Disulfurous acid, metabisulfurous acid or pyrosulfurous acid is an oxoacid of sulfur with the formula H2S2O5. Its structure is HO−S(=O)2−S(=O)−OH. The salts of disulfurous acid are called disulfites or metabisulfites. Disulfurous acid is, like sulfurous acid (H2SO3), a phantom acid, which does not exist in the free state. In contrast to disulfate (S2O2−7), disulfite has two directly connected sulfur atoms. The oxidation state of the sulfur atom bonded to three oxygen atoms is +5 and its valence is 6, while that of the other sulfur is +3 and 4 respectively.
