# Open-circuit potential of an electrochemical cell

I'm studying electrochemistry without a physics background--never taken it for extenuating reasons--for a research project using Bard's Electrochemical Methods. I'm having trouble understanding the concept of open-circuit potential (OCP): the electric potential measured in a cell when disconnected from a circuit. First off, why can you measure OCP at all? It's a result of thermodynamics--i.e. the sum of the standard potentials of the half reactions at the given electrodes. But wouldn't the lack of a current preclude any measurement of potential? I mean, a voltmeter, with its very high resistance, still allows some current flow. Since you can measure OCP, does that mean the half-reactions are taking place in small quantities?

Second, Wikipedia describes OCP as "the voltage that must be applied to a solar cell or a battery to stop the current." If a given cell has an OCP of 0.5V, that means I apply 0.5V to stop the current once a circuit is completed? But if there exists 0.5V without a current, applying an external 0.5V won't then create a total potential of 1V? In my head I want to apply -0.5V to negate the current flow.

• The answer to your first question is yes. The higher the input impedance of the voltmeter or electrometer, the better for minimizing having the half reactions take place. For the rest, see my answer here: chemistry.stackexchange.com/a/153076/79678. You apply the external potential to oppose the cell potential, not augment it.
– Ed V
Jul 10, 2023 at 22:04
• @EdV Ahh ok. So if you have two electrodes just sitting there, nothing's going on. But measuring the OCP requires completing the circuit. Without a complete circuit, the difference in potential still exists, but no current = no rxn. It's as if I have a zipline at the top of a hill. The potential to zipline down exists, but without actually connecting the thing, I can't ride it. Jul 10, 2023 at 22:19
• Well said! With the typical voltmeter mode in a digital multimeter, the input resistance is 10 M ohm. Using this to measure an approximation of the open circuit potential is fine for most purposes since, e.g., 0.5 V divided by 10 M ohms is only 50 nA. But for a standard reference electrode, e.g., Ag/AgCl, no more than a couple of pA can be drawn without polarizing the electrode. So then you use an electrometer voltmeter, commonly used with pH electrodes. They typically have 1 T ohm input resistance.
– Ed V
Jul 10, 2023 at 22:26