Potassium metabisulfite

Potassium metabisulfite

OTC grade K2S2O5, sealed in a plastic bag to reduce the sulfur dioxide smell.
Names
IUPAC name Potassium metabisulfite
Other names Dipotassium disulfite Potassium pyrosulfite
Properties
Chemical formula	K2S2O5
Molar mass	222.31 g/mol
Appearance	White solid
Odor	Pungent, sulfur dioxide
Density	2.34 g/cm3
Melting point	190 °C (374 °F; 463 K) (decomposes)
Solubility in water	45 g/100 ml (20 °C)
Solubility	Insoluble in alcohols, halocarbons, hydrocarbons
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	2,000 mg/kg (rat, oral)
Related compounds
Related compounds	Sodium metabisulfite
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.

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 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.

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.

References

Relevant Sciencemadness threads

• Potassium metabisulfite to K2CO3 or KOH