Quote Originally Posted by Alessandro Serrao View Post
Rudeofus: then why with permanganic acid a metabisulfite clearing bath is used?
I ask myself: what's the purpouse of the clearing bath?
I don't know for sure, but removing the products that are created when Permanganate/Dichromate are reduced seems like a plausible answer to me. Note that Dichromate is used both for intensifiers and for bleach, which means depending on exact formulation and post treatment you can leave the green stain in the emulsion or not.

Quote Originally Posted by Alessandro Serrao View Post
Is it to reduce CrVI to CrIII and to form Ag-sulfates complexes that are readily soluble in water?
You don't need Silver Sulfate complexes and I'm not even sure these exist. Silver Sulfate is water soluble on its own.

Quote Originally Posted by Alessandro Serrao View Post
Sodium sulfite is Na2SO3, sodium metabisulfite is Na2S2O5. Looking at their chemical formulas I see that metabisulfite anion has five negatively charged oxygens (three of which are more negatively charged than the other two), the same as the anion in sulfite.
Metabisulfite S2O52- is the anhydride form of Bisulfite HSO3-: Na2S2O5 + H2O <=====> 2 NaHSO3

which is the half way protonized version of Sulfite SO32-: NaHSO3 + H2O <=====> Na+ + H3O+ + SO32-

So yes, once in aqueous solution they are more or less the same except for the fact that Bisulfite/Metabisulfite is much more acidic.