sodium sulfite

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A white crystalline or powdered compound, Na₂SO₃, used in preserving foods, silvering mirrors, developing photographs, and making dyes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry The sodium salt of sulfurous acid, Na2SO3, used as a food preservative, in photography and in many industrial applications.