Quote Originally Posted by Rudeofus View Post
Sodium Metabisulfite is a dehydrated form of Sodium Bisulfite: 2 NaHSO3 <----> Na2S2O5 + H2O

If you dissolve them in water, they will be the same. In aqueous solution you can convert the Sodium Metabisulfite to Sodium Sulfite by adding 2 moles of NaOH per mole of Na2S2O5.
This is how the Sulphite is made in a concentrated form in Rodinal manufacture.

Ian