Standard Electrode Potentials

Consider two containers having different fluid levels. When connected, the liquid flows from the higher to the lower level. By measuring the individual fluid levels, the flow direction can be predicted.

Comparatively, in a galvanic cell, knowing individual electrode potentials helps predict the oxidant, reductant, and flow of electrons. Here, each half-cell has its corresponding electrode potential. However, the individual electrode potential cannot be measured directly but only the potential difference between the two half-cells. So how is it calculated?

To determine individual electrode potentials, one electrode is assigned a potential of zero, and the other electrode’s potentials are measured relative to it.

The electrode with a potential of zero is the hydrogen electrode under standard-state conditions, also called the standard hydrogen electrode, or SHE. The SHE is operated at 25 °C, consisting of an inert platinum electrode partially submerged in 1 molar hydrochloric acid and exposed to a stream of hydrogen gas at 1 atmosphere.

When a zinc electrode is connected to the SHE, its mass reduces, indicating its oxidation to Zn²⁺ ions. Simultaneous increased production of hydrogen gas signifies a reduction of hydrogen ions.

Thus, zinc is the anode, and SHE the cathode. Zinc’s standard electrode potential of −0.76 V indicates a greater oxidative potential than the SHE.

However, when a copper electrode is connected to the SHE, its mass increases as Cu²⁺ is reduced to copper. Hence, copper is the cathode, and SHE the anode. Copper’s standard reduction potential of +0.34 V indicates a greater reductive potential than the SHE.

Standard electrode potentials provide a measure of oxidations or reductions to occur—the more positive, the greater the tendency for a reduction under standard conditions.

A redox reaction’s direction and spontaneity are determined by examining the individual standard electrode potentials.

A positive summation of the electrode potentials implies a spontaneous reaction. Thus, for a copper-zinc galvanic cell with zinc as the anode and copper as the cathode and a cell potential of +1.10 V, both half-cell reactions occur spontaneously in the forward direction.

Lastly, the standard electrode potential is an intrinsic property and unaffected by any change in the half-reaction stoichiometry.