What does a REDOX/ORP Sensor measure, and how does it measure it?
A redox Sensor is a precious-metal Sensor. It is used for potential measurement, but it must not take part in the chemical reaction process, which means it has to withstand chemical shock. Therefore, only precious metals such as platinum, gold or silver can be selected here.
As a reference Sensor, the Ag/AgCl reference system is used, just like in pH measurement.
When a platinum needle Redox Sensor is inserted into a chlorine-containing solution, an interfacial layer forms between the surface of the platinum needle and the water surface, known as the “Helmholtz double layer.” This interfacial layer acts as a capacitor, with one terminal connected to the platinum needle and the other terminal connected to the reference sensor, just like in pH measurement. This capacitor charges due to the electrochemical potential difference between the platinum needle and the solution. The potential of the solution depends on the logarithmic concentration ratio Log COX/CRED and the sum of the potential differences of all ions in the water.
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 redox buffer concentrations, this effect can be neglected.
This effect can also be illustrated by the two-tank example mentioned earlier: one tank 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 ion-exchange process.
The surface of the Redox Sensor 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 double layer, electrochemical equilibrium will never be attained. Under such circumstances, the measured value will drift continuously, and, under certain conditions, chemical changes may also occur at the electrode surface.
Regarding pH
Regarding Conductivity
On Redox Reactions
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