Measurement of Electrode Potential — Definition

Definition

Imagine you have a piece of metal, say zinc, and you dip it into a solution containing zinc ions, like zinc sulfate. What happens at the surface where the metal meets the solution? There's a dynamic equilibrium established.

Zinc atoms from the metal might lose electrons and go into the solution as zinc ions, leaving electrons behind on the metal (oxidation). Conversely, zinc ions from the solution might gain electrons from the metal and deposit as zinc atoms on the metal surface (reduction).

This exchange of charge creates a potential difference between the metal electrode and its surrounding electrolyte solution. This potential difference is what we call the 'electrode potential'.

Now, here's the tricky part: you can't just connect a voltmeter to a single electrode and measure this potential directly. A voltmeter measures the *difference* in potential between two points. So, to measure the potential of a single electrode, we need a reference point.

This is where the 'Standard Hydrogen Electrode' (SHE) comes in. The SHE is a special electrode that has been universally agreed upon as the reference point, and its potential is arbitrarily set to exactly zero volts under specific 'standard conditions'.

What are these standard conditions? For the SHE, it means hydrogen gas at $1,\text{atmosphere}$ pressure bubbling over a platinum electrode immersed in a solution of $1,\text{Molar}$ hydrogen ions (like $1,\text{M HCl}$) at a temperature of $298,\text{K}$ ($25^circ\text{C}$).

When we want to measure the electrode potential of, say, our zinc electrode, we connect it to the SHE to form a complete electrochemical cell. The voltmeter then measures the potential difference between the zinc electrode and the SHE.

Since the SHE's potential is zero, the measured cell potential directly gives us the electrode potential of the zinc electrode relative to the SHE. This measured potential is specifically called the 'standard electrode potential' if the zinc electrode is also under standard conditions (e.

g., $1,\text{M Zn}^{2+}$ solution, $298,\text{K}$). By convention, these potentials are usually reported as 'standard reduction potentials', indicating the tendency of the species to gain electrons.