Ah! I haven't written T_E\ X in many years!Originally Posted by skahde
Trying to guess hat happens in such a solution is something I would leave to my University techer - professor Foss. He was nominated to the Nobel price for his work on sulfur chemistry!
The S_n^2- ion is very soluble, unlike S_8 (which, to the uninitiated, means solid sulfur). The "n" is not limited to 1-8, there are chains as long as 14 (although they are a lot less stable than shorter chains)!
Since a polysulfide chain can "drop" a sulfur atom without changing oxidation state, the reaction with silver is 2Ag + S_n^2- -> Ag_2 S + S_(n-1)^2-.
Then since the silver is a solid, and the sulfur reacts "dirctly" with the metallic silver, the only influence of higher concentration would be to increase the amount of sulfur available for the reaction - so it should speed up with higher concentration.
Since this doesn't happen, there must be some other reaction involved. My initial idea is that the lower concentration gives a proportionally higher concentration of thiosulfate (which will form in any aqueous sulfide solution). As we know the thiosulfate is a strong complexing agent for silver - that's why it's so good for fixing. So a relatively higher concentration of thiosulfate could "mobilise" the silver, causing it to react more easily with hte sulfide?
A similar effect is the "development" of van Dyke prints in the fixer - the dissolving silver reacts with the Fe^2+ to form more solid silver. The darkening in the fixer is abrupt and very obvious; a case of fixer working as developer.