Difference between revisions of "Sodium metabisulfite"

Revision as of 01:11, 15 August 2015

Sodium metabisulfite, also known as sodium pyrosulfite is an inorganic compound, used as a preservative and reducing agent. Its chemical formula is Na₂S₂O₅.

Properties

Chemical

When mixed with water or heated sodium metabisulfite releases sulfur dioxide, decomposing to sodium sulfite in the process.

Na₂S₂O₅ → Na₂SO₃ + SO₂

The same thing happens when a strong acid is added.

Na₂S₂O₅ + 2 HCl → 2 NaCl + H₂O + 2 SO₂

Sodium metabisulfite itself can be used as a reducing agent as well as the sulfur dioxide it produces. Perhaps the most famous example of this is the reduction of chloroauric acid to produce very pure elemental gold. It can also be added to a solution of copper(II) chloride to precipitate copper(I) chloride.

The little-known copper compound Chevreul's Salt can be produced by boiling a mixture of copper(II) sulfate and sodium metabisulfite solutions. Both of these reactions should be done outside or in a fume hood as sulfur dioxide is released, though to a much lesser extent in the case of Chevreul's salt.

Physical

Sodium metabisulfite is a white salt. It has a faint sulfur dioxide smell.

Availability

Sodium metabisulfite is available as a wine preservative, either alone or mixed with potassium metabisulfite, product known as Campden tablets. The metabisulfite salt is usually stored in an air-tight bag inside the outer paper or plastic bag, to keep it dry and reduce the sulfur dioxide smell. Some stump removers, namely the one maunfactured by Bonide (found at Home Depot), consist of sodium metabisulfite. Other stump removers may be potassium nitrate or other chemicals, so it is important to read the label/MSDS.

Preparation

Sodium metabisulfite can be prepared by evaporating a solution of sodium bisulfite saturated with sulfur dioxide:

2 HSO₃^- ⇌ H₂O + S₂O₅^2-

Projects

• Precipitate gold from gold compounds
• Generate sulfur dioxide for use as a reducing agent
• Bleach neutralization
• Chevreul's salt synthesis

Handling

Safety

It may cause allergic reactions to people who are sensitive to sulfites. Sulfur dioxide is very irritating and unpleasant as well as toxic.

Storage

Sodium metabisulfite should be kept out of hot or humid environments and away from water and acids while indoors.

Disposal

Sodium metabisulfite can be neutralized with bleach.

References

Relevant Sciencemadness threads