Quote Originally Posted by Michael R 1974 View Post
Ian, I never got a straight answer from anyone on this, but why would KRST last so long? It has ammonium thiosulfate (ie rapid fixer) in it, and sodium sulfite (presumably as a presevative). Why would it last forever if fixer doesn't?
It's mainly the percentages. A Rapid fixer concentrate has a very high concentration of Ammonium Thiosphate and this is prone to breaking down and precipitating sulphur.

KRST has a lower level of thiosulphate and a different pH which helps as well. That combined with the Sulphite helps prevent or slow down considerably the break down.