Oxidation/Reduction Potential, also known as redox (REDuction-OXidation) potential, or simply redox, is a common, single-value measurement for water quality, often used with aquaculture, pools, hot tubs and drinking water. ORP also plays an important role in sewage wastewater treatment and food processing facilities. 

Follow the Electrons

ORP is a complicated measurement. In its simplest form, we typically think of redox reactions as the transfer of electrons, although a more accurate description would be a change in oxidation state. In more practical terms, think of ORP as the ability, or potential, of water to break down the contaminants it contains. The higher the ORP reading, the stronger the water’s ability to destroy contaminants such as pathogenic microorganisms, chemicals, disinfectants, etc. Digital meters measure ORP in positive and, in some special applications, negative readings, from -2,000mV (millivolts) to +2.000 mV. Unlike pH and EC-TDS, ORP measurements are not affected by water temperature. It is also important to note that changes in ORP levels are not instantaneous; however, the higher the ORP reading, the faster the contaminant breakdown occurs.


LOSS of negatively charged electrons resulting in an increase in oxidation state.


GAIN of negatively charged electrons resulting in a decrease in oxidation state.

Why Does ORP Matter?

There is a strong correlation between bacterial activity and ORP levels. Therefore, ORP readings are good indicators of disinfection levels, and when targeted levels are achieved, can assure reliable disinfection. For instance, chlorine, bromine, and ozone are oxidizers and make good water sanitizers because they “steal” electrons (which increases the ORP level), thereby altering the chemical makeup of unwanted bacteria, algae and organic material and causing them to die.

How Does an ORP Meter Work? 

An ORP meter is actually a millivolt meter, measuring tiny voltages that are generated when its probe is placed in water containing oxidizing and reducing agents. The meter’s probe houses two electrodes; one for measuring (usually made of platinum) and one for reference, made of silver wire. The voltage measured across the circuit formed by the two metal electrodes has a direct relationship with the potential of the oxidizer or reducer to perform a chemical task (disinfection).

Negative ORP Readings?

Readings below zero are utilized for some health/medical quality waters, and you’re probably asking, “What happened to the higher, the better?” Fair question. Negatively charged water is brought to this “unnatural” condition by special equipment, and the process begins with water that has already been purified. To take negative ORP readings, the probe requires special conditioning - an oxidized coating must be formed on the diode. For more details about conditioning and storage, see “Negative Probe Preparation” below.

Preparation and Conditioning of Probes for Negative ORP Readings

ORP meters will read positive ORP solution right out of the box with little or no preparation. These meters are used in food preparation, wastewater treatment facilities, water quality testing for industrial applications and many other uses where only a positive (+mv) ORP is expected and needed.

When a negative (-mv) ORP reading is desired this usually applies to a medical water quality environment created by machine. The Super negative readings generated by these machines are below -50 and can go as low as negative 1000mv. To effectively read a negative ORP state of water the platinum probe used in most ORP meters must be in an unnatural condition, which is oxidized. 

Platinum does not oxidize easily. This is the reason platinum is used in jewelry and many types of industrial electronic applications. Originally, this oxidation process was accomplished by letting the probe sit in very acidic water (2.5 pH or lower) for 5 to 7 days however, this proved to be an inconsistent method to oxidize a probe. In many cases the probes did not get a consistent and complete coding of oxidation and this contributed to inconsistent readings. This problem has been corrected by the use of a conditioning and oxidation kit Milwaukee Instruments model number MA9025.

This kit has two 2oz bottles solutions and requires about 45 minutes to perform the oxidation procedure. This procedure is covered in detail in the instructions that come with the kit. After oxidation of the probe it is mandatory that the probe be put in white distilled vinegar and stored in this very acidic liquid all the time to retain the oxidation coating on the probe. Never let the probe dry out or set for an extended time in solution that is above 2.5pH. In addition, it will be necessary to re-oxidize probe about every 30 days so you want to retain the liquid in the kit to be reused. The oxidation of the probe does not affect the performance of the probe for reading positive ORP solutions in any way.

All ORP probes used in this manner will need to be replaced about every 10 to 12 months due to the corrosion of the platinum on the probe diode.