Potassium metabisulfite
 OTC grade K2S2O5, sealed in a plastic bag to reduce the sulfur dioxide smell.
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| Names | |
|---|---|
| Other names
Potassium pyrosulfite
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| Properties | |
| K2S2O5 | |
| Appearance | White solid |
| Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder, with a pungent sulfur odor, used as a reducing agent.
Contents
Properties
Chemical
Potassium metabisulfite decomposes if heated at 190 °C, yielding potassium sulfite and giving off sulfur dioxide fumes:
- K2S2O5(s) → K2SO3(s) + SO2(g)
Physical
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.
Availability
Potassium metabisulfite is available as a wine preservative, at various agricultural 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.
Preparation
Potassium metabisulfite can be prepared by evaporating a solution of potassium bisulfite, saturated with sulfur dioxide:
- 2 KHSO3 ⇌ H2O + K2S2O5
Projects
- Food and beverage preservative
- Generate sulfur dioxide for use as a reducing agent
- Reducing agent, used to precipitate gold from gold compounds
- Bleach neutralization
Handling
Safety
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.
Storage
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.
Disposal
Potassium metabisulfite can be neutralized with bleach.
