Rocco Moretti wrote:

Dr Engelbert Buxbaum wrote:

They usually contain a small amount of sodium thiosulphate, which
destroys chlorine catalytically. Thus a very small amount of dechlor can
destroy a huge amount of chlorine, but the less you use the longer it
takes.

How does that work then? If it works catalytically (isn't used up),

The reactions I've seen on the web show the reaction as
Cl2 + H2O = HOCl + H+ + Cl-

Yes, it must start there. Presumably the hypochloric acid gets
destroyed, something like 2 HClO - 2 HCl + O2, but I have no idea about
the exact sequence of events, may be some kine of peroxy-acid
intermediate with the thiosulfate?

The other thing I have never quite understood with these chlorine
destroyers is how they stabilise the small amount of thiosulfate in
there. In the lab such a dilute solution would be prepared fresh daily.