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.
