The redox potential is an electrochemical measurement that assesses the tendency of a solution to undergo oxidation or reduction chemical reactions. In simple terms:

• Oxidation: The process in which a substance loses electrons.
• Reduction: The process in which a substance gains electrons.

Since a redox reaction involves a transfer of electrons from one substance to another, it is easy to deduce that it is an electrical phenomenon.

The redox potential is measured with an instrument called a potentiometer, which consists of a reference electrode and a working electrode. The reference electrode has a known potential and is made of noble metals such as platinum, silver or gold, which do not react with the medium and can accurately measure changes in electron transfer. On the other hand, the working electrode is placed in the solution to be measured. The potential difference between the two electrodes is expressed in millivolts (mV).

The redox potential can be interpreted as an indicator of the chemical activity of the water. Higher values usually indicate oxidising conditions (such as the presence of dissolved oxygen or free chlorine), while lower values indicate reducing conditions (such as the presence of organic substances or sulphides).

The use of redox potential in water treatment offers multiple benefits, as it allows the evaluation of water quality and the optimisation of processes such as disinfection, removal of pollutants and control of microorganisms. Some of its most relevant applications are explained below:

The redox potential is a key parameter in disinfection systems as it helps determine the effectiveness of disinfecting agents such as chlorine, ozone and hydrogen peroxide. These agents work by altering the redox balance, killing pathogenic microorganisms such as bacteria and viruses. For example:

• A redox potential above 650 mV is usually sufficient to ensure effective disinfection of drinking water.
• In swimming pools, values of 700-750 mV ensure adequate control of bacteria and algae.

The redox potential is used to monitor and regulate chemical reactions that remove pollutants such as heavy metals, nitrates and phosphates. For example:

• Under oxidising conditions, certain metals such as iron and manganese can precipitate and be separated from the water.
• Under reducing conditions, nitrates can be transformed into gaseous nitrogen through denitrification processes.

In biological treatment systems, such as wastewater treatment plants, the redox potential is used to manage the metabolism of micro-organisms responsible for the degradation of organic matter. For example:

• Low redox potential values favour the activity of anaerobic bacteria, which are necessary for sludge digestion.
• Higher values promote the activity of aerobic bacteria, essential for the reduction of organic compounds and nitrification.

The monitoring of redox potential offers a number of advantages in water treatment, among which the most important are

• Accuracy: Allows to detect changes in water quality and adjust treatment processes in real time.
• Efficiency: Optimises the use of chemical agents and reduces operating costs.
• Sustainability: Minimises environmental impact by favouring more controlled and regulated processes.

Despite its benefits, the use of redox potential is not without its challenges. Some of the main obstacles are:

• Water composition: The presence of multiple compounds can complicate the interpretation of the redox potential.
• Technological requirements: Specialised equipment, such as probes or sensors, is needed for precise measurements.
• Education and training: Water treatment personnel must be trained to understand and apply this parameter correctly.

The redox potential is a very powerful and versatile tool for water treatment, improving the quality, efficiency and sustainability of processes. Although it poses certain challenges, its correct application and monitoring can significantly improve water treatment processes, ensuring a safe and reliable supply for the population. As we move towards a more technological and environmentally conscious future, the redox potential presents itself as an indispensable ally in the management of this vital resource.