine first and stir it in evenly.
Campden tablets
Campden tablets are pressed tablets of potassium metabisulfite or sodium metabisulfite, sold for small batches. They are convenient, but tablet weights and dose instructions differ by brand. For larger volumes, weighed powder is cheaper and easier to dose accurately. Brewers also use Campden tablets to remove chlorine and chloramine from brewing water.
Legal limits and labelling
Total SO2 in wine is capped by law, and the caps differ by country and wine type. In the United States, the Alcohol and Tobacco Tax and Trade Bureau (TTB) allows up to 350 mg/L, and wine with 10 mg/L or more must carry a “contains sulfites” statement. The European Union and many other markets set lower limits by wine style. Check the rules of the market where the wine will be sold.
Other uses of potassium metabisulfite
- Food preservation (E224): fruit juices, fruit products, vinegar and other foods where national food law permits sulphites, as a preservative and anti-browning agent.
- Brewing: dechlorinating brewing water and protecting beer from oxidation.
- Water treatment: removing residual chlorine, where a potassium salt is preferred to a sodium one.
- Photography and laboratories: as an antioxidant in developers and as a reducing agent in analysis.
- Pharmaceuticals: as an antioxidant excipient, using pharmacopoeial grades.
- Textiles and industry: as an antichlor after bleaching and as a general reducing agent.
Potassium metabisulfite vs sodium metabisulfite
| Potassium metabisulfite | Sodium metabisulfite | |
|---|---|---|
| Formula | K2S2O5 | Na2S2O5 |
| Molar mass | 222.32 g/mol | 190.11 g/mol |
| Theoretical SO2 | 57.6% | 67.4% |
| Food additive number | E224 | E223 |
| Typical choice | Wine, cider, mead and foods where added sodium is unwanted | Water treatment, textiles, mining and bulk industrial uses; also food |
Kilogram for kilogram, sodium metabisulfite delivers more SO2, so it is often the economical choice in industry. Winemakers usually prefer the potassium salt because it does not add sodium to the wine. If you switch from one to the other, recalculate the dose from the SO2 content. Our guide to sodium metabisulphite covers the sodium salt in detail, and the sodium thiosulfate guide covers another common dechlorinating agent.
Grades and what to specify
- Technical grade: for industrial reducing, antichlor and water treatment uses.
- Food grade (E224): meets food additive specifications such as FCC, JECFA or the relevant national standard, with limits on heavy metals, arsenic and iron. This is the grade for wine and food.
- Pharmaceutical grade: meets a pharmacopoeial monograph, such as BP or Ph. Eur., where required.
- Laboratory or analytical reagent grade: for analysis and research.
Whatever the grade, put these points in the order: the assay or minimum SO2 content, iron, heavy metals and arsenic, water-insoluble matter, the specification or monograph, the particle form (powder or crystals) and the packing. The plant dose is set by SO2, so two lots that are both labelled “potassium metabisulfite” are only interchangeable if their SO2 content matches.
Safety, storage and handling
- Eyes: the powder is classified as causing serious eye damage. Wear eye protection and gloves when weighing it.
- SO2 gas: solutions give off sulphur dioxide, and contact with acids releases it quickly. Avoid breathing the fumes, and work with good ventilation.
- Sulphite sensitivity: some people, particularly some asthmatics, react to sulphites, which is why foods and wines containing them must be labelled in many markets.
- Storage: keep it in tightly closed containers in a cool, dry place, away from acids and oxidising agents. Moisture and air slowly turn it into sulphate, and caked powder or powder with a weak smell has usually lost strength.
Follow the safety data sheet (SDS) supplied with the batch for classification, first aid and transport details.
Enquire with Scapex Exports
Scapex Exports is an exporter and supplier of industrial, food and fine chemicals from India. Related products in our catalogue include potassium metabisulphite, food-grade potassium metabisulphite, potassium sorbate and sodium metabisulphite. We supply to an agreed specification, with a certificate of analysis and a 16-section GHS MSDS for every batch, a certificate of origin and full export documentation.
Tell us the grade, the specification you need, the quantity, the packing and the destination port, and we will reply with a quotation.
Frequently asked questions
What is potassium metabisulfite?
Potassium metabisulfite is K2S2O5, a white crystalline powder (CAS 16731-55-8, food additive E224) that releases sulphur dioxide in water. It is used mainly as a preservative and antioxidant in wine, cider, juice and food, and as a reducing agent in industry.
How much potassium metabisulfite per gallon of wine?
Potassium metabisulfite delivers about 57% of its weight as SO2. To add 50 mg/L of SO2 to one US gallon (3.785 L) you need about 0.33 g. Measure free SO2 and adjust to the wine's pH rather than dosing by habit.
Are Campden tablets the same as potassium metabisulfite?
Campden tablets are pressed tablets of potassium metabisulfite or sodium metabisulfite. The active substance is the same, but the tablet weight and dose depend on the brand, so read the label.
Is potassium metabisulfite safe?
Used at permitted levels it is an approved food additive (E224). The powder can cause serious eye damage and releases irritating SO2 gas, especially with acids, and some people, particularly some asthmatics, are sensitive to sulphites.
What is the difference between potassium and sodium metabisulfite?
Both release SO2 and work the same way. Sodium metabisulfite gives more SO2 per kilogram (about 67% against 58%), while potassium metabisulfite avoids adding sodium, which is why winemakers usually prefer it.
Does potassium metabisulfite kill yeast or stop fermentation?
At normal doses it suppresses wild yeast and bacteria but does not reliably stop an active fermentation by itself. To stabilise a finished sweet wine it is usually combined with potassium sorbate.
Sources: PubChem compound record for potassium metabisulfite (CAS 16731-55-8); molar mass and SO2 content calculated from IUPAC standard atomic weights; US eCFR, 27 CFR 4.22 and 27 CFR 4.32 (TTB wine rules on sulphur dioxide and sulphite labelling); EU food additive list (E224, Regulation (EC) No 1333/2008); supplier safety data sheets for potassium metabisulfite.