OTC grade K2S2O5, sealed in a plastic bag to reduce the sulfur dioxide smell.
| IUPAC name
| Other names
|Molar mass||222.31 g/mol|
|Odor||Pungent, sulfur dioxide|
|Melting point||190 °C (374 °F; 463 K) (decomposes)|
|45 g/100 ml (20 °C)|
|Solubility||Insoluble in alcohols, halocarbons, hydrocarbons|
|Safety data sheet||Sigma-Aldrich|
|Lethal dose or concentration (LD, LC):|
LD50 (Median dose)
|2,000 mg/kg (rat, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder, with a pungent sulfur odor, used as a reducing agent.
- K2S2O5(s) → K2SO3(s) + SO2(g)
Potassium metabisulfite is a white powder, with a pungent smell of sulfur dioxide. It is extremely soluble in water (450 g/l at 20 °C), and will hydrolyze if added in water. It is insoluble in ethanol and several other organic solvents.
Potassium metabisulfite is available as a wine preservative, at various agricultural stores and sometimes even food stores, either pure or mixed with sodium metabisulfite, product known as Campden tablets. Potassium metabisulfite is stored in an air-tight bag inside the outer paper or plastic bag, to keep it dry and reduce the sulfur dioxide smell.
Potassium metabisulfite can be prepared by evaporating a solution of potassium bisulfite, saturated with sulfur dioxide:
- 2 KHSO3 ⇌ H2O + K2S2O5
- Food and beverage preservative
- Generate sulfur dioxide for use as a reducing agent
- Reducing agent, used to precipitate gold from gold compounds
- Bleach neutralization
Potassium metabisulfite may cause allergic reactions to people who are sensitive to sulfites. It also gives off sulfur dioxide vapors, that are very irritating and unpleasant, as well as toxic.
Potassium metabisulfite must be kept out of hot or humid environments and away from water and acids while indoors. Storing it in resealable bags, as it's generally sold, is a good choice.
Potassium metabisulfite can be neutralized with bleach.