The potential at the ring electrode in a reduction-oxidation experiment will depend on the specific electrochemical reaction that is taking place and the ratio of the areas of the disc and ring electrodes, known as the collection ratio.

In general, during a reduction reaction on the disc electrode, electrons are gained and the potential at the disc electrode becomes more negative. As a result, the potential at the ring electrode will typically become more positive due to the increased concentration of electrons in the solution. This is known as the cathodic shift.

Conversely, during an oxidation reaction on the disc electrode, electrons are lost and the potential at the disc electrode becomes more positive. As a result, the potential at the ring electrode will typically become more negative due to the decreased concentration of electrons in the solution. This is known as the anodic shift.

The magnitude and direction of these shifts will depend on the specific electrochemical reaction and the collection ratio, which is the ratio of the surface areas of the ring electrode and disc electrode. The collection ratio can be used to calculate the potential at the ring electrode based on the potential at the disc electrode and the concentration of the electroactive species in the solution.

Overall, the potential at the ring electrode in a reduction-oxidation experiment is dependent on both the specific reaction and the collection ratio of the electrodes.

In selecting a ring potential for rotation ring-disc electrode measurements, the goal is to ensure that the potential difference between the disc and ring electrodes is large enough to enable the detection of the electrochemical reaction of interest, while minimizing any potential interference from other reactions that may occur.

One common approach is to set the ring potential to a value that is outside the potential range of the electrochemical reaction of interest, while ensuring that the potential difference between the disc and ring electrodes is sufficient to generate a measurable current. This can be achieved by performing preliminary experiments to identify the potential range of the electrochemical reaction and selecting a ring potential that is sufficiently removed from this range.

Another consideration in selecting a ring potential is to avoid any potential interference from other electrochemical reactions that may occur in the solution. This can be particularly important in complex solutions or mixtures where multiple reactions may occur simultaneously. In these cases, it may be necessary to carefully select the ring potential based on knowledge of the electrochemical behavior of the other species present in the solution.

Overall, the selection of a suitable ring potential for rotation ring-disc electrode measurements will depend on the specific electrochemical reaction of interest, the properties of the solution being studied, and the desired sensitivity and accuracy of the measurement.