Potassium and Thiosulfate Fixer

I have read repeatedly that potassium ions do very bad things to the speed of thiosulfate based fixers, but I have never seen an explanation why this is the case. Apart from simple curiosity, knowing about the mechanisms at work here would give me some hints about which concentration of K+ would be acceptable. Since sodium and potassium do very similar things in many chemical reactions, I'm a little bit lost here. Can anyone "in the knows" shed some insight here?

I recall that in run up, or during WWII the germans were quite scarce of Sodium, amd had lots of avialable Potassium at a much better price.
So a bunch of German fixer formulae from that period lists K Thio.
Time showed that K Thio was not a good thing for the longevity of the negatives, from what I have read from more than one source.

I have not attempted to independently verify this, or figure out why.

I am usally quite chemically curious, but not knowedgable to tease out why all things reaction wise work, or don't, or what the side effects are.

I've read that as well however it's odd because Kodak, Ilford, Agfa etc used Potassium Metabisulphite in some of their fixers and Potassium Alum as a hardener.

So while Mike's right about not using Potassium Thiosulphate in fixers smaller amounts of Potassium salts in Sodium and Ammonium Thiosulphate fixers are quite nornal in some fixer formulae.

Ian

Whatever the reason is, sodium scarcity sounds hardly realistic. Germany held a long coast line almost until the very end of WWII and there were also rock salt deposits within Germany. Sodium must have been abundant and available, at least in the quantities required for photographic fixer.