Understanding Oxidation Reduction Potential (ORP) Systems
By Lori McPherson Walchem Corporation

Optimize Reactions By Measuring Oxidation-Reduction Potential
Oxidation-Reduction reactions are as important in nature as acid-base reactions. Though the measurement of pH is commonly used and relatively well understood for acid-base reactions, the measurement of Oxidation Reduction Potential (ORP) in oxidation-reduction systems is generally misunderstood and is often under-utilized. Common oxidation-reduction operations include water disinfection, odor control in scrubbers, cyanide destruction, chrome reduction and metal etching. Commonly used oxidizers include chlorine, bromine, ozone, sodium hypochlorite and hydrogen peroxide, while sodium bisulfite, sodium metabisulfite and sulfur dioxide are well-known reducing agents.

Basic Oxidation Reactions
The most basic reaction of oxidation is that of free oxygen with a metal. For example, raw metal iron in contact with oxygen will be oxidized to ferrous oxide (FeO) and further to ferric oxide (Fe2O3) according to the following reactions:

2 Fe + O 2 2 FeO

4FeO + O 2 2 Fe 2 O 3

   This can also be accomplished in an aqueous solution, with the chemicals in their disassociated ionic forms:

4Fe + 2 H 2 O + O 2 4 Fe + 4 OH

   The oxidation of the ferrous to ferric ion occurs with the loss of an electron to the oxygen. Oxidation has since been universally defined as the loss of an electron. With any oxidizing event, a reducing event must occur simultaneously; i.e. the lost electron from the oxidized element is transferred to another element, which is then defined as reduced (gains an electron).

Oxidation Reduction Potential (ORP)
ORP is related to the concentration of oxidizers or reducers in a solution, and their activity or strength. It provides an indication of the solution's ability to oxidize or reduce another material. Because oxidizers and reducers are relatively unstable in solution, those present in a system have generally been intentionally added for a specific purpose.
    These chemicals have the ability to oxidize (accept electrons) or reduce (donate electrons). When present in a solution, the addition of an oxidizer will raise the ORP value, while the addition of a reducer will lower the ORP value. The greater the concentration of an oxidizer or reducer in the solution, the faster the rate of reaction will be. The actual ORP value of a solution depends on both the concentration and activity of the oxidizer present.
    Table 1 provides a comparison of commonly used oxidizers and their oxidation potentials relative to that of chlorine. It shows that ozone, for example, is one-and-one-half times as active an oxidizer as is chlorine. Thus, on a per molecule basis, one needs less ozone than chlorine to achieve the same ORP value.
    For most purposes, water is generally considered "neutral" with regards to its ORP value. Water solutions are actually very weak oxidizing solutions, a result of dissolved oxygen that is nearly always present. Consider the reaction between water and low carbon steel, where corrosion (oxidation of the steel) will occur. Well water or city-supplied water will generally have an ORP value of between 200 and 250 mV, a range that is considered the "zero" point (or standard potential). This value is actually achieved from a standard offset created in the design of standard industrial ORP electrodes (platinum measuring surface with silver/silver chloride reference) of approximately 190 mV, plus additional oxidizing capability from the dissolved oxygen and trace chlorine residual in the range of 10 to 60 mV. The addition of oxidizing chemical will increase this value to greater than 250 mV, while the addition of reducing chemicals will suppress it to less than 200 mV.
    The ORP value of the solution quantifies the true ability or potential that the solution has to oxidize or reduce. In most applications, this property is more important than the absolute concentration of the oxidizer or reducer in the solution.

    When using ORP for process control, it is important to realize that it is the presence of the oxidizer or reducer that is being monitored, and not the chemical it is reacting with. This is extremely important to remember in such processes as cyanide destruction or chrome reduction.
    The ORP value of a process solution will generally relate to a rate of oxidation or reduction. Although many reactions, such as disinfection, can occur almost immediately, others, such as cyanide oxidation, require significant retention time to ensure nearly complete destruction.

Applications
The common ORP applications include water disinfection, odor control (air/fume scrubbing), cyanide destruction, chrome reduction, metal etching, and dechlorination. Some examples of these processes follow:

Disinfection
In water disinfection applications, the ORP value of the solution is more meaningful than milligrams per liter (mg/L) measurements of free residual or total chlorine. This is because the equilibrium between two forms of the chlorine in the water shifts with changing pH. The molecular form of free chlorine in water is HOCl, or hypochlorous acid, a strong, fast-acting oxidizer. As the pH increases, the HOCl converts to its ionic form, OCl (the hypochlorite ion), which is a weaker, slower acting oxidizer. Figure 1 shows the speciation curve. As a result, the pH has a significant affect on the oxidizing strength of any chlorine solution. Monitoring mg/L of chlorine alone would not indicate oxidization strength. Further, if the chlorine is combined with an amine or a stabilizer, the "total" chlorine concentration is also affected. These mixtures also do not provide significant oxidizing capability.
    To maintain free chlorine in its most active form, solution pH should be maintained between 7.4 and 7.6. An increase to a pH of 8.0 will convert 80 percent of the free chlorine to the hypochlorous ion form. This is 80 to 300 times less effective as an oxidizer, depending on the specific bacteria involved. It should be noted that a free chlorine measurement alone cannot guarantee disinfection. An ORP of 650 mV has been proven by the World Health Organization to provide instantaneous E. coli destruction regardless of whether it is a result of 0.3 mg/L free chlorine at 7.6 pH, or 0.4 mg/L free chlorine at 7.8 pH.

Figure 1

    The data in Tables 2 and 3 show the affect of the alkalinity of sodium hypochlorite on the pH and ORP values of a solution. Table 2 represents values when a 12.5% solution of NaOCl is added to water. NaOCl (bleach) solutions typically have pH values of 12.5 to 13.0 that can cause notable changes in the pH of the systems to which they are added. Note how quickly the pH changes with the initial additions of NaOCl. As the pH rises, the ORP decreases as the inactive OCL is formed.
    Table 3 is derived from a two-percent solution of NaOCl where sulfuric acid is added to counteract the alkalinity and reduce the pH value. Although the concentration of free chlorine has not changed, the oxidation potential of the solution increases significantly.

Other Applications
ORP is often used for odor control processes. Sodium hypochlorite (NaOCl /bleach) is added to the scrubbing solution to oxidize or disinfect the odors. Acid or caustic may also be added depending on the specific gases involved. An ORP measurement system in the recirculation line can monitor the consumption of the NaOCl and automatically replenish when necessary as shown by Figure 2.
    Other common applications for ORP include cyanide oxidation and chrome reduction - both are industrial waste treatment processes commonly found in the plating industry. Cyanide oxidation is a two-stage process utilizing pH and ORP control in each stage to safely convert cyanide to cyanate (Stage 1), and then to carbonate (Stage 2). Both stages utilize chlorine or sodium hypochlorite to oxidize the cyanide or cyanate. In chrome reduction, toxic hexavalent chrome [Cr(VI)] is reduced to a significantly less toxic trivalent chrome [Cr(III)] which can then be precipitated as chromium hydroxide, a solid that is easily handled. A choice of reducing agents is available for this process depending on the user's preference to handle liquids, powders or gases.
    Dechlorination processes for eliminating chlorine prior to industrial or municipal discharge help protect biological treatment processes and marine life from excess oxidizers. ORP controls the addition of reducing agents to "neutralize" the chlorine. This process is commonly found in plants such as rendering where high concentrations of chlorine are used to disinfect waste prior to discharge.

ORP Measurement Systems
The measurement of ORP is very similar to that of pH. Platinum is sensitive to the activity level of the electrons in the same manner as pH-sensitive glass is sensitive to the presence (activity) of the hydrogen ion. The ORP electrode is nearly identical to a pH electrode, having a platinum surface (generally either a platinum rod or band) for the measuring half of the electrode, and a silver-silver chloride reference wire in a potassium chloride reference electrolyte for the reference half as shown by Figure 3. In practice, there are many other configurations that can be used as well for measuring ORP.

Figure 2


    The electrode can be considered a battery, where voltage flows from the measurement side to the reference side of the electrode. The function of the reference electrolyte is to complete the circuit to the solution being measured. For this reason, it is important that the reference junction remain clean and free flowing. Maintenance of an ORP electrode is similar to that for a pH electrode. The electrode should stay in solution at all times; the system requires routine cleaning and calibration to compensate for electrode degradation; and, the electrode will have to be replaced on a regular basis (every 1-2 years, depending on the application).
    Cleaning an ORP electrode is generally best done with a 5% hydrochloric acid solution. This solution is most effective for solubilizing the various hard-water deposits that may form in the reference junction. It is important to keep the reference junction free-flowing, to enable the reference electrolyte to carry the reference voltage back to the solution.
    Calibration should be performed on a regular basis, with the frequency dependent upon the cleanliness of the application and the desired accuracy of the system. Once per month is an average calibration frequency for most processes. Calibration is best accomplished in freshly made saturated solutions of quinhydrone (a dry, light-sensitive powder) in buffer solutions of pH 4.0 and 7.0.
    Quinhydrone is a weak reducing agent whose activity changes with pH. By placing the quinhydrone in known pH solutions, one can produce standard ORP solutions of 90 mV (7.0 pH), and 265 mV (4.0 pH). These two solutions are used to monitor the offset (standard) and span (slope) of the electrode as it degrades over time. The quinhydrone should be stored in a cool, dark place, and fresh solutions should be used prior to any calibrations.

Figure 3

Conclusion
ORP is invaluable for measuring the oxidizing (or reducing) strength of a solution. Automatic control of ORP using on-line measurement and control systems can provide consistent process control especially for applications such as disinfection. Chlorine chemistry and its oxidizing capability are affected by external factors, especially the pH of the solution. ORP measurement indicates the true oxidizing strength of the solution, automatically incorporating the affects of these external factors. Calibration and maintenance is similar to that for pH control: routine cleaning of the sensor and 2 point calibration in buffer solutions. Annual replacement of the sensor is generally required.