Water Technology - Jan 2012

FEATURE Making the case for ORP What ORP is and when and how to use it instead of free chlorine 20 Water Technology January 2012 Oxidants acquire electrons through the process of reduction i e they are reduced Reductants lose their electrons through the process of oxidation i e they become oxidized How is ORP measured ORP sensors are basically two electrochemical half cells A measurement electrode in contact with the solution being measured and a reference electrode in contact with a reservoir of highly concentrated salt solution When the solution being measured has a high concentration of oxidizers it will accept more electrons than it loses so that the measurement electrode develops a higher electrical potential than the reference electrode A voltmeter placed in line with the two electrodes will display this difference in potential between the two electrodes Once the entire system reaches equilibrium the resulting net potential difference represents the ORP A positive reading indicates an oxidizing solution and a negative reading indicates a reducing solution The more positive or negative the value the more powerful the oxidants or reductants the greater their concentrations or both What does ORP measure ORP can be used to determine the efficacy of chemical disinfectants that work via the oxidation or reduction of the structures of microbial contaminants For example Though the measurement of free chlorine concentration is often indicated for the disinfection of water and disinfectant byproduct control there is a better way Because free chlorine works through oxidation ORP instrumentation can be used to monitor and control its effectiveness ORP measures the actual oxidation power of the solution specifically the strength and number of oxidation and reduction reactions in solution This yields a clear picture of the efficacy of the chlorine present regardless of the concentration or ratio of chlorine species in solution Measuring ORP directly reflects the sanitizing power of free chlorine or any other oxidizing or reducing chemicals The measurement of ORP is precise empirical and requires no user interpretation making it ideal for water quality and industrial process control What is ORP ORP is the acronym for Oxidation Reduction REDOX Potential ORP is a differential measurement of the mV potentials built up when electrodes are exposed to solutions containing oxidants and reductants ORP describes the net magnitude and direction of the flow of electrons between pairs of chemical species called REDOX pairs In a REDOX pair one chemical loses electrons while the other chemical gains electrons The chemical in the REDOX exchange that acquires electrons is called the oxidant HOCL OCl ClO2 bromine hydrogen peroxide etc The chemical in the REDOX exchange that gives up electrons is called the reductant Li Mg2 Fe2 Cr2 etc By Heather Rekalske For more information on this topic go to www watertechonline com and enter keyword s ORP free chlorine oxidation WHO FIGURE 1 ORP Sensor Construction The reference electrode cell is usually constructed of silver silver chloride The measurement electrode is usually constructed of platinum