# Equilibrium cell potential question

In an attempt to wrap my head around the basics of electrochemistry I'm working my way through Wesley R. Browne's 'Electrochemistry' primer.

With regards to the equilibrium potential of an electrochemical cell he states: "The equilibrium potential is defined as the potential at which the forward and reverse electron transfer rates are equal and the net current flow is zero. ... When the electrode potential is more positive or more negative than the equilibrium electrode potential, the forward and reverse rates are no longer equal and a net current will flow."

My understanding is that the only equilibrium potential possible for a two-electrode electrochemical cell is 0V, as anything above or below 0V will result in the electrons 'wanting' to go to one electrode or the other = a net current.

However, in a three-electrode setup (working electrode, reference electrode and counter electrode) the potential of the working electrode is stated relative to the reference electrode (eg, 'vs Ag/AgCl'). In this setup, would the equilibrium potential of the working electrode be 0V relative to the counter electrode (as it's the counter electrode with which the vast majority of electrons are exchanged) but therefore not 0V relative to the reference electrode and thus you would be correct in stating that the equilibrium potential for working electrode X is (for example) +0.21V vs an Ag/AgCl reference electrode?

Any and all help appreciated!

• Maybe this will help a bit, but potentiostats always seem a bit strange! – Ed V Apr 21 at 15:28
• One thing that might be useful to remember is that the current depends on the voltage and the resistance. If you place a reference voltage electrode behind a large resistance then it will not draw significant current. That's in part what the link Ed V posts explains. – Buck Thorn Apr 21 at 17:23
• You must have two half-cells to get a voltage. So in a three-electrode setup there could be three different equilibria (O volts). Number electrodes 1, 2, and 3. So 1-2, 1-3, and 2-3. – MaxW Apr 21 at 17:44
• Thank you for the responses all. @MaxW yes there would be - so in that case the voltage could be 0V between the WE and the CE (they're at their equilibrium potential) whilst simultaneously the voltage could be (for example) +0.21V between the WE and RE. – Technocratic Apr 22 at 14:31
• If there is current flowing through the solution impedance between the WE and CE, then there is a non-zero potential difference. – Ed V Apr 22 at 16:31