Basics of ORP

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3.1. Concept of ORP

Fig. 3.1a
Fig. 3.1a
Fig. 3.1b
Fig. 3.1b

Have you ever wondered what gives a sapphire its deep blue color? It comes from a simple REDOX reaction between the titanium (IV) and iron (II) impurities. The REDOX reaction can be seen as:

Ti4+ + Fe2+ → Ti3+ + Fe3+

However in order for the reaction to occur energy must be supplied. This is easily 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 fuel the REDOX reaction, thus allowing only the blue light to pass thru the crystals, resulting in the deep blue color seen in sapphire crystals.

ORP (Oxidation-Reduction Potential) is the measurement, in millivolts, of a solution's capacity for electron transfer (oxidation or reduction). ORP measurement may also be called REDOX for REDuction OXidation. The name reflects that fact that within a chemical reaction reduction and oxidation are complementary; one cannot occur without the other. If one species undergoes oxidation (loses electrons) then another species must accept those electrons and is said to be reduced (gains electrons).

With redox reactions we speak in terms of the strengths of the oxidizing and reducing agents. Oxidizing agents have the capacity or potential to acquire electrons and become reduced. Reduction means the gain of electrons by an atom, leading to a decrease in the oxidation state of the element.

Cu2+ + 2e- → Cu

Reducing agents donate electrons and therefore become oxidized. Oxidation means the loss of electrons from an atom, leading to an increase in the oxidation state of the element.

Fe → Fe2+ + 2e-

Since reduction and oxidation reaction occur simultaneously, the formulas for the two half reactions shown above, (the reaction between iron and copper (ii) sulfate solution) are combined and result in the following:

Fe + CuSO4 → FeSO4 + Cu

Fig. 3.1c
Fig. 3.1c

The Copper, Cu, being the oxidizing agent while the Iron, Fe, is the reducing agent. Another example is the reaction between hydrogen and fluorine in the process of making hydrogen fluoride (HF). The hydrogen (H2) is being oxidized and fluorine (F2) is being reduced:

H2 + F2 → 2 HF

The two half-reactions are as follows:
the oxidation reaction is:

H2 →2H+ +2 e-

and the reduction reaction is:

F2 + 2e- → 2F-

When a chemically inactive metal electrode is placed into a solution where an oxidation-reduction reaction is taking place, an electric potential appears at the electrode. This potential is called the oxidation-reduction potential.

While a pH value can be obtained within seconds, a stable ORP value can take up to several minutes, if not hours, to reach the final equilibrium due to the type of reactions and their reaction rates. The ORP measurement behavior is strongly influenced by the metal surface condition. For example, a new, unconditioned ORP electrode will show different values than an ORP electrode that has been conditioned and considered in use.

3.2. The ORP Scale

A simple working definition for ORP is a solution's capacity for electron transfer known as oxidation or reduction, given in millivolts. The measurement of ORP is the reading of the voltage potential between the measuring electrode and a reference electrode. Depending on the solution being measuring, the ORP electrodes will serve as either an electron donor or an electron acceptor. 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.

Shown in Figure 3.2 is a section of the typical ORP scale. The full range is typically 1500 mV to -1500 mV. Just like with pH, all ORP electrodes are designed to produce 0 mV at pH 7. When we look at the pH scale an acid is defined as a substance that is capable of liberating hydrogen ions and a base is a substance capable of absorbing hydrogen ions.

Fig. 3.2

Fig. 3.2

Therefore every acid has its complementary base. 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. Some common liquids and their respective ORP values are shown in Figure 3.2. Soda is known to have a pH value of around 2.00; shown here the respective ORP value for soda is approximately 400 mV. Indicating that a positive mV reading (or below a pH 7) is associated with the charge of the Hydrogen ion, H+, and the solution is said to be acidic. Where as a negative mV (or above pH 7) is associated with the charge of the Hydroxyl ion, OH-, and the solution is said to be alkaline or basic.

An ORP system can be defined in the same manner. Unlike pH, ORP values are affected by all oxidizing and reducing agents, not just acids and bases which only influence a pH measurement. Since ORP is the direct measurement of electrons in transit during Oxidation-Reduction reactions, under oxidizing conditions, the measuring probe loses electrons to the solution, which creates a positive potential; in a reducing environment, electrons are donated to the probe, producing a negative potential. Since a reducing agent is capable of accepting an electron and an oxidizing agent is capable of losing an electron; 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.

For example:

Acid Permanganate solution is strongly oxidizing: it strongly attracts electrons from the REDOX electrode, so the REDOX potential is highly positive.

Opposite to that would be, Sulfite solutions are strongly reducing. It pushes electrons into the electrode, so the REDOX potential is strongly negative.

While pH is a specific measure of the Hydrogen ion concentration in solution, ORP only provides relative measures of chemicals and cannot discriminate one from another. even though ORP is non-ion specific, it is an inexpensive and useful method for controlling and monitoring the activity of various compounds such as chlorine, ozone, bromine, cyanide, chromate, and many others.

3.3. Measuring the ORP Value

ORP is measured in milivolts (mV), with no correction for solution temperature. Similar to pH, ORP is not a measurement of concentration directly, but of activity level. ORP is the measure of the ratio of the activities of the oxidizing and reducing species in a solution. The ORP value of a particular material results in either a positive or negative mV output; the value is determined by the size of the atom of the material and the number of electrons found in its outer electron shell. The response speed of the process measurement varies with the concentration of the REDOX in the system; higher concentrations are faster and lower concentrations are slower.

Once again, the German physical chemist and physicist, Walther Hermann Nernst found that a potential difference occurs between a metal object and a solution that contains ions of the same metal, when the object is immersed in the solution.

In electrochemistry, the Nernst equation is an equation that can be used (in conjunction with other information) to determine the equilibrium reduction potential of a half-cell1) in an electrochemical cell2). It can also be used to determine the total voltage (electromotive force) for a system.

Nernst formula:

Eh = E0 + RT/nF log Aox/Ared

In which....

Eh = is the Oxidation Reduction Potential value of the reaction
E0 = is the standard potential that is particular to the reaction series and that has a constant value, that is not affected by Aox/Ared, temperature, etc.
RT/nF = is the Nernst number
Aox = is the activity of the oxidant
Ared = is the activity of the reductant

Note 1. A half cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally-occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a potential difference between the electrode and the electrolyte.
Note 2. An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy.

Some Standard Reduction Potentials in Aqueous Solution at 25°C

  Element Reduction Half-Reaction Standard
Potential E0(V)
Strength of
F2 F2 + 2 e- → 2 F- 2.870 Strongest
Strength Of
Au Au3+ + 3 e- → Au 1.420
Cl2 Cl2 + 2 e- → 2 Cl- 1.358
Br2 Br2 + 2 e- → 2 Br- 1.065
Hg Hg2+ + 2 e- → Hg 0.851
Ag Ag+ + e- → Ag 0.800
I2 I2 + 2 e- → 2 I- 0.535
Cu Cu2+ + 2 e- → Cu 0.340
H2 2 H+ + 2 e- → H2 0.000
Pb Pb2+ + 2 e- → Pb -0.126
Sn Sn2+ + 2 e- → Sn -0.136
Ni Ni2+ + 2 e- → Ni -0.230
Cd Cd2+ + 2 e- → Cd -0.403
Fe Fe2+ + 2 e- → Fe -0.409
Cr Cr3+ + 3 e- → Cr -0.740
Zn Zn2+ + 2 e- → Zn -0.763
Al Al3+ + 3 e- → Al -1.706
Mg Mg2+ + 2 e- → Mg -2.375
Na Na+ + e- → Na -2.710
Ca Ca2+ + 2 e- → Ca -2.760
K K+ + e- → Lk -2.292
Li Li+ + e- → Li -3.040

The reference point for all oxidation or reduction reactions, are compared to the hydrogen ion/ hydrogen (H+/H2) reaction; which has a standard potential, E0, of 0 mV.

3.3 Eq: Oxidized atom, free electrons, reduced atom

Tables for standard potentials, E0, as seen in Table 3.3a, for various reactions and their half reaction can be found in various General Chemistry Textbook reference materials3).

The tables are usually written as reduction reactions, showing the free electrons and the oxidized atom on the left and the reduced atom on the right hand side of the reaction equation.

A typical industrial ORP measurement loop is similar to that used for pH measurement. It includes a high input impedance analyzer a reference electrode, measuring electrode, and system ground.

The reference electrode is typically a standard pH reference electrode, normally, a silver/silver chloride wire in a potassium chloride electrolyte solution. It may be either free flowing or gel filled. The measuring probe is typically platinum though some other inert metals have been tested.

When measuring ORP an important feature to remember is that unlike pH, temperature compensation is not normally used for ORP measurements. Temperature does have two distinct effects on ORP measurements; however it is not compensated for because:

  • The isopotential point (the point of thermal independence) of an ORP system is only relative to the particular redox reaction and therefore there is no "standard" isopoint for the overall ORP reaction.
  • Since ORP is non-ion specific measurement, the chemistry of the redox reaction can be quite complex, especially if several ionic species involving varying numbers of electrons transferred contribute to the reaction / oxidation reduction potential.
  • Most ORP measurements are done at constant temperatures, such as in process measurement and control.

Note 3. Oxtoby, Nachtrieb, Freeman. 1994. Chemistry Science of Change. Philadelphia: Saunders College Publishing.

3.4. Composition of the Measuring Electrode

ORP/Redox is a potentiometrical measurement of the oxidizing/reducing power of a liquid. An ORP measuring electrode is similar to that of a pH measuring electrode, except it is normally constructed of an inert (noble) metal.

The most common metal used is platinum. Platinum, which is considered the standard, has excellent chemical resistance but suffers slightly from Chemisorption1) of oxygen; which slows down the measurement response time. Meaning that the surface can absorb organic compounds and it may be attacked by sulfides and cyanides in strongly reducing solutions, i.e. such as solutions with redox potentials less than -500 mV.

The Oxygen bonds to the surface in strong oxidizing solutions and hydrogen bonds to the surface in strongly reducing solutions. Some anti-corrosion chemicals added to cooling towers and pasteurization processes perceive the electron active surface of the platinum as corrosion and passivates it. All of these surface reactions can result in slow response. Any surface coatings that insulate the platinum surface from the solution will decrease the speed of response.

The basic measurement principle is that the measuring electrode will give up electrons to an oxidant or accept electrons from a reductant, without interfering with the chemical reactions that are taking place within the solution.

The metallic electrode can be classified into three distinct types of ORP electrode.

  1. The first consists of a metal in contact with a solution of the same metal ions. i.e. a silver electrode placed in a solution of silver nitrate, which will develop a potential proportional to the silver ion activity.
  2. The second consists of a metal electrode coated with a sparingly soluble salt of metal, in contact with a solution containing the anion of the metal salt. i.e. a silver-silver chloride electrode in a potassium chloride solution.
  3. The third kind, and most common, consists of a noble metal in contact with a solution containing both the oxidized and reduced forms of an oxidation-reduction system. This is typically a platinum.

An ORP measuring electrode can either be a separate electrode (as seen in Figure 3.4a), a combination ORP/Reference electrode (as seen in Figure 3.4b), or a combination ORP/pH measuring electrode (as seen in Figure 3.4c). The choice depends on the application as well as customer installation requirements.

Note 1. Chemisorption (or chemical adsorption) is adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds.
Chemisorption means to take up and chemically bind (a substance), in this case Oxygen, onto the surface of another substance.

Fig. 3.4a: Separate ORP Measuring Electrode Fig. 3.4b: Combination ORP/Reference electrode Fig. 3.4c: Combination ORP/pH measuring electrode
Fig. 3.4a: Separate ORP
Measuring Electrode
Fig. 3.4b: Combination ORP/Reference electrode Fig. 3.4c: Combination
ORP/pH measuring electrode

3.5. Composition of the Reference Electrode

ORP can be measured relative to any practical or theoretical reference electrode such as Ag/AgCl, or SHE (Standard Hydrogen Electrode) like described in section 2.5.2 and 2.5.5.

3.6. The Measuring Circuit

Fig. 3.6a
Fig. 3.6a

An ORP probe is really a millivolt meter, measuring very tiny voltages generated across a circuit formed by a measuring electrode (the positive pole of the circuit normally platinum), and a reference electrode (the negative pole), with the process solution in between. The difference in voltage between the two electrodes is what is actually being measured by the analyzer.

These voltages give us an indication of the ability of the oxidizers or reducers within a solution. The speed of response varies with the concentration of the redox system; high concentrations are fast and low concentrations are slow.

The Figures 3.6a/b shows that following potentials are of the most significance:

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

The sum total of these potential differences is measured by the signal convertor.

Et=E1 +E2 +E3

It is also important that within the analyzer being used for detection, there must be a high impedance (resistance) in order to measure the very tiny voltages (or charge build up) being generated by the constant acceptance and giving up of electrons on the ORP electrode.

The ideal conditions described above cannot always be completely realized in practice. A small potential difference may exist in the reference and is acceptable because most mV changes measured for ORP within solutions are large.

3.7. Standard ORP vs pH Compensated ORP (rH)

Fig. 3.6b
Fig. 3.6b

ORP measures the ratio of the activities of the oxidizing and reducing species in a solution. This is a measure of the solutions ability to oxidize or reduce another substance. As an oxidizer is added to the process, it "steals" electrons from the surface of the ORP measuring electrode, causing it to become more positively charged. Continuing to add oxidizer generates a higher and higher positive voltage. The role of an ORP system is to measure these tiny voltages generated across a circuit formed by a measuring electrode (the positive pole of the circuit, normally platinum), and a reference electrode (the negative pole, reference electrode), immersed in the solution.

ORP system are typically rugged, but do have some limitations. For example, when ORP is used with a chlorine-based sanitation system, it will not indicate the chlorine concentration in parts per million. It will however, indicate the effectiveness of the chlorine as an oxidizer. ORP can be used to indicate the activity of chlorine in a solution. Since addition of chlorine increases the oxidizing capability of water, measurement of the ORP provides a useful indicator of the quantity of active chlorine present. This is very important when the chlorine is being used as a biocide agent to control algae growth in the process. One drawback however, is that pH changes also affect the oxidizing potential of the available chlorine and the resultant ORP value.

Below a value of 1.9 pH, chlorine exists as a diatomic molecule (Cl2) in water. As the pH increases above 1.9, chlorine oxidizes water to produce HOCl and the ORP millivolt reading will go down. As the pH value continues to increase HOCl further dissociates into OCl- above a pH of 7.3.

HOCl being more active that OCl- has a higher ORP value. So, as the pH increases, an ORP sensor detects a decrease in value which reflecting the decrease in HOCl. Therefore, if we were using ORP to monitor the diatomic Cl2 level, we would have a large mV reading to start with, but as it oxidizes with water reacting with the Calcium Thiosulfite to form a salt, the mV reading would decrease.

Since ORP varies with pH changes, as well as changes in the chlorine levels, we must compensate for the effects of any pH changes. This can be done by measuring the pH and ORP independent of one another and then calculating the effect of the pH change on the ORP using formulas and graphs. A simpler and more direct method is to compensate for the pH changes by replacing the standard Ag/AgCl reference electrode normally used with a pH measuring electrode. This is known as pH Compensated ORP (rH).

Since the pH measuring electrode output changes as the pH of the process changes it acts as a moving reference effectively cancelling out any change in pH and leaving only the mV value which is due to changes in diatomic chlorine (Cl2) levels.

3.8. Standard Maintenance and Calibration

Maintenance and calibration for an ORP system has similarities with those methods and procedures used for both pH and conductivity systems. The problems and maintenance procedures associated with the ORP reference electrode, for example, are handled the same as they are for a pH loop. The maintenance of the ORP measuring electrode however, is handled in much the same way as a conventional conductivity sensor. Calibration of the ORP system is also similar to how a conductivity loop is calibrated.

Cleaning the measuring electrode will improve accuracy and the sensors response time. The frequency of maintenance, which includes cleaning and calibration, is determined by how the process affects the electrode. Methanol, isopropyl alcohol or a detergent can be used to remove oily or organic coatings while those applications where removal of any anti-corrosion chemicals or mineral deposits, soaking the electrode in 10% nitric acid for about 10-15 minutes is a good starting point.

If the various forms of chemical cleaning are not sufficient to achieve an accurate measurement and response time, a last resort would be to polish the platinum surface with a 600 grit wet-dry emery cloth or a 1-3 micron alumina polishing powder to remove any surface pitting or stubborn coatings.

After any cleaning procedure, the electrode should be allowed to soak in clean tap water for at least 30 minutes to allow residual chemicals to dissipate and the sensor to recover. After calibration, when the electrodes are placed back in the process, they should be allowed to equilibrate for at least 15 minutes. For optimal operation, the sensors should be installed in an area with good agitation as process flows past the electrode helps to keep the platinum sensing element clean.

ORP electrodes should need less frequent calibration than a typical pH sensor since the redox potential is a characteristic of the interaction between the platinum and the redox equilibrium. However, it is prudent to periodically verify the performance of the measuring system by placing the electrode in a solution with a known potential and calibrate is needed to correct for the reference side of the sensor.

Standard ORP Solutions

When verification or calibration of an ORP sensor is required, there are two types of Standard Solutions that are commonly used. The first are premade solutions designed to provide a specific stable mV value, typically one that falls within the process ORP range. The second type of solutions, and probably the most common, are those that are made using the standard pH 4 and pH 7 buffers with quinhydrone crystals mixed in until saturation is reached. either of these pH buffer solutions can be used for calibration of an ORP measuring system and are very practical if pH loops are also being maintained. Preparation and use of both types of solutions are discussed below:

Quinhydrone1) Solution

To create an ORP solution using a pH buffer (either 4.0 or 7.0) stir in a small amount, approximately < 0.5 gm, of quinhydrone into 200 mls of solution. Quinhydrone is not very soluble, so only a small amount will dissolve in the buffer changing the solution to an amber color. If all of the quinhydrone does dissolve, then continue to add small amounts and stir again. Saturation is achieved when a small amount of quinhydrone remains un-dissolved after mixing.

Whether it is a 4.0 or a 7.0 buffer you are using, Table 3.8a shows the mV reading you should obtain depending on which reference electrode is being used. As an example, a quinhydrone/pH 4.0 solution should give a 253 mV (± 30 mV) at 25°C for a reference electrode that has 3M KCl internal fill.

Note 1. The quinhydrone powder poses a moderate health risk, causing irritation of the lungs with prolonged exposure. The premade calibration solutions are fairly innocuous unless ingested in large amounts. Both types should be handles carefully following good laboratory practices.
Note 2. SCE = Saturated Calomel electrode
Note 3. SHE = Standard Hydrogen electrode

Table 3.8a mV value of ORP solution made with pH Buffers and Quinhydrone

Reference electrode ORP value (mv)
pH 4 Buffer solution pH 7 Buffer solution
20°C 68°F 25°C 77°F 30°C 86°F 20°C 68°F 25°C 77°F 30°C 86°F
Ag/AgCl (1M KCl) 236 231 226 61 54 47
Ag/AgCl (3M KCl) 257 253 249 82 76 70
Ag/AgCl (sat. KCl) 268 263 258 92 86 79
Calomel (sat. KCl) --- 218 --- --- 41 ---
SCE2) 223 218 213 47 41 34
SHE3) 470 462 454 295 285 275

Pre-Made Stabilized ORP Solutions

Reference electrodes with different internal fill solutions will have different mV outputs when they are put in the same Standard Solution. This is because the Standard Solution was prepared with one specific reference fill solution in mind. Table 9.2 lists in the left-most column, some of the most commonly used reference electrode fill solutions. Across the top of the table are the possible reference fill solutions that Standard Solution was prepared against. To use the chart below, you have to know what (1) reference solution is used in the reference electrode and (2) what reference solution the known premade solution is being compared to. For example, if you have a premade 250 mV solution that is referenced to SHe (Standard Hydrogen electrode) and the reference electrode in your measuring loop uses a 1 M KCl fill solution, then on the transmitter you would nOT read 250 mV, but instead you would read only 19 mV at 25° C. This is the 250 mV value on the solution minus the 231 mV value shown as the difference between the SHe and the 1M KCl references. This would be 19 mV.

Note 1: SCE = Saturated Calomel electrode
Note 2: SHE = Standard Hydrogen electrode

Table 3.8b mV offset between Various Reference Electrode Solutions

  To SCE1) ToAg/AgCl) (3MKCl) ToAg/AgCl (sat. KCl) ToAg/AgCl) 1M KCl) To SHE2)
20°C 68°F 25°C 77°F 30°C 86°F 20°C 68°F 25°C 77°F 30°C 86°F 20°C 68°F 25°C 77°F 30°C 86°F 20°C 68°F 25°C 77°F 30°C 86°F 30°C 86°F 30°C 86°F 30°C 86°F
From SCE1) +34 +35 +36 +45 +45 +46 +13 +14 +16 +241 +241 +241
From Ag/AgCl (3M KCL) -34 -35 -36 +11 +10 +9 -21 -22 -23 +205 +205 +205
From Ag/AgCl (sat KCL) -45 -45 -45 -11 -10 -9 -32 -31 -30 +202 +199 +196
From Ag/AgCl (1M KCL) -13 -14 -16 +21 +22 +23 +32 +31 +30 +234 +231 +228
From SHE2) -247 -244 -241 -213 -209 -205 -202 -199 -196 -234 -231 -228

Stir in a small amount of approximately < 0.5 gm, of quinhydrone into 200 mls of a pH buffer solution. Quinhydrone is not very soluble, only a small amount will dissolve in the buffer changing the solution to an amber color.

When verification or calibration of an ORP sensor is required, there are two types of Standard Solutions that are commonly used. The first are premade solutions designed to provide a specific stable mV value, typically one that falls within the process ORP range. The second type of solutions, and probably the most common, are those that are made using the standard pH 4 and pH 7 buffers with quinhydrone crystals mixed in until saturation is reached. either of these pH buffer solutions can be used for calibration of an ORP measuring system and are very practical if pH loops are also being maintained. Preparation and use of both types of solutions are discussed below:

Proper calibration

The following steps are commonly used for calibration of an ORP loop.

  1. Clean the ORP & Reference electrodes FIRST.
  2. Make FRESH Buffer Solutions with either Quinhydrone Crystals (See section 3.8), or pour a fresh sample of a premade stabilized ORP solution.
    A. ORP Buffers are best made just before using.
    B. Never keep (store) ORP calibration solutions.
  3. Perform a single (1) point calibration.
    A. Pick a solution with a mV value closest to the control point.
    B. If possible adjust (heat) the solution to one of the temperatures shown in the mV tables above.
    C. Allow stabilization time then adjust to the correct value.
  4. Rinse the electrode between calibration measurements if checking at a second buffer value.

If a short span is found (less than a +150 mV change between the first solution and the second solution), the platinum/gold measuring surface may be coated and the electrode should be re-cleaned and re-calibrated.

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