EQUILIBRIUM ELECTRODE POTENTIAL2

"The concept of equilibrium potential is probably easiest to demonstrate with a simple metal/metal-ion electrode system. When a metal (e.g., silver) is immersed in a solution containing its ion (e.g., silver nitrate solution) metal ions will cross the metal/solution interface. They will pass from the phase where the ""chemical energy"" of the ion is large to the phase where the ""chemical energy"" of the ion is smaller. Depending on the conditions of the system, this can occur in either direction. However only the positively charged (e.g., silver) cations can pass through the interface. The negatively charged electrons cannot pass into the solution, and the anions (e.g., nitrate) cannot pass into the metal. Consequently, charge accumulation occurs at the interface forming an electrical double layer. When the metal ions move preferentially from the metal into the solution, the metal surface becomes negatively charged because of the accumulation of the electrons left behind, while the solution layer near the metal surface becomes positively charged because of the accumulation of silver ions. In the opposite case, the metal surface becomes positively charged, while the solution layer near the metal surface becomes negatively charged because of the accumulation of nitrate ions tat are left behind. In either case, this process produces a potential difference between the two phases that will slow and eventually stop the passage of the metal ions. At ""equilibrium"" the chemical driving force and the opposing electrical force are equal. The potential difference between the metal and the solution phases under these conditions is the ""equilibrium potential difference."" This potential difference cannot be measured because there is no way to make an electrical connection to the solution phase without setting up another electrode potential. Consequently, electrode potentials are always measured against a reference electrode whose potential is known on an arbitrary scale. See standard hydrogen electrode. "