Oxidation Reduction Potential (ORP)

What is ORP?

Oxidation Reduction Potential (ORP or Redox Potential) measures an aqueous system’s capacity to either release or accept electrons from chemical reactions. When a system tends to accept electrons, it is an oxidizing system. When it tends to release electrons, it is a reducing system. A system’s reduction potential may change upon introduction of a new species or when the concentration of an existing species changes.

ORP values are used much like pH values to determine water quality. Just as pH values indicate a system’s relative state for receiving or donating hydrogen ions, ORP values characterize a system’s relative state for gaining or losing electrons. ORP values are affected by all oxidizing and reducing agents, not just acids and bases that influence pH measurement.

How is it used?

From a water treatment perspective, ORP measurements are often used to control disinfection with chlorine or chlorine dioxide in cooling towers, swimming pools, potable water supplies, and other water treatment applications. For example, studies have shown that the life span of bacteria in water is strongly dependent on the ORP value. In wastewater, ORP measurement is used frequently to control treatment processes that employ biological treatment solutions for removing contaminants.

Hach's ORP Product Offering

Hach offers Meters, Probes, Kits, Testers, and Process Sensors to measure ORP. See below for more details.


Hach probes for ORP are designed to work with Hach Meters.

IntelliCAL™ ORP Gel-Filled Probe – Hach

HQd ORP Product Line

Probe options include rugged, refillable, and standard. These probes are used for Drinking Water, Wastewater, Laboratory, Industrial, Environmental Monitoring, and Rugged Field applications.

Sension+ Meters and Probes from Hach

Sension+ ORP Product Line

Probe options inlclude portable, benchtop refillable, and benchtop low-maintenance. This line of products is used for Drinking Water, Wastewater, Laboratory, Industrial, Environmental Monitoring, and Beverage applications.

ORP110-GS Standard ORP Probe

H-Series ORP Probe

The H-series standard ORP probe is used for Food Processing, Beverage, Academic Teaching, Pharmaceutical, and Industrial applications.



Meters that work with ORP probes can be found under their respective product lines above

Hach MP-6 Portable Meter

Probeless MP Meters

Hach's MP meters are designed to work without probes. For ORP, select from the MP-6 Portable Meter (pH, ORP, Conductivity, Resistivity & TDS) or the MP-6p Portable Meter (pH, ORP, Conductivity, Salinity & TDS). Use these meters for clean water applications. Great for pool and spa monitoring, water treatment professionals, and other spot testing applications.



Quick, reliable ORP testers for a wide range of applications.

Hach ORP Testr® 10

ORP Testr® 10

Ideal for water treatment applications including ozone systems, free chlorine activity, drinking water and water pollution.

Hach Pocket Pal™ ORP Tester

ORP Pocket Pal™ Tester

Offers an accurate and affordable way of testing for redox potential, with a range of ±999 mV.


Process pH/ORP Sensors

Hach's process ORP sensors plug-n-play with Hach' Digital sc Controllers.

Regenerateable Differential pH and ORP Sensor Controller

Analog and Digital Differential ORP Sensors 

The Differential Electrode Measurement Technique uses three electrodes instead of the two used in conventional sensors to increase measurement accuracy and eliminate sensor ground loops.  Available in convertible (PEEK® or Ryton®),insertion, and sanitary body styles.  Lower maintenance with replaceable salt bridge.

Hach Online Process ORP Sensor - General Purpose Analog ORP Sensor

1-1/2" Differential pHD/ORP Sensors

These field-proven differential electrodes area available in LCP and Ryton ® body materials.  They provide greater reliability and reduced maintenance

Digital 3/4 Inch Combination pH Sensor, Convertible Sensor Style, Ryton Body Material, General Purpose Glass pH Electrode, Temperature Compensation - Pt 1000 ohm RTD

Digital ORP Combination Sensors

Digital combination pH and ORP sensors are available in convertible, insertion, and sanitary mounting styles. Choose from rugged dome electrodes or “easy-to-clean” flat glass electrodes.

Application Articles for Process Probe

PDF Applying oxidation reduction potential sensors in biological nutrient removal systems

PDF Advanced Online Instrumentation Helps Facility Meet BNR Goals

More Information