Water Measurements Explained: ORP vs pH


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Clean water is an ever-growing concern from tap water to manufacturing to natural waterways. We can’t live without water, yet understanding the characteristics of clean versus unhealthy water flow swiftly past us in common day-to-day activities. 

Two parameters of water quality that are most well known are pH and oxidation-reduction potential (ORP). These two measurements are insightful on their own, but even more powerful together to determine overall water quality in any application from hydroponics to environmental sampling to brewing a fine imperial stout. 

Other water measurements like electrical conductivity, dissolved oxygen, are important as well; but for this article, we will focus on the comparison between ORP and pH. In this way, one can apply this knowledge (and hopefully these measurements!) to improve and monitor water quality for the backyard pool or a water treatment facility. Everyone deserves high water quality so let’s take matters into our own hands and discuss ORP versus pH.

For more information on other water quality characteristics, like pH, chlorine, dissolved oxygen, conductivity, visit the Atlas Scientific blog.

What is pH in water?

pH is a concentration measurement. pH probe kits measure the concentration of hydrogen ions in a water-based solution, and the output value defines the words acid and base. This pH scale ranges from 0 to 14, packed with insight about the nature of the test sample. 

Most commonly, pH and water are discussed together since pure water is the poster boy with a neutral pH of 7 (middle-point on the scale). As the hydrogen ion concentration increases, an acid is produced and yields pH values below 7—vice versa for bases with pH values higher than 7 on the pH scale. 

For drinking water, healthy values of pH span from 6.5 to 8.5 according to the Environmental Protection Agency (EPA) on municipal drinking water. Water quality can be affected by many factors like pollution. Additionally, water outside these pH ranges can produce unwanted health effects such as skin irritation in households or animal poisoning in natural waterways, depending on how severe the pH becomes. 

That said, it’s best to understand and measure pH to prevent any unwanted side effects from compounding over time. A variety of different types of pH probes exist, but they are all after the same hydrogen ion concentration. The differences between the probes will relate to the intensity of the measurement environment and the accuracy of measurement required. 

What is oxidation-reduction potential (ORP) in water?

ORP is the electron exchange potential that relates to active oxidant or reductant, primarily oxidant in our discussion around high water quality. Oxidizing agents steal electrons from other molecules and reducing agents give up electrons to other molecules. 

The oxidation and reduction reactions, or redox reactions, can become complicated very quickly; so just remember that oxidizing agents are the substances that provide the sanitation value and contribute to higher ORP values.  

Substances like dissolved oxygen and chlorine will contribute to higher oxidation and higher ORP; these substances have higher electronegativity (oxidizing agents) and will normally pull electrons from other substances like unwanted contaminants and other bacteria. 

ORP is susceptible to other active ions and total dissolved solids (TDS) that do not contribute to sanitation, and in this way, ORP is not the cure for all water quality measurements. However, it provides a great signal of sanitation, water quality, and therefore creates warning signs to investigate drops in water quality.

The ORP characteristic is measured in millivolts (mV) by an ORP sensor kit, with higher readings correlating to a more sanitary water system. ORP readings in the positive millivolt range will correlate to an oxidative water state or a more sanitary state. 

So, negative readings (reductive state) will not be as common in the clean water quest. Realistically you should only encounter negative ORP readings in a laboratory setting, utilizing a lab-grade sensor with high range and accuracy for extreme oxidizers and reducers.

Overall, ORP will give you a great understanding of the general water quality, with positive millivolt readings in the 650 mV to 750 mV range being ideal for water sanitation. The best ORP value for drinking water is at least 650 mV to ensure adequate sanitation. 

This ORP value depends on both naturally occurring and external chemicals used to balance water quality. Depending on the scenario and the goal ORP value, corrective actions will change because a natural stream will not require the level of sanitation that a backyard swimming pool does. In this way, understanding and measuring ORP levels is very beneficial to identifying water quality for any application.

ORP vs pH: What is the difference?

Both pH and ORP probes utilize a similar combination sensor technology where they compare a reference solution to the actual sample. pH sensors measure the hydrogen ion concentration and ORP probes instead measure the electron potential in the water. pH is a concentration measurement and ORP is not. Two atomic-scale measurements, but indicate slightly different information. 

ORP probes will produce values in mV whereas pH yields unitless values on the pH scale from 0 to 14. However, pH probes record voltage values and convert those to pH values based on the reference solution. Overall, both ORP and pH probes use small electrical signals to achieve output measurements.

ORP measurements, as discussed above, relay information about the overall sanitation level of the water, with 650 mV to 750 mV being ideal. Since pure water is inherently a pH of 7, water applications tend to stick closely to neutral 7 for healthy conditions. Therefore, measuring pH is very important to water quality by understanding when values fall outside the neutral level (6.5 to 8.5 for drinking water).

How do oxidation-reduction potential (ORP) and pH interact in water?

pH and ORP are two separate water quality measurements that are somewhat inversely related (in terms of high and low on the scales); they are loosely related based on the chemicals and water content that affects both of them. For example, the chlorine will increase ORP levels, but if pH is too high (basic) it will neutralize the active chlorine and ORP will decrease. Since chlorine is a large contributor to high ORP, the loss of chlorine will greatly reduce ORP levels in a swimming pool and wastewater treatment environment where chlorine is the main contributor to high ORP and sanitation.

On the other hand, when various acids like muriatic acid are added to water, pH will decrease (acids are below 7 on the pH scale) but ORP will increase since this will be a strong oxidizing agent contributing to sanitation and high ORP levels.

In several instances, ORP will go up as pH decreases and vice versa. This is great to note, but overall the best practice is to stay within a neutral pH level. Neutral pH is best for water quality and allows adequate ORP levels for sanitation that create high water quality. 

Lastly, if you’d like to measure this relationship instead of memorizing it, check out Atlas Scientific’s industrial pH/ORP/temperature probe to conduct three measurements with one probe—and our two favorite measurements pH and ORP. 


pH is an important water measurement since pure water is a neutral pH of 7, and water applications should strive for this 6.5 to 8.5 pH range to maintain healthy conditions for all applications. Once pH falls beyond this range it can be detrimental to life and difficult to adjust.

Oxidation-reduction potential (ORP) is an equally important measurement to determine the high or low water quality of any application. These probes provide insight into the overall sanitation level (positive ORP readings in the 650-750 mV range are ideal for sanitation) and safe consumption. 

Both pH and ORP probes come in different grades from lab-grade ORP to industrial-grade pH, depending on the level of accuracy and intensity of the measurement environment. Choosing which measurement is right for you will come down to your application intensity and frequency of measurement. For standard measurements every so often, use the consumer-grade probes; for continuous monitoring in more extreme conditions, definitely choose the industrial-grade probe, and so on.

If you are unsure exactly which ORP or pH device will best suit your needs, or you would like to learn more about other water quality measurements, do not hesitate to reach out to the world-class team at Atlas Scientific.

pH Probes

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