This test method covers the apparatus and procedure for the electrometric measurement of oxidation-reduction potential (ORP) in water. The test method described has been designed for the routine and process measurement of oxidation-reduction potential. ORP is defined as the electromotive force between a noble metal electrode and a reference electrode when immersed in a solution.� The ORP electrodes are inert and measure the ratio of the activities of the oxidized to the reduced species present. In addition, the ORP electrodes reliably measure ORP in nearly all aqueous solutions and in general are not subject to solution interference from color, turbidity, colloidal matter, and suspended matter. The apparatus to be used in the measurement of ORP includes: pH meter, reference electrode, oxidation-reduction electrode and electrode assembly. Materials necessary in the procedure for ORP measurement include chemical reagents, aqua regia, water, buffer standard salts, phthalate reference buffer solution, phosphate reference buffer solution, chromic acid cleaning solution, detergent, nitric acid, redox standard solution or ferrous-ferric reference solution, and redox reference quinhydrone solutions.