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What is ORP?

ORP is a less common analytical measurement, thus it is frequently misunderstood. ORP is an abbreviation for “Oxidation Reduction Potential”. It is also commonly referred to as a “Redox” as the sensor is used to measure redox reactions.

ORP is a measurement of the net voltage potential of excess oxidizers or reducers present in a liquid. Based on this description it is helpful to understand the chemistry behind these particles. An oxidizer is a molecule with an excess of electrons and has a negative charge. A reducer is a molecule with a deficit of electrons and has a positive charge.

Any aqueous solution that has an excess of either oxidizers or reducers will have a voltage potential. An excess of reducers will have a net negative charge while an excess of oxidizers will have a net positive charge. A neutral liquid will have no charge.

When these molecules meet up, a redox reaction occurs that results in the transfer of electrons.

Perhaps the most common redox reaction people are familiar with is combustion. Let’s consider combustion of gasoline. The main component of gasoline is a hydrocarbon known as octane. When octane is combined with oxygen (an oxidizer) and enough heat is added, then a redox reaction occurs as shown by the equation below.

2C8H18 + 25O2 → 16CO2 + 18 H2O

The octane and oxygen are converted to carbon dioxide and water vapor. Now you know why you frequently see condensed water dripping out of the exhaust pipe of your automobile.

How Do ORP Sensors Work?

The construction of an ORP sensor is similar to a pH sensor. Both products have a reference electrode and a measurement electrode. The reference electrode contains an Ag / AgCl element in KCl electrolyte. The reference electrode has its own reaction that produces a stable voltage potential of approximately 220mV. It also has a porous liquid junction (sometimes referred to as a diaphragm) that allows the electrolyte to come in contact with the liquid media. This liquid junction is part of the voltage circuit for the sensor.

The measurement electrode is the main differentiator between a pH sensor and an ORP sensor. ORP sensors do not have the specialized glass membrane used in pH sensors. Instead, the measurement electrode of an ORP sensor has a metal band or wire in contact with the process liquid. The metal contact point completes the sensor’s voltage circuit that includes the reference electrode and the liquid junction. There is no potential generated by the metal; however changes in the concentration of oxidizers or reducers will influence the overall voltage potential of the circuit. Normally a noble metal such as platinum or gold is used for the measurement electrode so that it will not get damaged due to the redox reaction. All Hamilton ORP sensors use platinum for their measurement electrode.

It is important to remember that the reference electrode generates 220mV. This means that in a truly neutral solution with no potential voltage, the ORP sensor would still output 220mV. For proper measurement this voltage is subtracted out by the pH transmitter electronics during calibration.

What is the ORP unit of measure?

The ORP sensor is not specific to any single oxidizing or reducing molecule. Due to this, the sensor output is a raw millivolt value with no other unit of measure. Increasing voltage is associated with a higher concentration of one type of molecule or the other. Because of this, ORP is frequently used as a control parameter for a redox reaction. For example, chlorine is a strong oxidizer frequently used for disinfection.

When bleach is added to the liquid, the ORP value immediately increases. Over time, as chlorine reacts with biological organisms and other molecules, the ORP value will decrease. An ORP setpoint is used to control chlorine addition to optimize the disinfection. While ORP is technically not directly measuring chlorine, its measurement is a good indicator of the strength of the reaction.

What effect does temperature have on ORP?

Temperature has a direct effect on the ORP value of a solution. An increase in temperature can speed up the dissociation of molecules in the liquid thus increasing the ORP value and redox reaction rate.

Unlike pH, where the glass membrane potential is affected by temperature and should be compensated; the metal measurement electrode has no output. Due to this, ORP is not a temperature compensated measurement. Temperature is used during calibration. The value of an ORP calibration solution at different temperatures is predictable and can be compensated for as part of the calibration process to increase accuracy.

What effect does pH have on ORP?

pH is a measurement of the hydrogen ion concentration in an aqueous solution. Since ORP is affected by any positive or negative charged particle it can be confirmed that pH changes will directly influence the ORP value.

While these two measurements change together the rate of change may not correlate exactly. The redox reaction must be taken into account. Let’s consider chlorine mentioned above. At values below 1.9 pH chlorine is Cl2 in water. As the pH increases, chlorine reacts with water to form hypochlorous acid HOCl and the ORP value decreases. The formula is shown below.

Cl2 + H2O → HOCl + HCl

At 7.3 pH and greater hypochlorous acid HOCl reacts to form Hypoclhorite OCl-. and the ORP value will continue to drop.

HOCl → OCl- + H+

The stages from Cl2 (strong oxidizer) to HOCl (mild oxidizer) to OCl- (weak oxidizer) impact how proficient chlorine is as a disinfectant. For this reason pH and ORP are used in tandem to control the process.

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