pH meters and rH (redoxpotential) meters. pH meters are used to measure the factor expressing the acidity or alkalinity of a solution or mixture (pure water being the neutral standard). rH meters are used to measure the oxidising or reducing power of a. Solution. These instruments operate on a number of different principles; the most common type employs the electrometric system, in which electrodes are used to set up a potential difl ierence which is proportional to the pH or rH of the solution. In addition to measuring, these instruments may also be used for automatic control.