Understanding Oxidation-Reduction Potential (ORP)
ORP, or Oxidation-Reduction Potential, is a measurement expressed in millivolts (mV) that indicates a water sample's ability to either donate or accept electrons during a chemical reaction. A positive ORP value signifies an oxidizing environment, meaning the water tends to accept electrons. Conversely, a negative ORP value indicates a reducing environment, meaning the water tends to donate electrons.
In the context of drinking water, a higher positive ORP value is often associated with better sanitation and disinfection. This is because disinfectants like chlorine are strong oxidizing agents, and their presence raises the ORP, effectively neutralizing pathogens and organic material. ORP is a quick and reliable indicator of overall water quality and the effectiveness of sanitation, especially in industrial or municipal applications.
How Purification Affects ORP
The process of water purification fundamentally alters the water's chemical makeup by removing dissolved solids and other impurities. This has a direct and significant impact on its ORP.
Purification's Impact on Redox Balance
Purified water, especially highly purified forms like distilled or reverse osmosis water, has had most or all of its dissolved solids removed. The removal of these ionic species, which act as both oxidizing and reducing agents, leaves the water with a significantly altered redox balance. The result is an ORP that is much lower and highly unstable, as it becomes extremely sensitive to even minor environmental changes, such as the introduction of air or slight contamination.
The Case of Reverse Osmosis (RO)
Reverse osmosis systems force water through a semi-permeable membrane to remove total dissolved solids (TDS). Since this includes minerals, salts, and often residual chlorine, the resulting RO water typically has a very low ORP, often close to 0 mV or even slightly negative. For this reason, ORP is a critical monitoring parameter in RO systems, as a low ORP reading can indicate biological activity or an overfeed of dechlorination chemicals.
The Case of Distilled Water
Distillation involves boiling water and condensing the steam back into a liquid, removing almost all dissolved minerals and contaminants. The ORP of freshly distilled water is essentially neutral, but it is extremely unstable. As soon as it is exposed to air, it begins to absorb atmospheric carbon dioxide, forming carbonic acid. This process can cause the ORP to fluctuate and can introduce some level of conductivity, even though the water remains highly pure.
Other Purification Methods
Filtration methods using activated carbon may also alter ORP by removing chlorine and other oxidizing agents, leading to a lower reading. Water softeners, which use ion exchange, do not typically remove disinfectants and therefore may not dramatically change ORP unless other chemical changes occur.
ORP Values: Comparing Purified vs. Tap Water
To better understand the effect of purification, consider the typical ORP values across different types of water:
| Water Type | Typical ORP Range (mV) | Key Characteristics Affecting ORP |
|---|---|---|
| Municipal Tap Water | +200 to +650 | Contains disinfectants like chlorine or ozone, which are strong oxidizing agents and raise the ORP significantly. |
| Standard Purified Water (RO) | -50 to +150 | Lacks minerals and disinfectants, resulting in a lower and less stable ORP. Value depends on post-treatment and exposure to air. |
| Distilled Water | Near 0, unstable | Contains almost no dissolved solids or gases initially, making the ORP highly sensitive to any atmospheric contact. |
| Alkaline Ionized Water | -100 to -750 | Produced through electrolysis, which creates water rich in dissolved hydrogen gas (H2), a reducing agent, along with a higher pH. |
Factors Influencing Purified Water's ORP
Several factors can influence the ORP reading of purified water, beyond the initial purification process:
- pH: The pH is one of the core factors that influences ORP, especially in the presence of disinfectants. An increase in pH generally leads to a decrease in ORP, as the efficacy of many oxidizers is pH-dependent.
- Dissolved Gases: The amount of dissolved oxygen and other gases in the water affects its redox state. Pure water, being less buffered, is more susceptible to these changes from atmospheric exposure.
- Temperature: ORP is temperature-dependent. As water temperature increases, the solubility of dissolved oxygen decreases, which typically lowers the ORP value.
- Measuring Equipment: The accuracy and stability of the ORP sensor itself are critical. Measurements in low ionic strength, pure water can be particularly challenging and prone to drift without proper equipment.
Health Claims and Scientific Facts
Marketing surrounding so-called "antioxidant water" often cites a negative ORP as proof of health benefits. While water with a negative ORP can contain dissolved hydrogen (H2), a known therapeutic antioxidant, the ORP reading itself is not a reliable measure of H2 concentration. The ORP value is a relative measurement and is disproportionately influenced by pH rather than the actual amount of H2. Furthermore, a negative ORP does not guarantee health benefits, as other factors and the actual antioxidant agent's concentration are more important. It is also important to note that major health organizations do not regulate ORP as a parameter for drinking water quality.
For a deeper understanding of why ORP should not be used as a proxy for hydrogen concentration, consulting authoritative sources like research papers is recommended. A valuable resource on this topic is the article by LeBaron et al. published in Frontiers in Food Science and Technology.
Conclusion
Purified water, whether produced through reverse osmosis or distillation, has a low and inherently unstable ORP. The removal of dissolved solids, including both oxidizing and reducing agents, leaves the water in a near-neutral redox state. This low ORP is a chemical consequence of the purification process, not a definitive indicator of health benefits or safety. While ORP is a useful tool for monitoring the effectiveness of purification and disinfection processes, especially in industrial settings, its value for consumer-level drinking water is often overstated. The health effects of water are better understood by measuring specific substances like dissolved hydrogen, not by relying on a single, relative ORP reading that is highly susceptible to other environmental factors like pH and temperature.
