Oxidation Reduction Potential Basic Principles and Techniques

Slide 1: Introduction

Slide 2: Many of you might already use ORP measurements every day, but you may not fully understand what Oxidation Reduction Potential means, and we wanted to provide you with a basic understanding of how the ORP measurement is taken and what is required for the measurement to be successful.

Slide 3: So today … READ SLIDE

Slide 4: ORP

Slide 1: Introduction

Slide 2: Many of you might already use ORP measurements every day, but you may not fully understand what Oxidation Reduction Potential means, and we wanted to provide you with a basic understanding of how the ORP measurement is taken and what is required for the measurement to be successful.

Slide 3: So today … READ SLIDE

Slide 4: ORP is a key measurement and control parameter in a wide variety of applications. We will go into some of the different applications in more detail later, but at a quick glance….. READ SLIDE

Slide 5: If you are already familiar with a pH measurement, you will find that ORP is similar in the fact that they are both electrochemical measurements that require a special high impedance input. Also pH and ORP normally use the same standard Ag/AgCl reference electrode design.

However ORP differs from pH because it uses a different measuring electrode that responds to the oxidizing or reducing activities of the ions within the solution instead of the hydrogen ion activity, as seen in pH.

Slide 6: So now let’s take a moment and discuss what ORP or Oxidation Reduction Potential actually is. Another name for this type of measurement is known as REDOX, which refers to the fact that within the process being measured, reduction and oxidation are occurring simultaneously. One cannot occur without the other. If one species undergoes oxidation (or loses electrons) then another species must accept those electrons and is said to be reduced (or gains electrons).

Slide7: Within a REDOX reaction there are always two types of species, an oxidizing agent and a reducing agent.

In the reaction shown here, between iron and copper sulfate, there are two half reactions that occur at the same time. (1) One is the reaction that takes place on the copper element, and the second is the reaction that takes place on the iron element. (2) In this reaction the copper is considered to be the Oxidizing agent because it has the capacity or potential to acquire or gain electrons and become reduced.

While, (3) the iron element is considered to be the Reducing agent, because it donates electrons and therefore become oxidized.

Slide 8: So, I know this sounds a little confusing because it is almost as if the terms are backwards.

However (1) the oxidizing agents are reduced because, they gain electrons by an atom which leads to a decrease in the oxidation state of the element.

(2) While the reducing agents are oxidized because, they lose of electrons, leading to an increase in the oxidation state of the element.

It is also important to remember that ORP is a quantitative measurement. Therefore, it responds to all ionic content within a solution, and cannot distinguish between the individual process components. Similar to what is seen when you are measuring conductivity.

Slide 9: Now here is an everyday example that you probably would not have thought about. Have you ever wondered what gives sapphire crystals their deep blue color? Surprisingly enough it actually comes from a simple REDOX reaction between the titanium (IV) and iron (II) impurities that exist within the crystals naturally.

(1) However in order for the reaction to occur energy must be supplied to fuel the reaction. This is accomplished when ordinary white light passes thru the crystals. The reaction between the titanium and iron absorbs the red, orange and yellow light regions of the spectrum to feed the REDOX reaction. Thus allowing only the blue light to pass thru the crystals, (2) resulting in the deep blue color seen in sapphires. 

Slide 10: You could say that a simple working definition for ORP is a solution’s capacity for electron transfer known as oxidation or reduction, given in millivolts. (1) ORP is similar to pH in that pH indicates how acidic or basic a solution is based on the hydrogen ion activity within the solution and ORP indicates the reduction-oxidation status of a solution based on the collective electron activity within the solution.

Slide 11:  Shown here, is a section of the typical ORP scale. The full range is typically 1500 mV to -1500 mV.  (1) Most of you are probably already familiar with what a pH scale looks like. When we look at the pH scale, an acid is defined as a substance that is capable of liberating hydrogen ions (2) and a base is a substance capable of absorbing hydrogen ions.  When you look at the pH scale, at 0 mV a solution is neutral (it is neither acidic or alkaline), but as you move above 0 mV the solution is considered to be acidic, and when you move below 0 mV the solution is considered alkaline or basic.

(3) An ORP system can be defined in the same manner.  Since a reducing agent is capable of accepting an electron and an oxidizing agent is capable of losing an electron; (4) it can be said that the stronger the reducing agent the more negative the ORP value, and the stronger the oxidizing agent the more positive the ORP value.

Slide 12: For example:

(1) An Acid Permanganate solution is strongly oxidizing: meaning it strongly attracts electrons from the ORP electrode, so the REDOX potential is highly positive.

(2) Opposite to that would be Sulfite solutions which are strongly reducing. They push electrons into the electrode, so the REDOX potential is strongly negative.

Slide 13: Like pH, ORP is based on the known Nernst Equation to determine the equilibrium reduction potential of an installation that contains a conductive electrode and a surrounding conductive electrolyte within a process solution. When a chemically inert or Nobel metal electrode is placed into a solution where an oxidation-reduction reaction is taking place, an electric potential appears at the measuring electrode. This potential is called the oxidation-reduction potential.

Slide 14: What equipment is required in order to measure ORP? You will need an analyzer, a sensor and a cable.

The Analyzer converts the raw input from the sensors and calculates displays and transmits the ORP mV value.

The ORP Sensor includes two elements: a measuring electrode, and a Reference electrode.

• These can be either separate electrodes or contained in an All-in-One type sensor.

Unlike pH, since ORP is the direct measurement of electrons in transit during REDOX reactions, regardless of temperature; temperature compensation is not normally used in ORP measurements, and therefore there is no need for a temperature electrode.

The cable used depends on the sensors being used. They can be either separate or disconnected at the electrodes, as seen with a variopin, or integral on the sensor as in the All-in-One style. If separate electrodes are used typically a holder is required to install them in the process.  

Slide 15: Shown here you can see the basic ORP Measurement circuit consisting of a reference and measuring electrode. Also the voltage potentials that are present, which affect the accuracy of the measurement are shown.

The total voltage of this circuit is the Algebraic sum of these potentials:

• E1 is the Potential between the ORP metal surface and the process
• E2 is the Potential between reference electrode and the electrolyte
• E3 is the Potential that develops at the surface of the electrolyte and the process

The difference in potential between the two electrodes is what is actually being measured by the analyzer. These voltages will give us an indication of the ability or activity, of the oxidizers or reducers within a solution. The response time for the reading varies with the concentration of the redox system. Higher concentrations are faster to respond than lower concentrations.

Some of the common Problems that can occur with this measurement are similar to those seen with pH:

•  Coating of the metal measuring electrode will cause a sluggish reading
•  Since the reference electrode that is used in ORP is the same reference electrode used in pH you can expect to have the same problems. You can have coating or plugging of ref. Junction, any pressure spikes in the process can be seen in the reference. As well as any ions such as (sulfide, cyanides, bromides) will react with Ag+ in the reference electrode to form silver sulfide causing what is known as poisoning of the reference electrode.   

Slide 16: The First component of the ORP measuring system is the Measuring Electrode. 

ORP is a potentiometrical (POE-TIN-SHE-O-METRIC) measurement of the oxidizing/reducing power of a liquid. So the ORP measuring electrode is similar to that of a pH measuring electrode, except it is normally constructed of an inert or noble metal. The most common metals used are platinum, gold and silver. Platinum, is considered the standard and it has excellent chemical resistance but suffers slightly from Chemisorption. This is where when placed in strongly oxidizing solutions Oxygen bonds to the surface of the noble metal and in strongly reducing solutions hydrogen bonds to the surface of the noble metal.  These surface coatings will insulate the platinum from the solution which will decrease the speed of response.

(1) Gold electrodes have properties that make them a good recommendation for either strongly oxidizing solutions or very strongly reducing solutions. However Gold is not recommended for use in application solutions that contain Cyanide, Chloride or Bromide, because gold is not resistance to the corrosion that can occur because of these chemicals.

Slide 17: The Measuring electrode life and operational accuracy are adversely affected by the process chemical make-up so it is important to choose the right metal for the process.

Slide 18: Since we cannot measure the voltage (or charge build up) being generated by the constant acceptance and giving up of electrons on the ORP measuring electrode directly, we need to introduce a reference voltage and a way to complete the measuring circuit. (1) Just as you see in a pH the ORP reference electrode performs 2 functions:  It provides a constant reference voltage and completes the measuring circuit.

(2) Nernst discovered that when you have a Ag/AgCl pin immersed in KCl electrolyte, as long as Cl- concentration remains constant, this combination will produce a constant mV potential.

(3) To connect this voltage to the process (and then to the metal electrode to complete the measuring circuit) a liquid junction or diaphragm is required.  (4) There are various types of junctions such as: Ceramic, Porous Teflon, sleeve, simple pore, Double-Junction.

The KCl within the reference electrode can be either in a liquid or a Gel form.  In Gels, the KCl will diffuse out of the junction at a slower rate, but cannot be refilled. If a liquid KCl solution is used, it will diffuse and flow out of the junction. Liquid KCl (or flowing references) solve some of the problems we’ll discuss in a moment.

Slide 19: Typically the same reference electrode that is used with a pH measurement is also used with an ORP measurement and therefore the same problems exist.

The Reference Electrode because it is often open (via the junction) to the process, is the weakest link in the measurement loop.  Shown here are just some of the typical problems that are seen with the reference electrode.

Slide 20: So far we have discussed what is ORP and how ORP is measured, now let’s take a moment and go over what maintenance is need to maintain your ORP loop.  

Slide 21: Some basic DO’s and DON’T’s to assure the most accurate calibration. You always want to remember to start by cleaning your electrons. Always use fresh buffer solutions, and pick a buffer solution closest to the control point. Just with pH calibrations you have to allow ample time for the reading to stabilize. It is not recommended to calibration to grab samples.  

Slide 22: A variety of cleaning solutions can be used depending on the coating effects of the process on the electrodes.  Typically a 10% solution of nitric acid works well to remove any anti-corrosion chemicals or mineral deposits.

First, rinse off the electrodes/sensor in plane water to remove any heavy process coating using a soft brush.

Immerse the electrodes in the cleaning solution for 10 minutes, agitating them regularly, to clean off any remaining deposits.

Rinse the electrodes thoroughly with clean water, and allow the electrode to soak in tap water for at least 30 min.

Place the electrodes in a new clean ORP buffer solution and allow it ample time to stabilize.  If the displayed value is within ±30 mV of the buffer value, the electrodes are clean and do not require calibration and the system can be placed back on line. If the value is outside the tolerance, then a calibration is required.

Slide 23: Typically this is done about once a month but more aggressive processes will require more frequent maintenance and calibration. ORP is a one point calibration; however two buffer solutions are normally used, one to do the calibration and one is used as a check.

It is always recommended to use fresh buffer solutions to avoid the possibility of introducing errors from contaminated or aged solutions.  You also need to note that buffers supplied as liquids have a limited shelf life and should be used within the specified timeframe on the bottles.

• You have already Cleaned the Electrodes
• Make or pour buffer solution
• Perform a single point calibration
• Rinse the electrode

Then as a final check for correct calibration immerse the sensor in the second buffer solution to see if the reading is accurate. If it is not, the calibration should be repeated.

It is good practice to note that if a span is found of +150 mV change between the first solution and the second solution when using the quinhydrone solution or a span of 30 mV for premade solutions, the platinum/ gold measuring surface may be coated and the electrode should be re-cleaned and re-calibrated.

Slide 24: There are three common types of ORP buffers used today.

You can either purchase a premade stable ORP mV solution, or you can make your own ORP solution using standard pH Buffers.

When making an ORP solution there are two known types. The first is known as a Quinhydrone solution, and the second is known as a Light’s solution, but it is not used very often because you have to mix in sulfuric acid.

The most common used solution is the Quinhydrone solution.  This is where quinhydrone is mixed into the normally used pH 4 or 7 buffer solution until it is saturated (meaning some crystals remain undissolved), resulting in a known mV value solution.  

Slide 25: The top chart provides the common mV values you can expect when using standard pH buffer solutions, with quinhydrone mixed in. The bottom chart shows the standard expected mV reading when using the seldom used Light’s solution mixture.

(1) So a quick example would be if (2) I used my standard 4.0 pH buffer solution and dissolved Quinhydrone into it at 25 degrees Celsius and my reference electrode uses a 3M KCl solution, (3) then I can expect to see a mV reading of 253 on the display of my analyzer. 

Slide 26: When using a premade ORP solution, before you perform your calibration you have to know two things:

What is the reference solution in the electrode that I am using? Is it 3M KCl or is it 1M KCl.

and

What is the particular reference system that my pre-made ORP standard solution is based on?

To make sure we are comparing apples to apples, we have to convert the value to reflect the reference system we are using (generally Ag/AgCl). The label shown here in the slide is an example of a Hamilton’s premade ORP buffer solution. (1) As you can see the label shows you two different mV values. There is a 475 mV and there is a 680 mV.  (2) The * indicates that depending on which reference you are using which mV value you can expect to see.   

(3) This chart here shows you what you will have to either add or subtract to the value of the premade solution, to determine what can expect to be seen on the analyzer Display if the values are not already given. To determine the correct milivolt of the pre-made solution we have to account for the difference between the electrode reference value and the solution reference value. 

Let’s go over an example. As the chart shows, if you have a pre-made solution of 350 mV that used a (4) 1M KCl reference solution and the electrode being used contains a (5) 3M KCl solution, when a calibration is done at 25 degrees Celsius the unit will not read 350 mV. (6) Instead it will read an additional 22 mV showing a total of 372 mV on the display of the instrument.  Yes the mV difference from one molar KCL solution to a different molar KCl solution is minimal, however if the premade solution uses the standard hydrogen electrode or SHE as the reference solution, this difference could mean a significant difference of 200 plus mV.

Slide 27: Here are some troubleshooting Tips for the most common problems. If you are getting a slow response, this could be due to coating of the metal electrode or plugging of the reference junction. If you are seeing a noisy response this could be because of the reference electrode is poisoned or the KCL could be depleted. Noisy or drifting measurements could also be caused by old cable to a plugged reference junction.  

Slide 28: READ SLIDE……

For this scenario there are two possible problems:

                  The first is that the Platinum is saturated or fouled. If so, this is difficult to correct. You can try to reactivate the Platinum by inserting it in a strongly oxidizing or strongly reducing solution for 8-24 hours.

                  The second problem is known as Electrical polarization. For this you need to check how the sensors are connected. If the Platinum is connected to the solution ground terminal, then you can try to moving it to the measuring terminal instead, leaving the reference wired to its correct terminal. That way you make the ORP input high impedance and less susceptible for polarization. However this would mean that you would have to use a separate electrode to measure pH and ORP.

Slide 29: READ SLIDE…

Two different things could be happening.

                  First would be that the customer believes that everything should match exactly and but in reality the accuracy for the ORP can be as much as ± 40 millivolts.

                  The second could be that the concentration of the electrolyte being used in the measuring system is different than that of which was used when making the ORP solution. Remember the example we discussed a few slides back when discussing calibration? Let’s take a look at it again. (1)

                  As the chart shows, if you have a pre-made solution of 350 mV that used a (2) 1M KCl solution and the electrode being used contains a (3) 3M KCl reference solution, when a calibration is done at 25 degrees Celsius the unit will not read 350 mV. (4) Instead it will read and additional 22 mV showing 372 mV on the display instead. 

Slide 30: As a rule of thumb the Reference electrode is the weak link in the measurement system in about 80% of all applications. So selection of the appropriate style Reference is critical to reduce potential electrode problems, extend the life and to maintain measurement accuracy.

Slide 31: So far we have discussed, what is ORP and how is it measured? What equipment is needed and how do you maintain your ORP system? Now let’s take a moment and look at some applications. As with any measurement the goals are always the same… how I can get an accurate, reliable and resourceful measurement with the least amount of maintenance. For many years ORP measurements have caused their fair share of frustration. However, if applied properly ORP can provide critical information. 

Slide 32: ORP can be used to monitor and control in many industrial aqueous systems to correlate the millivolt readings to the sanitization strength of the chlorine present in the water. Various chemicals are commonly used to control the slime producing microbes, but the most commonly used chemical is chlorine. For Chlorine to function properly as a biocide it must be present in its oxidizing forms, either in the hypochlorous acid form or the hypochorite ion form. Since addition of chlorine increases the oxidizing capability of water, ORP provides a useful indicator of the effectiveness of the chlorine present in the water.

Slide33: However, since the equilibrium between the three species is pH dependent any changes in pH will be seen as a mV change and interrupted as an ORP change when a standard ORP measuring electrode and reference electrode are used.  So, to provide an accurate indication of the true active chlorine present we must compensate the ORP measurement for the effects of the varying pH. One way to do this is to replace the Ag/AgCl reference electrode normally used with a pH measuring electrode as a reference electrode. This is what is known as pH Compensated ORP. 

Slide 34: With a traditional ORP measurement system, you cannot differentiate between an oxidant change and a pH shift.  By using a pH Measuring electrode as the reference electrode, it allows you to recognize the difference between the two. Since the pH electrode is acting a reference electrode, the stable mV value that it will generate will shift at the same rate as the mV value changes because of a change in the pH. 

Advantages to using pH compensated ORP is that the process control is more reliable, since the mV value given depends only on the reagent concentration and not on the pH, as well as there is less Maintenance; because the measurement does not require a reference cell that could lead to potential junction troubles.

Slide 35: Similar to Biocide control, ORP can also be used in the disinfection of water as a critical step in minimizing the potential transmission of pathogens. Many facilities have already established ORP as the primary approach to monitor disinfection parameters, because it offers many advantages of real time monitoring for a rapid and single-value assessment of the disinfection potential of the system.

Research shows that bacteria such as E. coli and Salmonella are killed within a few seconds if the ORP value is maintained between 650 to 700 mV.  Research also indicates that when water is left at the range for a few minutes yeast and other more sensitive types of spore-forming fungi can also be killed. The table shown here was taken from an article published by the University of California in 2004 which indicates that E. coli and Salmonella species were killed within 30 seconds. 

Slide 36: However, a disadvantage of this method is that a combination of ORP and ozone concentration monitoring is necessary to ensure proper water disinfection. This is because resistant spore-forming decay pathogens and human parasites are highly tolerant to chemicals such as chlorine, bromine, iodine and other weak oxidizers that are commonly used for water disinfection.  In clean, potable water, ozone has been proven to be a highly effective sanitizer at concentrations of 0.5 to 2 ppm.  The table at the bottom shows other applications and ozone targets.

Here the graph shows that the relationship between ozone concentration and ORP values are not linear; however ORP can also be used to give an indication of the buildup of residual ozone and the antimicrobial oxidative status of the water. The chart at the bottom just shows some collected ORP and Ozone values for various water applications.

Slide 37: Another application where ORP can be seen is the dyeing of indigo fabrics. Indigo it is natural form will not dye fabrics because it is insoluble in water. So during the dyeing process additives such as, sodium hydrosulfite, are used to convert the indigo into what is called "white indigo," which is then able to stain the fabrics. The submerged fabrics are removed from the dye bath, allowing the “white indigo” to quickly react with the oxygen in the air causing it to return to the oxidized natural form, making the dye solution on the stained fabrics to be insoluble in water again. During this dyeing process it is vital that the ORP be maintained at a low enough level to ensure that the indigo dye remains in its reduced “white indigo” form.

Slide 38:  The last two applications that we will talk about are found in waste water treatment. At electroplating facilities and certain types of chemical plants the waste water can contain toxic forms of hexavalent chromium such as chromate and dichromate. EPA regulations require that the hexavalent chromium in this wastewater must be reduced before the water can be discharged to the water system. This requires a two-step process: hexavalent chromium (CR⁶) is reduced to trivalent chromium (CR³); and CR³ is precipitated as chromium hydroxide. As the oxidation-reduction potential (ORP) and the speed of the reduction reaction are closely tied to the pH value, ORP and pH meters are used to ensure proper control of the reduction process through such means as the injection of reducing agents.

 For more detailed information we can send you the application note that we have on Chrome reduction.

Slide 39: The last application we will discuss today is Cyanide wastewater treatment. The waste materials can contain alkaline, rare earth metals, and other heavy metals such as iron, nickel, zinc, cadmium, copper, silver and gold. As well as sometimes can contain the deadly poison, cyanide. Cyanide-bearing wastewater found in mining and electroplating facilities and certain types of chemical plants are toxic and must be treated by oxidation with chlorine or chloride to bring the cyanide concentration within regulatory limits.

 Again for more detailed information we can send you the application note that we have on Cyanide treatment.

Slide 40: You will also ORP measurement in the production of Magnesium, and in the electro refining of various metals such as copper silver or nickel.

Slide 41: Let’s quickly go over what we covered today. READ SLIDE

Slide 42: I want to thank you for your attendance today and I believe we have time for a few of the questions that we have had come in.

Slide 43: If I do not answer to question directly I will contact you with an answer either via email or telephone call. However if we can be of assistance here is my contact information as well as our Liquid product manager’s contact information.