Hypo soda is popular name of sodium hyposulfite. It has 5 crystal water (Na2S2O3?H2O), so is also called as sodium hyposulfite, pentahydrate.

Hypo soda is colorless transparent crystal, easily soluble in water. Its aqueous solution is weak alkaline. In dry air above 33℃, it will effloresce and lose the crystal water. It is stable in neutral or alkaline solution, and will rapidly decompose in acidic solution.

Na2S2O3+2HCl====2NaCl+H2O+S↓+SO2↑?

Hypo soda has quite strong complexing capability, and can react with silver bromide into complex. Reaction fromula:

AgBr+2Na2S2O3====NaBr+Na3〔Ag(S2O3)2〕. For this property, it can be used as fixing agent. When developing photos, superfluous Hypo soda will react with the unsensitized silver bromide to turn to soluble Na3〔Ag(S2O3)2〕, so as to remove AgBr and fix the development.

Hypo soda also has strong reductibility, capable to reduce chlorine, etc.

Na2S2O3+4Cl2+5H2O====H2SO4+2NaCl+6HCl. So, it can be used as chlorine remover for bleached cotton textiles, and be used to remove iodine stain.

Moreover, Hypo soda can be used in leather bating, electroplating and extracting silver from ore, etc.