I think I have my answer...
From Grant Haist, "Modern Photographic Processing"
"Thiosulfate elimination can be accelerated by the use of two classes of chemicals: (1) compounds that act by physically displacing the thiosulfates from the photographic material by an ion-exchange action and (2) compounds that react with the thiosulfate, usually oxidizing it to a form that is more soluble or more diffusible during the subsequent washing in water. The first class of compounds, which do not react chemically with the thiosulfate, are usually called hypo clearing compounds or washing aids. The second class of compounds, the oxidizers, are called hypo eliminators [Note from Haist: the term "hypo eliminator" usually referes to chemical treatments that oxidize the thiosulfate]. Both types of compounds have as their objective the improvement of silver image permanence by the reduction of the quantity of silver thiosulfates and thiosulfate that is retained by the photographic material." (v1, p. 648)
MSDS for Kodak HCA:
Dead Link Removed
Key ingredients in working solution:
Sodium sulfite 1-5%
Sodium bisulfite < 1%
MSDS for Edwal HE:
Dead Link Removed
Key ingredients in working solution
Ammonium sulfite 5%
Sodium sulfite 1.6%
Sodium sulfite is from the class of (1) hypo-clearing agent, and is present in both products. As for ammonium sulfite, I can't find an exact mention about it in Haist. I suppose it behaves as a hypo-clearing salt, but someone with more chemistery experience than I might know.
Real hypo-eliminator formulas like Kodak HE-1 do not use sulfite compounds. HE-1 uses hydrogen peroxide and ammonia in water solution.
There is no difference between a "wash aid" and a "hypo clearing agent" in Haist, so I suppose Tom is using these terms as synonyms. However there is a difference between "hypo clearing" and "hypo elimination." Edwal's product therefore seems to belong more to the hypo clearing category, despite their "hypo eliminator" brand name.
Sorry for the tangent!
