Oxygen reduction potential (ORP) electrodes are analytical sensors for measuring oxidation-reduction potential (ORP). They are part of an ORP instrument, which also includes a metal half-cell, a reference cell, and a user interface.

Oxidation reduction is a type of chemical reaction in which electrons are transferred from one substance to another. The oxidized species loses electrons and the reduced species gains electrons. It also serves as a measure of how much oxidation or reduction takes place in the given conditions. Redox reaction refers to the exchange of electrons. Oxidation is the loss of electrons and therefore the solution is more positive. Reduction is the gaining of electrons which causes a negative charge. Both species must be present for the reaction to occur. The speed of an ORP instrument is directly related to the exchange current density derived from concentration, the oxidation reduction system, and the electrode. The speed diminishes when the sample ORP is similar to the electrode ORP. The listed disadvantages can be compensated for by rinsing the instrument's electrodes before readings, and by correlating the system by checking the oxidizer and reducer in a steady state system with a wet test and measuring pH.

ORP Assembly. Image credit: Hach

When the measuring electrode is exposed to oxidizing or reducing agents, electrons are constantly transferred back and forth on its measuring surface which generates a tiny voltage. The measuring or sensing electrode is made of platinum, gold, or graphite. The reference electrode is connected to a salt water solution and given a half cell potential of 0.0mV. This electrode is made of silver chloride or saturated calomel (SCE) because they provide stable and reliable performance. The ORP electrodes measure the voltage across a circuit formed by the two electrodes and the measurement can be made using the millivolt mode of a pH meter. The single voltage is called the oxidation-reduction potential, where a positive voltage shows a solution attracting electrons (oxidizing agent). ORP instruments can measure oxidation reduction potential in the range of -450 to +1100 mV. Readings toward the positive region of this range by these instruments indicate a strong oxidizing agent, while readings toward the negative region by indicate a strong reducing agent.

For more information please read IEEE GlobalSpec's How to Select Oxidation Reduction Instruments selection guide.

Specifications

With oxygen reduction potential (ORP) electrodes then, there are three main performance specifications:

• ORP/redox range- The ORP / redox range the instrument is capable of measuring. As seen in the chart below, reactions produce a positive or negative ORP depending on the type and quality of the reaction occurring. The oxidation scale can go from about -1000 to +1000. Sources with a strong negative ORP are safer to consume.
• Accuracy- The ORP / redox accuracy of the instrument (±mV). The size of the electrode influences the fluctuations in readings. A larger detection area means a better sensor. If the curves produced by the data collected overlap, it can be assumed that the reaction was carried out while the electrode was in equilibrium.
• Response time- Response time for ORP electrodes is usually expressed as a value such as 95% in 10 seconds.

Electrode

• Single cells or electrode pairs require a separate reference electrode. This type of device is the best choice for colloidal suspensions, iodides in sample, and high-percentage solids in fluid.
• Combination oxygen reduction potential (ORP) electrodes are also commonly available. These devices have two parts: the measuring electrode and the reference electrode. Combination electrodes are the most popular type of ORP electrode where the measuring electrode and the reference electrode are in the same housing.

Reference Solution

The reference solution options for an ORP electrode are either refillable or sealed. The trade-off between the two types is the amount of maintenance versus the length of product life.

• Refillable ORP electrodes require more maintenance, but also last longer and typically have higher accuracies.
• Sealed ORP electrodes do not require refilling, but have a limited life since the chemicals inside are used up and are not replaceable.

Junction Type

unction type is important physical specification to consider when selecting oxygen reduction potential (ORP) electrodes. There are two basic types:

• Single junction- Single junction reference electrodes contain a single electrolyte. The electrolyte provides a constant level of the ion sensed by the reversible reference element and forms a low potential liquid junction with the sample solution.

• Double junction- Double junction reference electrodes have an additional reference cell in front of the potassium chloride cell. This cell screens the sample from the potassium chloride reference cell. Typically, double junction cells have a longer time and provide a higher level of stability.

• ORP electrodes with other, unlisted junction types are also available.

Materials

There are two common choices for body material: glass, and epoxy or polymer.

Maintenance

Proper care and maintenance is critical to ensure accurate readings and a long lifetime. When preforming an ORP experiment, the oxidation reduction potential electrode must be prepared by removing the air bubbles and pretreated by soaking in a pre-treatment solution. A quinhydrone (HI 7091M) is used as a reducing treatment, and sodium hypochlorite (neutralized at pH7) with 1 M hydrochloric acid or bleach is used as an oxidizing treatment.

The electrode should be cleaned and rinsed after every use to prevent buildup. It should also be checked for scratches or cracks, and the surface should be cleaned and smoothed until all traces of the cleaning substances are removed. Upon storage, the electrode should be stored wet in a storage solution or pH 4 buffer and rinsed periodically. ORP electrodes should not be stored in distilled or deionized water.

Calibration

The video below explains the two-point check system for calibrating an ORP electrode. The desired result of a calibration is that the reading given by the electrode will give the specified differential valve with tolerances of +/- 10 mV

If the reading given by the electrode is not within the specified range the electrode by being dirty/coated or the reference junction is clogged. The electrode should be cleaned and calibrated before every use.

Configuration

The oxidation reduction potential electrode is available in several mounting options to fit the desired application. ORP tests are frequently done in laboratories as well as in remote water storage units or water supplies, therefore the mounting and portability are important to consider when selecting an instrument.

• Handheld devices are designed to be operated while held by hand. They are rarely able to establish a fixed point in a system and the "bounce" observed measurement may be as much as 25mV. This depends on if the device is stationary or in motion. The University of California Division of Agriculture and Natural Resources recommends that handheld ORP instruments be held in the source for 30 seconds.
• Portable devices have handles, a case, or wheels for ease of movement. They are not necessarily handheld.
• Insertion-style ORP electrodes are often inserted into process piping through a tapped hole in a pipe or bulkhead
• Flow-through styles fit directly into the pipeline and become an integral part via some connection, such as flanges or other fittings.

Features

Typical features for oxygen reduction potential (ORP) electrodes include:

• pH testing- ORP electrodes may also be able to test pH within the system so the ORP and pH can be tested at once.
• Temperature compensation- Temperature compensation is very important for an accurate pH measurement; however, it is not used for ORP measurement. Temperature does affect the reaction potentials for all chemicals but true ORP is the direct measurement of electrons moving during an oxidation-reduction reaction, regardless of temperature.
• Built-in temperature sensing- Devices have a built-in calibrator, which can be used for testing and readjustment.
• Submersible or water resistant design- Submersible ORP electrodes can be used in water testing applications.
• Suited for food or sanitary applications- ORPs used in food or sanitary applications can be easily sanitized between uses to prevent contamination.

Applications

Oxidation reduction potential (ORP) instruments are mainly used to measure the oxidizing ability of chlorine in swimming pools and spas, and to determine when the equivalence point has been reached in an oxidation-reduction reaction. ORP instruments are also used for cooling tower disinfection, groundwater remediation, bleaching, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection, and dechlorination.

Chemicals like chlorine, bromine, and ozone are all strong oxidizers. Because they are able to take electrons from other substances they are excellent sanitizers. The act of sanitation is caused by the alteration of the chemical makeup of the unwanted organism. As the oxidizers are reduced, their ability to oxidize is reduced until they are used up or combined with another substance. The oxidation process can be monitored by an ORP instrument and the user can identify the end of the sanitation process. Oxidation reduction potential instruments can possibly select an individual ORP value, expressed in millivolts, at which a predictable level of disinfection can be achieved and sustained regardless of variations in either oxidant demand or oxidant concentration.

Video Credit: WaterEducationA / CC BY-SA 4.0

Oxidation reduction potential instruments have proven their use as analytical tools in monitoring changes in a system rather than determining their absolute value (e.g., process control and titrations).

Standards

The value of ORP measurements has been popular since the 1960s as a way to gauge water quality. In 1971, the World Health Organization set a standard ORP value for drinking water disinfection at 650 millivolts. This means that when a body of water measures 650/1000 (about 2/3) of a volt, the sanitizer in the water is active enough to destroy harmful organisms almost instantaneously.

References

How to Select Oxidation Reduction Potential Instruments

Hach - Combination ORP Electrode

Image credit:

Emerson Process Management