ORP is typically measured to determine the oxidizing or reducing potential of a water sample. It indicates possible contamination, especially by industrial wastewater. ORP can be valuable if the user knows that one component of the sample is primarily responsible for the observed value. For example, excess chlorine in wastewater effluent will result in a large positive ORP value and the presence of hydrogen sulfide will result in a large negative ORP value.

ORP is determined by measuring the potential of a chemically-inert (platinum) electrode which is immersed in the solution. The sensing electrode potential is read relative to the reference electrode of the pH probe and the value is presented in millivolts (mV).

The determination of ORP is generally significant in water which contains a relatively high concentration of a redox-active species, e.g., the salts of many metals (Fe2+, Fe3+) and strong oxidizing (chlorine) and reducing (sulfite ion) agents. Thus, ORP can sometimes be utilized to track the metallic pollution in groundwater or surface water or to determine the chlorine content of wastewater effluent. However, ORP is a nonspecific measurement, i.e., the measured potential is reflective of a combination of the effects of all the dissolved species in the medium. Because of this factor, the measurement of ORP in relatively clean environmental water (ground, surface, estuarine, and marine) has only limited value unless a predominant redox-active species is known to be present.

The value of ORP in determining the content of environmental water is greatly enhanced if the user has some knowledge or history of the site. ORP data can typically become more useful if used as an indicator over time and/or with other common parameters to help develop a complete picture of the water quality being tested.