| IUPAC name
| Other names
Sulfurous acid, calcium salt (1:1)
|3D model (Jmol)||Interactive image|
|E number||E226 (preservatives)|
|Molar mass||120.17 g/mol|
|Melting point||600 °C (1,112 °F; 873 K)|
|0.0043 g/100 mL, 18C|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|(what is ?)|
Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfurous acid with the formula CaSO3. In presence of moisture it commonly forms the hemihydrate CaSO3.0.5H2O. Its uses as the pure compound are limited, such as a food additive, but it is produced in large quantities during flue gas desulfurization (FGD) with lime, which then exposed to air converts to common calcium sulfate (CaSO4, more exactly the gypsum form, CaSO4.2H2O) by oxygen scavenging, this route of CaSO4 production via CaSO3 intermediate being the main source of drywall wallboard.
As a food additive it is used as a preservative under the E number E226. Along with other antioxidant sulfites, it is commonly used in preserving wine, cider, fruit juice, canned fruit and vegetables. Sulfites are strong reducers in solution, they act as oxygen scavenger antioxidants to preserve food, but labeling is required as some individuals might be hypersensitive.
Wood Pulp Production
Chemical wood pulping is the removal of cellulose from wood by dissolving the lignin that binds the cellulose together. Calcium sulfite can be used in the production of wood pulp through the sulfite process, as an alternative to the Kraft process that uses hydroxides and sulfides instead instead of sulfites. Calcium sulfite was used, but has been largely replaced by magnesium and sodium sulfites and bisulfites to attack the lignin.
A laboratory way calcium sulfite can be produced is a precipitation reaction between calcium chloride and sodium sulfite.
CaCl2 + Na2SO3 → CaSO3 + 2NaCl
Flue gas Desulfurization
When coal or other fossil fuel is burned the byproduct is known as flue gas. This often contains SO2 sulfur dioxide from the initial sulfur contained in the fuel, whose emission is strictly regulated by the EPA to prevent acid rain, and must be scrubbed before the remaining gases are emitted through the chimney stack. The most economical way of scrubbing SO2 from flue gases is via Ca(OH)2 hydrated lime or CaCO3 limestone.
What happens in limestone scrubbing is the following overall reaction:
SO2(g) + CaCO3(s) + 1/2 H2O(l) → CaSO3(s) .1/2 H2O + CO2(g)
Similarly, what happens in hydrated lime scrubbing is the following overall reaction:
- Ropp, Richard (2013). Encyclopedia of the Alkaline Earth Compounds. Elsevier Science Technology. pp. 145–148.
- "USGS Gypsum Statistics and Information". USGS. Retrieved June 26, 2016.
- Hudson, JL (1980). Sulfur Oxidation in Scrubber Systems. University of Virginia.
- Miller, Bruce (2004). Coal Energy Systems. Elsevier Science Technology. pp. 294–299.