What does a REDOX/ORP electrode measure, and how is it measured?

A redox electrode is a noble-metal electrode. It is used for potentiometric measurements while remaining chemically inert—i.e., it must be able to withstand chemical attack. For this purpose, only noble metals such as platinum, gold, or silver are suitable.

As a reference electrode, the Ag/AgCl reference system is used, just as in pH measurement.

When a platinum redox electrode is immersed in a chloride-containing solution, an interfacial layer forms at the interface between the platinum surface and the solution–water interface; this layer is known as the Helmholtz double layer. This interfacial layer functions as a capacitor, with one plate connected to the platinum electrode and the other plate connected to a reference electrode, as in pH measurement. The capacitance charges in response to the electrochemical potential difference between the platinum electrode and the solution. The potential of the solution, in turn, depends on the logarithmic concentration ratio Log [COX]/[CRED] and the sum of the potential differences arising from all ions present in the solution.

At the same time, platinum also undergoes oxidation, and depending on the concentration of the oxidizing agent, a platinum oxide layer 3 to 4 atomic layers thick forms on its surface. This oxide layer conducts electrons, thereby hindering the redox measurement process. However, it also functions as an oxidative memory element, causing a delay in the measurement when the chloride concentration decreases. The more dilute the solution being measured, the longer this delay becomes. Under conditions of high-concentration redox buffer, this effect can be neglected.

This effect can also be illustrated by the earlier example of the two tanks: one is filled with water, while the other is empty. If the diameter of the connecting pipe is small, the process of leveling the water levels in the two tanks will be slower; conversely, a larger pipe diameter will result in a faster equilibration. Similarly, surface roughness on the electrode can introduce the aforementioned measurement inertia. This is because the pits and valleys on a rough surface can act as storage sites, thereby degrading the response time during the exchange process.

The surface of the redox electrode should be kept as smooth as possible.

Because the Helmholtz double layer behaves like a capacitor, a charging current flows whenever the potential changes, continuing until electrochemical equilibrium is reached. If the measuring amplifier does not employ a zero-current method to measure the potential of this composite layer, electrochemical equilibrium will never be attained. Under such circumstances, the measured value will continuously drift, and, under certain conditions, chemical changes may also occur at the electrode surface.