Measuring Techniques for Oxidation- Reduction Potential in High Purity Water

The measurement of Oxidation-Reduction Potential (ORP) can manner. The focus of this paper will be primarily on the ORP be used in the water/steam cycle of a power plant to infer (redox) measurement, although ECP and corrosion potential are information about system reactions to water chemistry. While the mentioned, since these are usually seen to be qualitatively, if not measurement of ORP is by no means a new measurement, the quantitatively, very similar. high purity water of the water/steam cycle is a relatively new application for ORP. This presents a challenge since little ORP, also called redox, utilizes a platinum measuring electrode information is available on proper measuring practices, such as with a Ag/AgCl reference electrode to measure the potential. calibration and sampling techniques. These techniques are Most fossil plants and the secondary side of some pressurized important to properly utilize the measurement in order to obtain water reactor (PWR) nuclear plants make this measurement at meaningful results. Attempts to correlate ORP findings with ambient temperature through a sampling system, similar to the published data or even historical results often result in confusion measurements of pH, dissolved oxygen, or conductivity. Other and frustration. However, by applying observations from high nuclear plants, especially those with a boiling water reactor purity water ORP installations as well as understanding the (BWR), make this measurement either in-situ (directly in the theory of the ORP measurement, measuring practices have been process) or in a high flow, at-temperature autoclave system developed that can help to ensure reliable results for accuracy which provides a sample which is exactly representative of the and comparison purposes. process water.

Corrosion Potential The ECP measurement typically utilizes a Ag/AgCl reference Most analytical measurements performed on a plant water/steam electrode and the system itself as the measuring electrode. By cycle quantify chemicals or contaminants present in the water. doing this, the “actual” potential of the system itself is Few measurements show the actual response of the system to determined. This measurement is utilized extensively within the the water. In the last two decades, extensive work has been nuclear industry. BWR plants often use the ECP as a control done to determine the system metallurgy response to the water point for the addition of hydrogen, and as such have found that chemistry. One of the more promising methods for determining this is a more accurate method for ensuring the correct dosage 1-3 this is a measurement of a potential which can be associated into the system. PWR plants often use the ECP on the with the water’s corrosivity to the system. primary side to observe variations in the primary water during startup, steady state, and shutdown conditions.1 This method for obtaining a measurement of the potential for corrosion utilizes a reference electrode, typically a silver/silver The measurement of corrosion potential utilizes a Ag/AgCl chloride (Ag/AgCl) half-cell, and a measuring electrode, typically reference electrode and a measuring electrode made of the platinum or some form of metal which is similar or identical to the same metallurgy as that of the system metal (usually some form system metallurgy (such as a steel alloy). The names given to of steel alloy). This is based upon the theory that utilizing a this particular measurement vary extensively – corrosion measuring electrode similar to the system metal should produce potential, oxidation/reduction potential (ORP), electrochemical a potential more representative of the system metal itself. This potential (ECP), or redox (another name for ORP). It is important measurement is typically used most by the nuclear industry, and to understand that these methods all measure the oxidizing or is usually measured in conjunction with either the ECP or ORP reducing potential of the system with respect to the water. measurements. However, each of the listed names above specify a method of obtaining the oxidation/reduction potential in a slightly different

Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 2

Oxidation-Reduction Potential electrodes Chemical reactions which involve the transfer of electrons F = Faraday constant between reactants are known as oxidation-reduction, or redox, R = Gas constant reactions. A species with lesser affinity for electrons in solution T = Absolute temperature, °C + 273.15 will lose electrons, increasing its electrical charge. This half- n = Number of electrons involved in the electron transfer reaction is termed the oxidation, because the initial reactions Aox & Ared = Activity of reactants in the process which were studied early on involved oxygen as the oxidizing agent. The species will become more positive in the oxidation It should be noted that the above equation is a limiting law with reaction. A species with greater affinity for electrons in solution respect to ORP. Many of the reactions associated with oxidation- will accept electrons, reducing its electrical charge. This half- reduction deviate from the equation. The equation can be useful, reaction is termed the reduction, since the species becomes less however, for determining approximate tendencies of the various positive. Equations 1 and 2 summarize the half reactions taking reactions with respect to the oxidation-reduction potential. place. Standard Hydrogen Electrode

Xm + pe-  Xm-p (reduction -- gain of electrons) (1) ORP values are reported in two ways. The first method shows the units as milliVolts (mV) followed by “SHE”, which stands for Yn  Yn+q + qe- (oxidation -- loss of electrons) (2) Standard Hydrogen Electrode. This refers to a method of reporting the potential with respect to a common standard, in this The Xm represents the species which is reduced to Xm-p by case the SHE. It is typical to report the potential as “mV SHE” gaining p electrons. The Yn represents the species which is when making the ORP measurement “at-temperature” (the same oxidized to Yn+q by losing q electrons. The species which gains temperature as the process). Equation 5 is used to convert an electrons is reduced, and acts as an oxidizing agent. The ORP reading based upon another reference electrode, such as species which loses electrons is oxidized, and acts as a reducing the Ag/AgCl, to the SHE reference: agent. 5 ESHE = Eobserved + Eref (5) In order for the whole oxidation-reduction reaction to occur, the two half-reactions must occur simultaneously. If the example where: above is simplified by assuming that both oxidation and reduction ESHE = the potential of the redox solution referenced to the SHE reactions involve a single electron, Equation 3 can be written. Eobserved = the potential observed utilizing the reference electrode

Eref = the potential of the reference electrode Xm + Yn  Xm-1 + Yn+1 (standard redox equation) (3) The SHE electrode is never used in industrial measurements, If a solution is strongly oxidizing, it has a deficiency of electrons since it requires a fixed partial pressure of hydrogen for its use. available and thus will attempt to acquire electrons. Likewise, if a By convention, its potential is usually defined as 0.00 V, solution is strongly reducing, it has electrons available and will regardless of temperature. The potentials developed by attempt to give up electrons. The tendency for a solution to electrodes other than the SHE are typically defined with respect donate or accept electrons can be sensed as an electrical to the SHE. The potentials are variable dependent upon potential on an inert electrode. Figure 1 shows an example of temperature, type of electrode half-reaction, and the ionic the typical range for oxidation-reduction potential (ORP) seen in strength of the reference electrolyte. An example of the Ag/AgCl the water/steam cycle in a power plant. The ORP is related to reference electrode’s potentials (Eref in Equation 5) at varying the solution composition by: temperatures with respect to the SHE are given in Table 1 (the temperatures listed represent those typical in a cooled sampling o 4 5 Em = E + 2.3 (RT / nF) log (Aox / Ared) (4) system). Conversions to the SHE at higher temperatures are available. where:

Em = Electromotive Force, defined as the oxidation-reduction The second method of reporting the potentials in this paper will potential (ORP) not have any designation following the mV value. This refers to a Eo = Constant that depends upon the choice of reference potential measured with respect to the Ag/AgCl electrode Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 3

(Eobserved in Equation 5) at sampling temperature, regardless of electrode. This is also visible mathematically in Equation 4 since whether the temperature was ambient or similar in temperature to the ratio of oxidizing to reducing activity will be less than 1 in a the process. This method is typically used when the reducing solution, logarithmically yielding a negative number. measurement is made at ambient temperature in a sampling While Equation 4 is useful for theoretical purposes, predicting the system. response of an ORP electrode pair by theoretical means is typically not practiced due to the complex interaction taking place ORP Measuring System with the system water. This can include a multitude of redox A two-electrode system, similar to that used with pH, is used to reactions (some reversible and some not), temperature measure ORP in the water/steam cycle. Figure 1 shows an variations, or a non-ideal electrode response. It is therefore more example of an ORP electrode system with a measuring useful to look at the ORP reading from a comparison standpoint electrode, solution ground, and reference electrode. The purpose rather than a theoretically accurate standpoint. of the reference electrode is to develop a stable potential in the ORP Calibration and Sampling in High Purity Water water from which the potential of the measuring electrode can be measured. The reference electrode that is used is identical to An ORP measuring system is very similar to a pH measuring that used with pH measurements. The most common type used system. The main difference is with the measuring electrode. with high purity water measurement consists of a silver/silver- Many of the problems that have plagued pH measurements in chloride (Ag/AgCl) wire surrounded by a potassium chloride (KCl) high purity water can also affect the ORP measuring system. In solution. A porous reference junction, typically ceramic, allows addition, new considerations become apparent when attempting contact between the KCl and the process water while minimizing to obtain reliable, consistent ORP readings in high purity water the leakage of the KCl electrolyte into the process. In for comparison purposes. Among the most important of these measurements made at-temperature, special Ag/AgCl reference concerns is the method used for sampling. There are two basic electrodes are available which can withstand the high choices – an at-temperature (not necessarily in-situ, which places temperatures of the process. the electrodes directly into the process line or component) measurement, or an ambient temperature measurement. The The measuring electrode consists of an inert metal, typically focus of this paper will be on the latter, although the former will platinum. It has been argued that the system metal is not be presented as well. platinum, and therefore the platinum electrode will not respond the same to the water chemistry as the system metals, such as Much research and testing has been done over the years with carbon steel or steel alloy. However, many plants have found the respect to pH measurements in high purity water at ambient platinum electrode to actually be equally or even more temperature, and as a result many of the standard techniques for responsive and sensitive to system changes.1,6-9 In addition, the minimizing problems with the pH measurement have been platinum electrode has been observed to maintain its documented. However, little exists on ORP measurements in responsiveness to ORP variations longer than the other electrode high purity water since it is a relatively new application for ORP. types.6 For these reasons, platinum electrodes have become the Fortunately, the measurement is similar enough to pH such that ORP measuring electrode of choice in high purity water for the many of the same high purity water pH techniques apply to high measurement of the potential for corrosion. purity water ORP.

If a solution is strongly oxidizing (electron accepting), it will Sampling Techniques withdraw electrons from the measuring electrode’s metal surface, The most obvious choice for ORP monitoring of the plant water creating a positive potential on the electrode surface. The ORP would be an in-situ installation. However, this method is not will thus be more positive in solutions which have a higher activity always acceptable since permanent placement of an ORP probe of oxidizing reactants than reducing reactants. The second term can result in a loss of sensitivity and responsiveness due to of Equation 4 demonstrates this mathematically in that a higher deposits that form on the surface of the probe. The necessary activity of oxidizing agents in relation to reducing agents leads to cleaning and/or calibrating that is required can expose the a ratio which is greater than 1, which logarithmically yields a system to oxygen. This method is typically only performed for a positive number. Likewise, in a reducing solution, a more short testing period, but is expected to give the most negative potential will exist since the solution has electrons representative value for the corrosion for potential in the system. available which are donated to the surface of the measuring Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 4

For these reasons, two other methods have been typically used responses observed with the ambient temperature, sampling when measuring the oxidation-reduction potential in the system measurement have been very qualitatively similar to water/steam cycle of a power plant. The first method makes use those observed with the autoclave method, with the quantitative of high temperature measuring and reference electrodes variations being a constant difference of the steady-state mounted in an autoclave located such that the time needed for potential between the two, a smaller magnitude potential for process water to flow to the point of measurement is a few water chemistry variations, and a slower reponse.2,12 Since the seconds.1-3,7-9 This method also assures that the ORP sensor is measurement of oxidation-reduction potential is often more being subjected to similar temperature water that has not useful from a qualitative point of view, the second method is often changed in contaminant and chemical concentrations from those preferable for long term monitoring, especially since this is the of the process water. This method has been shown to yield method used for most other on-line monitoring, such as pH, results that are very representative of those of the ECP dissolved oxygen, or oxygen scavenger concentration. measurement. However, measurements by this method can be more difficult to maintain on an on-going basis for the purposes It should be noted, however, that in some instances, the first of cleaning, calibrating, or replacing parts. method is the only acceptable means for obtaining the measurement. It has been shown that nuclear plants with a A second method measures the ORP in a sampling line cooled to boiling water reactor (BWR) cannot use the second method to room temperatures and pressures, similar to the method used for properly control hydrogen addition. This is because an exact pH or conductivity.2,6,10,11 From an electrochemical standpoint, potential control point has been defined for hydrogen addition so this method will yield results somewhat differently than the first as to minimize intergranular stress corrosion cracking (IGSCC). method, since the sampling flow is often transported in a fairly Using a measurement that is not in-situ may not properly control long, small diameter sampling tube to the point of analysis. the hydrogen addition.1-3,8 The autoclave method is favored in During this transport the chemistry of the water can vary due to the primary side of nuclear plants with a pressurized water oxygen ingress, hydrazine/oxygen residual reactions, and reactor (PWR). The sampling system method is typically used in temperature changes.2,8 One source showed that this method fossil fuel plants and the secondary side of PWR nuclear plants. yielded final results much slower (on the order of days) than the autoclave method.2 The same source showed that small Ambient Temperature, Sampling System ORP Measurements fluctuations in the oxiding or reducing nature of the process were The focus of the remainder of the paper will be on the ORP not picked up as easily with the sampling system method measurement made in the ambient temperature, sampling compared to the autoclave method.2,8 However, this method system method. Obeying certain rules for this second method does allow much easier access to the electrodes for calibration can be as vital to an accurate, on-line measurement as and maintenance purposes, which is useful for long term maintaining proper calibration techniques. Once again, practices monitoring. taken from experience with high purity pH measurements can be observed, due to the similarities between pH and ORP. Details Both methods have been shown to be responsive to system such as flow rate, streaming potentials, reference electrode changes. However, the differences in results between the two design, sample chamber type, and temperature control are appears to be greatly dependent upon the magnitude of changes among the considerations that are important to ensure that the in the process. How representative the sampling system method on-line ORP measurement is as “accurate” and consistent for shows the exact corrosion potential seems to be dependent upon comparison purposes as possible. a number of factors, including chemical concentrations, flow rate, temperature control, sampling system material, and total time Flowrates between the process water exiting the process and the ORP electrode response has been observed to be more sluggish measurement being made. In tests where ECP measurements at much lower flowrates, and therefore sample flow rates should have been compared to ORP measurements done in an be maintained at a minimum of 150 ml/min to improve response. autoclave arrangement, the responses have been very similar, It has been seen that increasing the flowrate much above 150 with the only differences being a constant differential (usually 100 ml/min will not tend to improve the responsiveness of the mV or less) of varying magnitudes between the two electrode significantly.13 However, higher flowrates are useful to measurements, as well as a somewhat faster response time with maintain the integrity of the sample, which can change during the ECP measurement, which is to be expected.9 The type of transport by residual chemical reactions. These chemical Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 5

reactions have been observed to affect the oxidizing and microsiemens/cm). However, this type of electrode is more reducing species of the sample.1,3,8 sensitive to flowrates and will typically produce a more “noisy” measurement in water of higher purity. Using this type also Higher flowrates can also help to minimize sample line leaks necessitates on-line calibration by the grab sample method with a which would allow oxygen ingress, causing dissolved oxygen portable ORP meter with a flowing reference electrode in order to levels to be higher in the sample line than those in the process.14 eliminate the reference potential shift that will occur if a buffer is This would cause the ORP reading to appear more oxidizing. used for calibration purposes. Stainless steel piping for sample lines have also been observed to minimize the extent of oxygen ingress. Another potential The use of a flow-through sample chamber is recommended for location for oxygen ingress can be in the sample chamber. Air best results in minimizing the previous listed problems. It is pockets were found in sample chambers at one plant, allowing recommended that a stainless steel construction be utilized, oxygen and carbon dioxide ingress, which affected the readings along with a flowing reference electrode, solution ground, and a substantially.6 separate platinum ORP measuring electrode. This type of mounting has been seen to yield best results with minimal error. Streaming Potentials While this improvement in measurement may not be significant, it can help minimize the differences in readings that would be seen It has been observed that because high purity water is a poor when comparing readings to historical data, other sample conductor, a large electrical resistance will be present between locations, or other plants. It can also help to guarantee that as the measuring and reference electrodes. This can cause ORP results in high purity water continue to be researched, readings to seem noisy, due to a heightened sensitivity to the minimal error will be associated with the actual measuring conductivity of the water. High purity water flowing through system. insulating (non-metallic) tubing or piping can generate static charges, known as streaming potentials. The relative strength of Temperature these potentials are associated with the water purity, flow velocity, and type of piping or tubing being used.15 These It is recommended that the temperature of the sample line be streaming potentials can appear in addition to the measured maintained within a few degrees Celsius for a variety of reasons. oxidation-reduction potentials and cause an error in the reading. While it is extremely difficult to predict which of the reasons listed In order to minimize the effect of the high purity water, it is again below may cause a change in the ORP reading with temperature recommended that stainless steel piping be used for all sample fluctuations, an understanding of each can help to realize the lines. Sample chambers should be stainless steel as well, in importance of temperature control. The most important reason is order to minimize static charge buildup and shield the electrodes that the potential measured in the water can vary due to the from external electrical noise. A solution ground has been dependence of the chemical reactions in the water on observed to minimize errors associated with streaming potentials temperature. This effect occurs with other measurements in high by allowing the meter circuitry to “float” to whatever ground purity water, such as pH or conductivity. However, there is no potential exists in the water.16 An additional means of minimizing ability at this point in time to provide “solution temperature this error is by the use of either a flowing reference electrode or a compensation” with the ORP measurement to offset any effects combination ORP electrode with an annular, diffusion reference of a variance of the chemical reactions with temperature. This is junction.15 due to a lack of information on the plant water which characterizes the many varied oxidation/reduction reactions that Probe Mounting can occur at any one point in time.

The type of probe mounting can be important as well. An in-line Another reason is associated with the second term of Equation 4. mounting of an ORP combination electrode is a common way of The “n” term in the equation is defined as the number of obtaining the measurement. This method is fine for most electrons involved in the process reaction. Process water installations. However, this type of electrode (typically called a typically will contain multiple reactions, each of which may combination electrode) often utilizes a sealed reference involve single or multiple electrons being transferred. The electrode. This type of electrode can work well for many second term can thus change with variances in activity of installations with higher conductivity (greater than 10 oxidants or reactants, temperature, or the number of electrons Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 6

per reaction. Because of this, it would be nearly impossible to typical, the reference junction potential of some reference determine the temperature compensation necessary for a given electrodes has been observed to vary by as much as 60 mV (20- process, unless there was only one oxidation/reduction reaction 30 mV is typical) when transferred from the calibration solution to taking place in the water. Temperature compensation is the high purity water. This effect can produce a significant offset therefore typically not incorporated as part of the ORP which can be incorrectly assumed to be attributed to the oxidizing measurement. Temperature changes can cause changes in the or reducing characteristics of the process solution. In addition to second term of Equation 4, however, although these typically are this offset, many reference electrodes can experience unstable no more than a few milliVolts per degree Celsius change in the potentials in high purity water due to variations in flow rates and ORP ranges usually associated with power plant water. conductivity of the water.

A third reason is associated with the reference electrode. It has been observed that these effects can be minimized by Reference electrodes should ideally have stable potentials over utilizing a flowing reference electrode for the on-line the temperature ranges experienced in power plant sample lines. measurement, which adds a significant amount of ions to solution However, the potential may change somewhat with temperature. in order to stabilize the potential.15 If a standard diffusion or Maintaining the same temperature in the sample line as that used “solid state” reference electrode is used, the offset could be for calibration purposes can help to minimize any change in value present and should be understood. Since many ORP systems of the reference potential. If the temperature will vary utilize the normal, diffusion type reference electrode, it is significantly, reporting the mV value with respect to the SHE will important that any calibration procedure take into account the ensure that any reference electrode temperature effects are offsets that can occur. taken into account.5 Standard Calibration Calibration Techniques A typical ORP calibration consists of only a one-point calibration, known as a standardization. Many users are familiar with the pH Many users who have tried measuring ORP in high purity water calibration, which utilizes a standardization and a slope. The report that they experience some difficulty in obtaining repeatable purpose of the standardization is to allow the meter to determine results. In addition to this is the seemingly impossible task of the relationship between the potential of the ORP electrode pair quantitatively determining the true ORP reading in the process, and the actual potential of a known standard. The slope due to the complexity of oxidation-reduction reactions in the calibration allows the meter to determine the response of the water and thus the associated potentials corresponding to the measuring electrode, which is associated with variations in the various reactions. For this reason, establishing an “accuracy” second term of Equation 4, since the reference electrode statement for the ORP measurement is not practical. However, potential should ideally remain the same. Standardization obtaining a consistent and reliable measurement is vital for point- calibration is common with ORP probes for purposes of to-point, plant-to-plant, or historical measurement comparisons. comparing results, not for purposes of accuracy. Since the In order to establish some simple rules for calibrating the ORP measuring and reference electrode responses can vary system such that repeatability and consistency issues are substantially depending upon type, age, or process water minimized, some of the problems experienced with calibration characteristics, it is important to calibrate to a known standard so must first be examined. that comparisons are meaningful. It should be noted, however, Reference Electrode that many users do not perform any sort of calibration at all; The reference electrode used with ORP measurements in a rather, they use the following procedure as a means to check the sampling line is identical to the type used for pH measurements. electrode pair. The basic purpose of the reference electrode is to establish a stable potential against which the measuring electrode potential It is not common practice for ORP meters to have a slope can be compared. The reference potential is developed based calibration like the pH meters do for a variety of reasons. The upon the electrolyte of the internal fill solution and the process first and most important reason is that the second term of solution. High purity water has relatively few ions by definition, Equation 4 will vary differently with the different types of reactions which can substantially impact the reference potential. If the which may be present in the high purity water. The second calibration of the electrodes is done in a standard solution which reason is that the ORP measuring electrode is expected to is much higher in conductivity than the process water, as is respond exactly to a process potential change in the process or else not respond at all. However, it was observed at one plant Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 7

that even in an ORP reaction with an expected response, the The above calibration procedure is useful to determine whether slope associated with the measuring electrode can vary by as the electrode pair is truly responding to process changes. Note much as 25% with a new electrode.6 In addition, an ORP that whenever the electrode pair is placed back into the process, electrode pair can be standardized even when the ORP ideally only the measuring electrode’s potential should change in measuring electrode is not responding to process changes. It is value, since it will be dependent upon the reactions taking place therefore important to at least perform a check with another in the water. The reference potential of the electrode pair should standard solution to verify that the measuring electrode is remain constant in order to maintain calibration accuracy. functioning correctly. This check typically involves a predictable However, since the calibration standard is highly ionic and the variation on the standardization solution. high purity water contains relatively few ions, the reference potential will change, especially if a diffusion type reference The most common standardization solution used with ORP electrode is used. This problem is commonly experienced with consists of a standard pH buffers with a modification to create a pH measurements in high purity water as well.15 known oxidation-reduction potential within the buffers. The recommended buffer modification is to use 1 liter of either 4 or 7 There are some possible solutions. One is to utilize a flowing pH buffers with 10 grams of quinhydrone (a dry, light-sensitive reference electrode. This type of reference electrode maintains a powder) mixed in.4,5 Note that the exact number of grams positive pressure on the reference electrolyte in order to minimize recommended may not always be the same in various sources; reference potential changes. Using this reference electrode will however, the important factor is that the buffer is saturated with ensure that any potential change between calibration and the quinhydrone. Although only accurate to about +/-10 mV after process water measurements will be minimal, typically no more preparation and typically only stable for a few hours, this than a few millivolts. This solution has been proven in many high calibration method is common since pH buffer is readily available purity water pH installations. and the solution with the quinhydrone is easy to prepare. Since both buffers will yield different values due to the difference in pH, Another solution is to use a diffusion type electrode and to it is useful to use both buffers in the calibration process -- one as perform an on-line calibration of the ORP reading, thus allowing the standardization buffer and the other as a check. Table 2 the calibration to take into account the reference potential contains expected ORP values in the reference solutions for associated with the high purity water, not the highly-ionic buffer three different temperatures. pH buffers greater than 7.00 should solution. To do this, a grab sample must be performed. Grab never be used for ORP calibration, since saturation of these samples are inherently difficult with high purity water buffers with quinhydrone will yield a negative ORP. These measurements, since any exposure to air can allow any solutions are very unstable, since oxidation by the air will occur dissolved gases within the process water to escape as well as quickly.5 letting oxygen and carbon dioxide within the air to change the ORP of the sample water. Since oxygen is a mild oxidizer, it can Typically, the meter reading should be within about +/- 30 mV of quickly change the ORP reading of high purity water. As a rule of the values listed in the table. Once the meter has been adjusted thumb, lower potentials will experience oxygen errors of a much to read the listed value, the probe should be checked in the other larger magnitude than samples with higher potentials. Since pH buffer solution. Note that if the measuring electrode is ideally most ORP measurements in a plant’s high purity water are in the responding to ORP changes, it should read within +/- 20 mV of negative range, it is possible that obtaining an accurate grab the listed value for the second buffer after stabilization. If the sample reading is very difficult, since the oxygen would so electrode does not respond as expected it may need to be quickly contaminate the sample.13 Due to the inherent difficulty cleaned. An ORP measuring electrode works best whenever the with performing this calibration, many users just ignore it and surface of the metal is fully exposed to the process. Over time, accept any error that will be present due to the reference reactants in the process can form on or react with the surface of electrode. the electrode, which will result in a loss of responsiveness to process potential changes. A mild abrasive solution can be used If a grab sample is attempted, proper technique is vital. A proper for most cleaning. If the electrode is still unresponsive, a sample technique is to let the sample line go into a container stronger solution, such as nitric or chromic acid, can be used for near the bottom, allowing the water to overflow the top of the cleaning. As a last resort, a fine-grained emery cloth can be container. A portable ORP meter with a platinum measuring used to polish the surface of the measuring electrode.4,5 electrode and a flowing reference electrode, calibrated with the Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 8

quinhydrone-based pH buffers, should be immersed deep Conclusion enough into the chamber to make sure that the probes are The measurement of ORP in a plant’s water/steam cycle is an exposed to process water that has not yet had exposure to air. important measurement to track changes in the system’s This will help to guarantee that the temperature of the grab response to the water chemistry. Comparison among various sample is the same as the water in the sample line, as well as measurement points is the common means used to observe ensuring that there is no exchange of gases between the water these changes. However, there are many things which can serve and air until after the ORP electrode has already made its to either mask any potential change or create a false appearance measurement. Once the portable meter has stabilized, the on- of a change to the ORP reading. In order to minimize these line meter should be adjusted to agree with the portable meter. If potential errors, it is important to practice the proper calibration the portable meter does not have a flowing reference electrode, it and sampling techniques. Along with practicing these will experience the same reference potential shift when it is techniques, a good understanding of the capabilities and calibrated in the highly ionic buffer solution, which means that no limitations of the ORP measurement is vital. When the accuracy benefits will be gained by performing the comparison measurement is understood and the proper techniques practiced, calibration. the measurement can be used to observe the system’s reaction to the water chemistry on both a historical and point-to-point basis.

References

1. W. Beyer, B. Stellwag, N. Wieling, A. Doerr, “On-line Monitoring of Electrode Potentials in the Steam Generator of a PWR”, Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, The Metallurgical Society, 1988

2. A. Molander, C. Jansson, “Conventional and In-situ Corrosion Potential Monitoring in a BWR”, presented at JAIF International Conference on Water Chemistry in Nuclear Power Plants, Fukui, Japan, April, 1991

3. P. Millett, “The Need for an Application of High Temperature Measurement Techniques in Power Plants”, presented at the 12th International Conference on the Properties of Water and Steam

4. Standard Practice for Oxidation-Reduction Potential of Water. ASTM D1498, Annual Book of Standards, Vol. 11.01, American Society of Testing and Materials, Philadelphia, PA

5. Analytical Instrumentation, Practical Guides for Measurement and Control, Instrument Society of America, 1996, pp. 457-482

6. Use of the Leeds and Northrup 7082 ORP Electrode (Internal Report). Doug Murray, Palo Verde Nuclear Station, April 1997

7. Pinacci, M. Ferrari, G. Buzzanco, “Corrosion Rate and Chemical-Physical Parameter Monitoring During the Startup of the Piombino Power Plant, Group 4”, CISE Technology Innovative SpA, 1988

8. A. Molander, B. Rosborg, P. Anderson, L. Bjornkvist, “Electrochemical Measurements in Secondary System of Ringhals 3 PWR”, Water Chemistry of Nuclear Reactor Systems 6, Volume 2, British Nuclear Energy Society, October 12-15, 1992

9. P. Hicks, R. Mouche, “Using Carbohydrazide in Nuclear Utility Applications”, Paper IWC-97-81, presented at the International Water Conference 58, Pittsburgh, PA (November 1997)

10. V. Mamet, V. Belyanin, V. Tyapkov, V. Yurmanov, “Measuring the Redox Potential for Assessing the Behaviour of the Iron Compounds in the Condensate-Feed Circuit of a Nuclear Power Station”, Thermal Engineering, 28 (2) , 1981

11. P. Sturla, “Bedeutung von Redoxpotentialmessungen beim Betrieb von Hochdruckkresilaufen”, VGB Kraftwekstechnik 59, March 1979, pp. 231-239 Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 9

12. S. Filer, A. Tenney, D. Murray, S. Shulder, “Power Plant ORP Measurements in High Purity Water”, presented to NUS 1997 International Chemistry On-line Process Instrumentation Seminar, Clearwater, FL (November 1997)

13. M. Gupta, A. Gupta, M. Suidan, G. Sayles, J. Flora, “ORP Measurement in Anaerobic Systems Using Flow-Through Cell”, Journal of Environmental Engineering, Vol. 120, No. 6, (November/December, 1994).

14. A. Tenney, “Sampling Considerations for Equilibrium Dissolved Oxygen Sensors”, presented to NUS 1996 International Chemistry On-line Process Instrumentation Seminar, Clearwater, FL (November 1996)

15. D. Gray, “Upgrade Your pH Measurements in High-purity Water”, Power, March 1985.

16. R. Hunt, “pH Measurements in High Purity Water,” Ultrapure Water, Sept/Oct 1985.

Measuring Solution Reference Electrode Ground Electrode

Ag/AgCl Internal Wire

Internal Wire

Reference Electrolyte

Stainless Steel Reference Junction Platinum Band

Figure 1: ORP Measuring Electrode with Ag/AgCl Reference Electrode Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 10

Temperature, Celsius 3 N KCl Saturated KCl 0 255.0 222.8 5 222.0 218.6 10 219.7 213.8 15 216.4 208.9 20 212.9 203.9 25 209.0 198.5 30 205.4 193.5 35 201.7 188.4 40 197.8 183.1 45 193.0 178.0 50 190.0 173.3 Table 1: Reference Electrode Potentials (mV) of Ag/AgCl Electrodes with Varied Temperature and Concentration of Electrolyte

Buffer Solution, nominal pH 4 7

Temperature, °C 20 25 30 20 25 30 Reference Electrode: Silver/Silver Chloride 268 263 258 92 86 79 Calomel 223 218 213 47 41 34 Hydrogen 470 462 454 295 285 275 Table 2: ORP Values (mV) for Reference pH Buffer Quinhydrone Solutions4

Measuring Techniques for Oxidation- Reduction Potential in High Purity Water 11

More Information For more information on ORP, visit www.honeywellprocess.com, or contact your Honeywell account manager.

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