Knowledge of redox is essential for monitoring chemical balance in water systems. But what is redox and how do you measure it? What do you need to pay attention to and which redox sensor is most suitable? JUMO is a manufacturer of proven analytical measuring equipment. In this blog, we answer the most frequently asked questions about redox and how to measure it.
Redox, also known as ORP (Oxidation Reduction Potential), is measured in liquids such as water. Measuring redox provides insight into a liquid's ability to break down contamination or, conversely, to create a stable, non-oxidizing environment. The term “redox” is a contraction of two fundamental chemical processes:
These two processes always occur simultaneously: when one substance loses electrons (oxidation), another substance must absorb them (reduction). Hence the term “redox.”
A redox reaction is a chemical reaction that occurs between two substances, involving the exchange of electrons: oxidation (release of electrons) and reduction (acceptance of electrons).
Example formula:
Consider the reaction between chlorine (Cl₂) and iron(II) ions (Fe²⁺) in water:
Reaction equation:
Fe²⁺ (aq) + Cl₂ (aq) → Fe³⁺ (aq) + 2 Cl⁻ (aq)
This equation can be split into two partial reactions:
Oxidation (iron loses an electron):
Fe²⁺ → Fe³⁺ + e⁻
Reduction (chlorine accepts electrons):
Cl₂ + 2 e⁻ → 2 Cl⁻
In this reaction, Fe²⁺ donates an electron (oxidation) and Cl₂ accepts electrons (reduction). Together, these two partial reactions form the complete redox reaction.
In water systems, such as swimming pools or drinking water installations, these types of reactions are essential for breaking down contaminants and ensuring safe water quality. By measuring the redox value, you gain insight into the extent to which these reactions can take place.
The redox value is expressed in millivolts (mV). It measures the electrical potential difference between a special measuring electrode (usually made of platinum) and a reference electrode, both of which are placed in the liquid.
When the redox value deviates from the desired standard, it is important to consider the specific context and the goals you want to achieve with your water treatment. In any case, take the following considerations into account:
If the redox value of water is too low, this means that its oxidizing capacity or disinfecting power is insufficient. This is often a problem when clean and safe water quality is desired.
Chlorine: The most common and effective method for swimming pools and spas. It immediately increases the number of oxidisers and thus the redox value.
Ozone: If you have an ozone generator, administering ozone can significantly increase the redox value, as ozone is a powerful oxidising agent.
Hydrogen peroxide or bromine: Depending on the system and preference, these agents can also be used to increase the redox value.
The pH value has a major influence on the effectiveness of disinfectants such as chlorine. Chlorine is most effective at a pH between 7.0 and 7.6.
Is the pH value too high? Then lower it with a pH reducer (such as sulphuric acid or hydrochloric acid). A pH value that is too high causes chlorine to work less effectively, resulting in a lower ORP (redox) value.
Organic contamination (leaves, oils, algae) consumes disinfectants and lowers the ORP value.
Ensure proper filtration: Run your filter long enough and clean or rinse it regularly.
Remove visible contamination: Scoop debris out of the water and clean surfaces consistently.
Good water circulation ensures that disinfectants are effectively distributed throughout the system, contributing to a stable and higher ORP value.
Checking pH values is essential for correct redox measurement
Sometimes it is necessary to achieve a lower redox value, for example in specific industrial processes or biological systems. This indicates a reducing environment.
A redox sensor, also known as an ORP sensor, is an electrochemical instrument that measures the redox potential of a liquid. This potential indicates how strongly a solution tends to give up electrons (oxidise) or take up electrons (reduce).
An ORP sensor operates on an electrochemical principle and typically consists of three main components:
Usually made of a precious metal such as platinum. Platinum is chemically inert and acts as a catalyst for electron exchange. It measures the electrical potential created by the balance between the oxidising and reducing substances in the measuring medium.
When there is a high concentration of reducing agents in the liquid, electrons move from the liquid to the platinum electrode, causing the electrode to become negatively charged.
This electrode provides a stable and constant reference potential against which to compare the measurement from the measuring electrode. This is often a silver/silver chloride electrode (Ag/AgCl).
The electrolyte is the conductive liquid that enables the electrical connection between the measuring and reference electrodes. The membrane is a small, porous component (ceramic, polymer, etc.) that forms the barrier between the internal electrolyte and the external measuring solution, for controlled ion exchange.
The redox sensor measures the voltage difference (in millivolts) between the measuring electrode and the reference electrode:
Oxidation and reduction reactions take place in the solution, during which electrons are exchanged between substances. The measuring electrode registers the electrical voltage generated by these reactions.
The reference electrode provides a stable reference point. The voltage difference between the two electrodes is measured and displayed as the ORP or Redox value.
The redox sensr is expressed in millivolts (mV).
The more frequently the sensor is used (especially for continuous measurements), the faster it wears out. Occasional use extends its service life.
Higher temperatures accelerate the degradation of sensor materials. A rule of thumb is that the service life is halved for every 25 °C increase in temperature:
With proper care and attention to maintenance and storage, the service life of the redox sensor can be significantly extended and reliable measurement guaranteed:
An ORP sensor, especially the reference electrode, should never be allowed to dry out. Always store the sensor with the protective cap filled with the appropriate storage solution (often a KCI solution or pH 4/7 buffer). Drying out can render the reference electrode unusable.
Clean the sensor regularly and remove coatings and dirt. Preferably after each use, but certainly on a regular basis.
Calibrate the sensor regularly. By calibrating the sensor regularly, you maintain accurate results and can recognize in good time when the sensor is no longer functioning correctly and needs to be replaced.
JUMO offers comprehensive calibration services, both in its own laboratories and on site.
Redox measurement is not only about understanding the chemical basis, but above all about translating that knowledge into practice. By gaining insight into what redox is, how the potential behaves, and which factors influence the value, it becomes clear how essential this variable is for water quality and process control. With a well-chosen and properly maintained redox sensor, you have an instrument that not only provides accurate measurements but also performs reliably over time. This makes redox measurement an indispensable link between theory, measurement practice, and the certainty of stable and safe processes.
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