Quote Originally Posted by miha View Post
Out of curiosity I went and checked the msds of sodium thiosulphate and under the Section 10 - Stability and Reactivity there is a paragraph Incompatibilities with Other Materials saying it's not compatile (among other things) with silver salts and iodine.

I'm missing a bigger picture here...
Yes it's not compatible with Silver salts and Iodine, it reacts with them to form complexes, which is exactly what we want in photography but we need to ensure that the silver/thiosulphate complexes are water soluble. Too high a level of silver and the complexes are less water soluble and too high a level of Iodide inhibts the process.