pH and oxidation reduction potential (ORP) instruments are used to measure or monitor pH and oxidation-reduction potential. pH is the negative logarithm of the hydrogen ion activity in solution. The pH value of a substance is directly related to the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations. It is one of the most common laboratory measurements because many chemical processes are dependent on pH. Oxidation reduction is a type of chemical reaction where electrons are transferred from one substance to another. The oxidized species loses electrons and the reduced species gains electrons. Both species must be present for the reaction to occur.