Principles of Electrochemical Potential Measurement
You want to measure electrochemical potentials – but how do they arise?
How do electrochemical potentials arise?
Voltages and potentials arise between an electron conductor (electrode) and an ion conductor (electrolyte) as soon as they are brought into contact with each other. Charges are exchanged until the system is balanced.
Example:
If an iron sheet is immersed in copper saline, a copper precipitation is formed on the iron. If you reverse the experiment (immerse a copper sheet in ferrous saline), nothing happens.
How do these two metals differ from each other?
An equilibrium between the metal and the liquid arises when they are brought into contact. Metal ions dissolve from the lattice of the metal, which is immersed in a liquid of its own or other metal ions. That is known as solution pressure, which is yielded by the difference between the lattice energy and the hydration energy. In return, metal ions are pressed back into the metal lattice due to the separation pressure.
In the example above, this means that iron ions go into the solution while the copper ions present in the solution absorb the remaining electrons in the iron sheet and separate as copper on the iron sheet. If you reverse the experiment, neither copper goes into the solution nor are iron ions separated. Thus, the solution pressure of iron is higher than the solution pressure of copper.
If the solution pressure dominates, an excess of electrons arises at the metal rod. If the separation pressure dominates, a shortage of electrons arises at the metal rod. A dynamic equilibrium is formed. The separation of the metal ions is thereby supported by the electric attraction of the negatively charged metal. The solution pressure, the separation pressure, and the electric pressure lead to a state of minimal energy in which electric work is done. The electric work can be seen in the form of voltage at the metal rod.
This absolute equilibrium voltage cannot be determined experimentally as such, since only voltage differences are experimentally accessible. Therefore, a second electrode is needed, which is called comparative electrode or, more accurately, reference electrode, but also called an indifferent electrode or indicator electrode. Its equilibrium voltage cannot be measured separately either. To compare the voltages of different metals in their solutions, they have to be measured against the same reference electrode. Therefore, you should use a reference electrode whose equilibrium voltage is quickly and reproducibly established.
Platinum Plated Platinum Electrode as Standard Reference Electrode
In the past, a platinum plated platinum electrode washed round with hydrogen gas in hydrochloric acid has established itself for this purpose.
If you would like to learn more about this or measuring potentials with the hydrogen reference electrode HydroFlex, you can find further information here.
Would you like to learn more about this topic?
Reference Electrodes of the Second Kind
In former years, other systems besides the reversible hydrogen electrode have become established as reference electrodes. However, they can only be used with an interior electrolyte and an appropriate electrolyte bridge. These are reference electrodes of the second kind.
Electrolyte Key – Salt Bridge
An electrolyte bridge, also known as a salt bridge, is used for the contacting of various electrolytes. Commercial reference electrodes rely on salt bridges.
Basics
Do you need help with basic terms like activity, concentration, potential, and voltage? Learn more here.
Pourbaix Diagram - helpful tools in electrochemistry
The potential of the RHE itself changes with the pH of the electrolyte by 59 mV/pH @25°C. This means that for any pH-dependent system changing its potential by 59 mV/pH, you will measure a stable pH-independent potential. The so-called Pourbaix diagram gives an overview of the pH dependence of potentials.