Reversible hydrogen electrode

A reversible hydrogen electrode ( engl. reversible hydrogen electrode, short RHE) provides a special electrode in electrochemical processes and consists of a reference electrode on the type of hydrogen electrode and differs in that the measured potential does not change with the pH of the standard hydrogen electrode so that it can be used directly in the electrolyte.

The name refers to the fact that the electrode is in the actual measuring electrolyte. It is not separated by a salt bridge. The hydrogen ion concentration is not 1, but corresponds to the measurement of the electrolyte; in this way it can be achieved that does not change the measured potential with the pH. The potential of the RHE therefore arises after the pH is adjusted to:

The following applies to the hydrogen electrode in which the reaction of:

, then the following dependence of the equilibrium potential of the hydrogen pressure and the activity of the oxonium ions:

In this case the standard potential is ( by definition this is equal to zero ), R is the universal gas constant, T the absolute temperature and F the Faraday constant.

In the electrolysis of water still occur surges, which means that the cell voltage required is due to kinetic inhibition higher than the equilibrium voltage. The over-voltage increases as the current density at the electrodes. The measurement of equilibrium potentials is therefore possible without power.

Design and function

In the commercially available Reversible hydrogen electrode ( Hydroflex ) could be parked some drawbacks of previous hydrogen electrodes. The benefits are due to the fact that there is no salt bridge is used:

  • There is no contamination by Cl - or SO42 - ions.
  • It now works on diffusion potentials at the salt bridge.
  • There are long- term measurements, because the salt bridge does not need to be constantly filled.
  • The temperature range is very extended (-20 ° C to 200 ° C ) because the boiling liquid, the salt bridge is missing.
  • The electrode includes an internal hydrogen supply for up to 1 year of operation time.
  • The construction of PTFE allows use even in strong alkaline media.
  • The porous platinum / palladium electrode is much more stable against contaminants than a superficial platinum plate.

Meaning and handling

The reversible hydrogen electrode has established itself in the meantime in the measurement in highly acidic or alkaline media. Here, this electrode has replaced the system mercury / mercury ( II) oxide. Firstly, because of the toxicity of mercury, on the other hand because the usual glass body of the reference electrodes are attacked by the concentrated alkalis.

In the figures shows how the well-known characteristics - here's a chart covering layer of platinum - shift in the potential position when the measurement with a reversible hydrogen reference electrode RHE or the better known calomel electrode are performed. In the sulfuric acid at pH0 can be seen that the difference between calomel and hydrogen electrode corresponds to about 250 mV. However, the representations against the reversible hydrogen potential are much clearer. One can immediately assign the evolution of hydrogen, but also the assignment of the different platinum surfaces.

  • Electrode ( electrochemistry )
Thermodynamic temperature Chloride Long-term experiment Mercury(II) oxide
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