pH electrodes and ORP electrodes are analytical sensors for measuring pH and oxidation-reduction potential (ORP). These electrodes can be for measuring either pH or ORP, or both. 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 in which 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.