The potential of a standard hydrogen electrode (SHE) is exactly 0.00 volts. This isn’t a measured value but a defined one: the scientific community assigned it zero to serve as the universal reference point for all other electrode potentials. Every voltage you see in a table of standard reduction potentials is measured relative to this zero baseline.
Why the Value Is Defined as Zero
You can’t measure the voltage of a single half-reaction in isolation. Voltage only exists as a difference between two points, so you always need two half-cells to get a reading. This creates a problem: if every measurement is relative, where do you start? The solution was to pick one half-reaction and declare it zero. The hydrogen electrode got the job because it’s relatively simple and reproducible. NIST confirms the SHE is assigned a value of zero volts at all temperatures, meaning the zero isn’t just for room temperature; it holds as a reference no matter the conditions.
This zero assignment also extends to how the potential changes with temperature. The temperature coefficient of the SHE is defined as zero, so when scientists measure how other electrode potentials shift with heating or cooling, those shifts are always expressed relative to the SHE’s unchanging baseline.
How the Electrode Works
The standard hydrogen electrode consists of a platinum wire coated in a layer of finely divided platinum (called platinum black) dipped into an acid solution with a concentration that gives hydrogen ions an activity of 1. Pure hydrogen gas is bubbled over the platinum surface at a pressure of 1 atmosphere and a temperature of 25°C (298.15 K). A quick note on pressure: IUPAC now recommends using 100,000 pascals (1 bar) as the standard pressure rather than 1 atmosphere, though the difference between the two is only about 1.3% and has a negligible effect on the potential.
The platinum serves two roles. Its surface catalyzes the reaction between hydrogen gas and hydrogen ions, helping electrons transfer efficiently. And because platinum is chemically inert, it doesn’t participate in the reaction itself, which keeps the measurement stable and repeatable. The reaction at the electrode can go in either direction depending on what it’s paired with:
- As a cathode (reduction): 2H⁺ + 2e⁻ → H₂
- As an anode (oxidation): H₂ → 2H⁺ + 2e⁻
How Other Potentials Are Measured Against It
To find the standard reduction potential of any other half-reaction, you build a cell with the SHE on one side and the half-reaction you’re interested in on the other. Since the SHE contributes exactly 0.00 V, whatever voltage the cell produces is entirely due to the other electrode. If the other electrode has a stronger tendency to gain electrons than hydrogen, its standard reduction potential is positive. If it has a weaker tendency, the value is negative.
For example, zinc has a standard reduction potential of −0.762 V. That negative sign means zinc gives up electrons more readily than hydrogen does. In a cell pairing zinc with the SHE, electrons flow from the zinc side to the hydrogen side. Conversely, a half-reaction like the reduction of permanganate ions registers at +1.507 V, meaning it pulls electrons much more strongly than hydrogen. These values populate the electrochemical series, a ranked list that predicts which substances will oxidize or reduce others in a given reaction.
To calculate the overall voltage of a full electrochemical cell, you subtract the anode potential from the cathode potential. Pairing zinc (−0.762 V) as the anode with permanganate (+1.507 V) as the cathode gives a cell potential of 1.507 − (−0.762) = 2.269 V. The SHE never appears in this final calculation, but it made both individual values possible in the first place.
The Reversible Hydrogen Electrode
In practice, many electrochemistry experiments don’t run at the exact standard conditions the SHE requires. The reversible hydrogen electrode (RHE) is a variation that accounts for the actual pH of the solution being studied. The potential on the RHE scale differs from the SHE scale by a factor that depends directly on pH. At pH 0 (the standard condition), the two scales are identical. At higher pH values, the RHE-referenced potential shifts by about 0.059 V per pH unit at room temperature. This makes the RHE especially useful for work in non-acidic solutions, where maintaining the SHE’s strict conditions would be impractical.
Practical Limitations
The SHE is more of a conceptual anchor than an everyday lab tool. Maintaining a steady flow of ultra-pure hydrogen gas, keeping the acid solution at exactly the right concentration, and preventing contamination of the platinum surface all require careful effort. Trace impurities in the solution can “poison” the platinum catalyst, degrading its ability to facilitate the hydrogen reaction and causing the potential to drift. For routine measurements, most labs use secondary reference electrodes (like silver/silver chloride or calomel electrodes) that are more convenient. These secondary references have their own known potentials relative to the SHE, so any measurement can still be converted back to the SHE scale.
It’s also worth noting that the 0.00 V assignment is arbitrary in an absolute sense. Researchers have estimated the “absolute” potential of the SHE, meaning the actual energy involved in the hydrogen half-reaction relative to an electron at rest in a vacuum, at roughly 4.28 V. That number matters for computational chemistry and gas-phase experiments, but for standard electrochemistry, the universally agreed-upon value remains exactly zero.