Sodium metabisulfite

From Sciencemadness Wiki
Jump to: navigation, search
Sodium metabisulfite
Names
IUPAC name
Sodium metabisulfite
Other names
Sodium disulfite
Sodium metabisulphite
Sodium pyrosulfite
Sodium pyrosulphite
Properties
Na2S2O5
Molar mass 190.107 g/mol
Appearance White solid
Odor Pungent, sulfur dioxide
Density 1.48 g/cm3
Melting point 170 °C (338 °F; 443 K) (decomposition begins at 150 °C)
Boiling point Decomposes
45.1 g/100 ml (0 °C)
65.3 g/100 ml (20 °C)
81.7 g/100 ml (100 °C)
Solubility Reacts with acids
Very soluble in glycerol
Slightly soluble in ethanol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Potassium metabisulfite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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 manufactured 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 + S2O52-

Projects

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

Handling

Safety

Sodium metabisulfite 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. Best way to store it is in plastic resealable bags.

Disposal

Sodium metabisulfite can be neutralized with bleach and then poured down the drain.

References

Relevant Sciencemadness threads