ORP(Oxidation Reduction Potental) Theory and Applications.

ORP(Oxidation Reduction Potential)

         Similar to the manner in which acidic or alkaline solutions are quantified by pH measurements, solutions can also be graded as oxidizing or reducing based on measurements of ORP(sometimes called "redox").

       When an oxidizing and / or reducing agent is dissolved into an aqueous sample, they may react with materials present and produce a voltage, or electromotive force(EMF), that is related to the ratio of oxidized to reduce species in the sample. An electron exchange can develop between this solution and an inert metal sensor immersed in the solution, and the voltage can be measured(When compared to a reference electrode) with a pH/mV meter. This type of measurement is known as redox or ORP. The units of measurement are in mV. At a glance, an ORP electrode may look very similar to a pH electrode. Like a combination pH electrode, both the sensor and the reference are housed in a common body.

        The scale of measurement may be positive(indicating oxidizing potential) or negative(indicating reducing). It should be noted that when zero mV is observed, it is really an oxidizing situation because the reference voltage(~200 mV for an Ag/AgCl with KCl electrolyte) is included in the observed mV value in some cases the user may wish to offset the reading to remove the reference contribution. The mV is then said to be approaching the absolute mV scale that references a SHE(standard hydrogen electrode). This type of calibration is called relative mV calibration.

        An ORP sensor must be chemically inert ; it cannot be oxidized or reduced itself. It must also have the proper surface characteristics to promote rapid electron exchange, a property known as high exchange current density. Two noble metals have proven to work well for this purpose pure platinum and pure gold are both used in the construction of ORP sensors.

        The platinum sensor is often preferred because it is mechanically simpler and safer to produce, Platinum can be welded to glass and has the same thermal coefficient. Sensors made of gold cannot be welded to the glass and are often placed in plastic supports applied to the glass or plastic tube by means of tiny elastomeric bungs. The gold or platinum sensor signal is carried through the electrode body, and together with the reference signal is conducted to the measurement meter via a coaxial cable with BNC connector.

         An ORP system does not have a high impedance source(like a pH bulb), but is a potentiometric device that produces a voltage. It also uses similar cables, connectors and calibration solutions. For this reason, a high impedance electronic meter(pH) with many user friendly features are a benefit for this measurement also.

         Because of the close relationship between pH and ORP, there is a scale that takes into account the ratio(mV) ORP/pH, the rh scale. The rH range varies from zero to fourty-two, where the extreme values represent the reducing effect of an atmosphere of pure hydrogen(rH=0) and to the oxidizing effect of an atmosphere of pure oxygen(rH=42), respectively.

The formula for obtaining value is as follows. 

   rH = mV/0.0992(273.15 + T)/(-2 pH)

         In this equation, where T is the temperature(degree celcius) of the sample, mV is the ORP(mV) reading, and pH is the pH value of the sample. 

        The rH scale is not used in the instruments available on the market. A direct mV reading from the electrode is preferred, within the +/- 2000 mV range, without compensation/correlation with the pH/temperature value.

ORP Applications

       ORP measurements are based on the potential difference measured between the platinum or gold electrode and a reference electrode. The identical reference system utilized for the pH electrode(Ag/AgCl) is also used for redox measurements. Redox electrodes are used to monitor many chemical processes particularly those involving reversible reactions. Common applications include the following.

Industrial wastewater Treatment

      The redox systems used in water treatment are the reduction of chromates and oxidation of cyanides. Waste hexavalent chromium is reduced to trivalent chromium by the addition of sodium bisulfite or sulphur dioxide. In the case of cyanide, chlorine or sodium hypochlorite is used to oxidize the cyanide, followed by the hydrolysis of cyanate to ammonia and carbon dioxide.

Water Sanitation

       ORP measurements are being increasingly used as an effective measure of the sanitizing activity in pool, spa and potable water. The kill time of E.coli bacteria in water depends on the ORP value. ORP is a reliable indicator of bacteriological water quality. Water having an ORP value equal to or higher than six hundred and fifty mV are well within accepted sanitization levels for pool and spa waters.

ORP and Sample Preparation Solutions

      ORP standard solutions allows users to test the precision of ORP electrodes. For example, by immersing the electrode in ORP solution, reading should fall within the 200 to 275 mV range(@20 degree celcius/ 68 degree farenhite)