[aeacfm]

hello all if i  have alot of ions in the solution like Ca++ Mg++,..also i has anions has the ability to reduce like sulfate or sulfide what is the priority of the anodic reaction , i mean what is reduces first and what is the rule control that ?
  wait there still another problem , if i want to make one ion that is not has the pririty how can i make it reduced or go to the anodic reaction before the other
i mean how can i control the process?

[FreeTheBee]

This is often not so straighforward. Standard potentials will tell you what half reaction should come first if thermodynamics were in control. In reality kinetics will complicate things. Overpotentials for different half reactions depend on the electrode material. So, if one anodic process dominates on gold, another one might do so on platinum.
There is not really a rule for this. Which is kinda nice in a way, since it allows people to have nice jobs working on electrocatalysis.

[aeacfm]

suppose we are at ordinay conditions ...

if so suppose we are at 120 degree centigrade .....