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Oxidation Reduction Potential (ORP)


“Oxidation–reduction potential (Eh) is a measure of the ability of chemical/biochemical systems to oxidize (lose electrons) or reduce (gain electrons). A positive value indicates an oxidized state, whereas a negative value indicates a reduced state.”

To provide examples, a slight oxidised state would be aerated water, a high oxidised state would be chlorinated pool water and a reduced state would be an environment where anaerobic microbes are active, such as a natural lake.

ORP is measured in millivolts (mV) with the value obtained by two electrodes usually contained in a single unit (sensor). The first (indicator electrode) is typically made of an inert metal such as platinum or gold, where the second (reference electrode) is typically made of silver & silver chloride or mercury & mercury chloride.
Electrons from the water interact with both electrodes, creating a voltage between them and a value is displayed on the main controller.

The response of an ORP sensor can degrade over time and as such requires the probe being tested for accuracy using what is called a standard solution. That is a solution preprepared in a laboratory with a known value at a given temperature. This same standard solution can be used to adjust the measured reading to match the value of the standard. This is known as a calibration of the electrodes (sensor).

ORP is used in the commercial swimming pool industry as a measure of the effectiveness of the sanitiser, which in most cases is chlorine, measured as ‘Free chlorine’.

However, there are two forms of Free chlorine (FCL); the strong sanitiser (Hypochlorous Acid, HOCL) and weak sanitiser (Hypochlorite, Ion OCL-), where the percentage of each in solution is determined by the pH of that solution. The lower the pH value the higher the percentage of HOCL. At a pH of 7.5 the two forms are equal at 50% each in solution.

Because HOCL has no molecular charge and also has a relatively low molecular weight it is better than OCL- in penetrating cell walls of pathogens, so as such is a more effective form of sanitiser. Also, as OCL- has a negative molecular charge, as do cell walls of a pathogen, the two are not attracted, therefore OCL- has little to no effect on the pathogen.

HOCL typically enters the cell wall of a pathogen via osmosis, disrupts the DNA and kills the pathogen. Having a relatively neutral (7.4) pH solution is the trick to allowing the HOCL to effectively operate this way as the pathogen takes solution (containing HOCL) into its cell, as per normal, as if it was just water (H2O) alone, and unaware of its own death warrant it signed in the process.

So why am I now blabbering on about forms of FCL when this article is about ORP? Well, FCL is a measure of the level of (both HOCL and OCL-) sanitiser in a given solution, which means you could have an acceptable level of FCL according to Health Regulations, but a high percentage of it could in fact be OCL- and as such a poor defence against pathogens!

Whereas ORP is a measure of how effective that sanitiser is and a more reliable measure to have in addition to FCL. “ORP ≥650 millivolts (mV), a value defined in the literature as adequate to kill viral and bacterial pathogens within seconds.”

A published scientific research study in the USA found that despite 132 pools and 30 spas were abiding by local Health Regulations (code). The findings were: “Compliance with the Minnesota Pool Code did not assure an ORP≥650 mV (p<0.01). Outdoor pools had significantly lower ORP values than indoor pools (p<0.001). ANCOVA and logistic regression models showed that ORP decreased with increasing cyanuric acid, increasing pH, and decreasing free chlorine.”

The NSW Health Department made changes to the Public Health Regulations 2022 in Part 3: Public swimming pools and spa pools, which includes; “Removal of ORP (Oxidation Reduction Potential) as a method of measuring disinfection effectiveness.” This change took effect in September 2022.
Whilst it’s unclear exactly why they have removed this from the new Public Health Regulation 2022, we can only have an educated guess that it is due to the extensive use of inferior quality ORP system in the commercial space that were not designed for that application and as such should not be relied on to ensure the health of the water for the public.


T. Bastain, et al. Do Traditional Measures of Water Quality in Swimming Pools and Spas Correspond with Beneficial Oxidation Reduction Potential? National Library of Medicine. 2009.

Apera Instruments. How to test ORP. 2018.

T.M. Cogan, Microbiology of Cheese; Encyclopedia of Dairy Science (Second edition), 2011.

NSW Health Department. Public Health Regulations 2022-Key changes. September 2022.

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