Oxidation reduction potential (ORP), also known as redox, is the measurement of a solution's oxidizing and reducing activity. The ORP process can be likened to stock-market activity: Whenever one material is oxidized, another material is reduced (for someone to purchase a share, another person must sell that share). Fire is an example of rapid oxidation and reduction. The carbon from a hydrocarbon combines with oxygen from the air to make CO2 while the hydrogen from the hydrocarbon combines with the oxygen to form H2O. The carbon and hydrogen have been oxidized while the oxygen has been reduced. Rust is a slower example of an oxidation/reduction reaction. Oxygen combines with iron to form iron oxides. In this process the iron is oxidized and, once again, the oxygen has been reduced. Fire and rust visually illustrate the basic characteristics of oxidation/reduction processes; namely, that the materials involved undergo chemical changes. More pertinent to water-treatment is the oxidation/reduction example of chlorine reacting with bacteria or algae. Bacteria and algae essentially are hydrocarbons, and chlorine is a powerful oxidizing agent. Even though it can't readily be seen, chlorine destroys bacteria and algae by literally burning their carbon and hydrogen into CO2 and H2O. When all of the oxidizing and reducing materials have reacted, an equilibrium is reached and there usually is a surplus of one of them. It is this surplus material that creates the oxidation or reduction potential of a solution. Measuring ORP ORP can be measured by colormetric or potentiometric means. Colormetric techniques take advantage of the fact that certain chemicals change their color as, for example, the amount of chlorine in water changes. Colormetric test kits are inexpensive but are subject to errors from the color of the water. They are not well-suited for monitoring or control applications. The principle of potentiometric operation is that whenever a metal is exposed to varying concentrations of chemicals, a millivolt-level (mV) electrical potential is generated. The millivolts generated are a function of the type of metal used, the type and concentration of the chemicals in solution and the solution's temperature. By selecting a particular metal, a correlation to the chemical type and concentration can be made and useful information obtained. In actual practice, a noble metal (a pure, elemental metal) is always used in ORP electrodes because it will not enter into unwanted chemical reactions that can lead to measurement errors. The use of a noble metal is important because, as noted above, the ORP value is a function of both the solution's chemicals and the type of metal in contact with that solution (even different noble metals can give different readings in the same solution). Platinum is normally the metal of choice; however, gold and other noble metals can also be used. The ORP potential generated at the platinum electrode varies as the chemicals in solution change. This signal is compared to that of a reference electrode (one so constructed that its potential remains constant even when the chemicals in solution change). The most commonly used reference electrode is a silver or silver chloride (Ag/AgCl) type. Unlike the pH electrode which responds only to hydrogen-ion activity, an ORP electrode responds to chemical reaction activity in which a material is converted from one oxidized state to another through electron transfer. There are many similarities between pH and ORP measurements. Why Measure? Many industries can benefit from the use of ORP measurements, including waste treatment applications and the pulp and paper industry. From a water treatment perspective, use of ORP for controlling water disinfection or the growth of algae with chlorine, chlorine dioxide, bromine and ozone in applications such as cooling towers, swimming pools, potable water supplies and a multitude of sterile water applications is of prime interest. Other oxidizers include fluorine and hydrogen dioxide. ORP measurement can be done in a variety of ways. A pH meter with a mV scale can be used simply by connecting an ORP electrode in place of the pH electrode. The mV signal generated by the electrode is representative of, for example, the residual chlorine in solution. This ORP potential is temperature-dependent; however, temperature compensation is not used because the compensation would vary for each different oxidation/reduction reaction occurring, and it is likely that several reactions are taking place at the same time. Another factor to consider when making ORP measurements is that they can be pH-dependent (remember that pH is a measure of hydrogen ions). For example, chlorine exists in solution as hypochlorous acid (OCl - ). Depending on the pH, this hypochlorous acid will shift its equilibrium to provide more or less free chlorine (this accounts for chlorine reacting more strongly at low pH values -- as the pH is lowered, more free chlorine is generated). Even though the concentration of chlorine remains constant, its oxidizing power is pH-dependent. To obtain accurate residual chlorine information, the pH must either be constant or adjusted. ORP measurement is slow when compared to a pH measurement. Whereas a pH electrode will respond in seconds, a new or cleaned ORP electrode can take several hours to initially equilibrate or re-equilibrate to a sample. Once equilibrated, the electrode's response time is measured in minutes, not seconds. ORP measurement can be defined as a measurement of oxidant demand relative to whatever ORP value is needed to accomplish a particular disinfection goal. Actual ORP levels required for bacteriological control will vary with use of the different oxidizers and makeup waters. Both concentration and activity of the oxidizer will affect the ORP levels. In addition, water chemistry -- which may inhibit an oxidizers performance -- can affect the ORP levels and affect the choice of oxidizing agents. For example, cyuranic acid is used in swimming pools to minimize the loss of chlorine. The cyuranic acid reacts with the hypochlorous acid to bind it in a form that reduces the free available chlorine. This chemical binding has the net effect of lowering the concentration of chlorine detected by the ORP electrode. Verifying Electrode Operation Calibration of ORP meters is not normally required. In fact, many ORP meters do not have calibration adjustments. However, measurement error can occur due to contamination or coatings on the electrode. Even though the meter cannot be adjusted, calibration verification can be helpful. To verify the operation of an ORP electrode, quinhydrone is added to pH buffers 4.0 and 7.0. When added to these buffers, two known, stable ORP solutions will be created. A 7.0 pH buffer with quinhydrone will produce a solution which will generate 90 millivolts with a platinum ORP electrode. A 4.0 pH buffer with quinhydrone will produce a solution of 265mV. If the electrode responds correctly in the samples, no further steps are required. If the values are incorrect, clean the electrode's metal measuring surface and reference junction with 5 percent hydrochloric acid. Scratches to the metal surface should be avoided. However, If the acid treatment is not effective, very lightly abrade the metal measuring surface with a 600-grit wet silicon carbide sandpaper using a circular polishing motion. After such abrasive cleaning the electrode may require several hours of soaking in a quinhydrone solution before providing stable readings. If these cleaning procedures do not restore the electrode's calibration the electrode will need to be replaced. Mike Ross is international marketing director for Sensorex, Stanton, CA, a manufacturer of pH and ORP electrodes. |
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