ORP Sensor: Oxidation Reduction Potential: Water Quality Probe

Why would I want to measure ORP?

Oxidation reduction potential, or ORP, is a measure of how chemically oxidizing or reducing a body of water is.  In general, the ORP of a solution determines what chemical reactions will tend to occur, indirectly affecting the types of flora and fauna that will thrive in the water.  In well-oxygenated natural waters at equilibrium, dissolved oxygen (DO) dominates the ORP of the solution, minimizing the importance of measuring ORP.  Under conditions of low DO, however, ORP can be important in determining which forms of a chemical will be favored (i.e., nitrate-nitrite-ammonia, sulfide-sulfite-sulfate, ferric-ferrous, etc.).  In addition, ORP is a good indicator of the presence of industrial oxidizing chemicals such as chlorine used to disinfect drinking and wastewater.

The true oxidation-reduction potential of a solution is a thermodynamic property that can only be determined by calculation involving every species present in the water.  Practically speaking, ORP is defined as the potential that develops at an inert electrode when it comes into equilibrium with the solution.  Unfortunately, not all compounds react reversibly or rapidly with the electrode, and adsorption of compounds on the electrode surface result in differences between true and observed ORP readings.  This can also lead to very long electrode response times for highest accuracy.  Knowing the limitations of measuring and interpreting ORP readings, useful information can still be obtained from many natural waters.

How is ORP measured?

ORP is measured as the voltage drop across the platinum membrane of an ORP electrode.  A reference electrode is used to complete the voltage-measuring circuit.  Because platinum does not react with ions in the water, it won’t give or take any electrons from those ions. The potential (voltage) created by this refusal is what you’re actually measuring as ORP.

Solinst Eureka’s ORP sensor, as with most other ORP sensors, consists of a polished platinum electrode sealed in glass, and is consistent with Standard Methods 2580 A.  Unlike an Ag-AgCl reference electrode whose potential is determined by the amount of chloride in solution, platinum is a noble, non-reactive metal whose potential is determined by the potential of the solution.  The sensor is inherently linear, and fastest response times are achieved when the sensor surface is clean and freshly polished.  The practical operational range of an ORP sensor is not limited by the electrode itself, but by the physical characteristics of natural waters.

The ORP sensor must be used with Solinst Eureka’s pH reference electrode, which is described in detail in the section on pH.  A temperature sensor is not required, but may be useful to interpret ORP data if accuracy greater than ±20 mV is desired, since the reference electrode shifts slightly with temperature.  No other sensors require the presence of the ORP sensor.

Since ORP is operationally defined as the potential at a platinum electrode, only a simple additive correction factor is made to the raw millivolts reading during calibration.  This corrects for the fact that various reference electrodes can have different filling solutions that shift their base potential.  It also allows the user to correct the reading to whatever reference electrode convention is used by their organization (NHE, SCE, 3 M KCl, saturated KCl, etc.).  No temperature correction is made to the readings.

ORP sensors are calibrated in an ORP buffer containing both the oxidized and reduced forms of a compound that rapidly reaches thermodynamic equilibrium with platinum.  Ferric-ferrous salts are often used (Lights or Zobel’s Solution), or solutions of quinhydrone at various pH values.  Calibration in a single solution is usually sufficient, though checking the reading at an alternate ORP is recommended.