Oxidation Reduction Potential (ORP)/Redox & Free Chlorine – Myron L Application Bulletin

Myron L Logo

Application Bulletin

WHAT IS ORP?
Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept electrons, reducers lose electrons. Examples of oxidizers are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine dioxide. Examples of reducers are sodium sulfite, sodium  bisulfate  and  hydrogen sulfide.  Like acidity  and  alkalinity, the  increase of one  is at  the expense of the other. A single voltage is called the Oxidation-Reduction Potential, where a positive voltage shows a solution attracting electrons (oxidizing agent). For instance, chlorinated water will show a positive ORP value whereas sodium   sulfite  (a  reducing agent) loses electrons and will show a negative ORP value. ORP is measured in millivolts (mV), with no correction for solution  temperature. Like pH,  it is not a measurement of concentration directly, but of activity level. In a solution of only one active component, ORP indicates concentration. As with pH, a very dilute solution will take time to accumulate a measurable charge. An ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. That charge is measured relative to the solution, so the solution “ground” voltage comes from the reference junction – the same type used by a pH sensor.
HISTORY OF ORP
ORP electrodes were first studied at Harvard University in 1936. These studies showed a strong correlation of ORP and bacterial activity. These tests were confirmed  by studies on drinking water  and swimming pools in other areas of the world. In 1971 ORP  (700  mV) was  adopted by the  World Health Organization (WHO) as a standard for drinking water. In 1982 the German Standards Agency adopted the ORP (750 mV) for public pools and in 1988 the National Swimming Pool Institute adopted ORP (650 mV) for public spas.
WHERE IS ORP USED?
As you can tell by the previous paragraphs, ORP is  used for  drinking  water,   swimming   pools   and spas. However,  ORP  is also  used for cooling tower disinfection, groundwater remediation, bleaching, cyanide destruction, chrome reductions, metal etching, fruit and vegetable disinfection and dechlorination. In test after test on poliovirus, E. coli, and other organisms,   a   direct   correlation   between   ORP and the rate of inactivation was determined. It is, therefore, possible to select an individual ORP value, expressed in millivolts, at which a predictable level of disinfection will be achieved and sustained regardless of variations in either oxidant demand or oxidant concentration. Thus, individual ORP targets, expressed in millivolts, can be determined for each application, which will result in completely reliable disinfection  of  pathogens,  oxidation  of  organics, etc. Any level of oxidation for any purpose can be related to a single ORP number which, if maintained, will provide utterly consistent results at the lowest possible dosage.* * Courtesy of US Filter/Stranco products.
WHY USE ORP?
ORP is a convenient measure of the oxidizer’s or reducer’s ability to perform  a  chemical task.  ORP is  not  only  valid  over  a  wide  pH  range, but  it is also  a  rugged  electrochemical  test,  which  can easily be accomplished using in-line and handheld instrumentation.  It is by far a more consistent and reliable measurement than say chlorine alone.
LIMITATIONS FOR ORP
As with all testing, ORP has certain limitations. The speed of response is directly related to the exchange current density which is derived from concentration, the oxidation reduction system, and the electrode. If the ORP of a sample is similar to the ORP of the electrode, the speed will be diminished. Carryover is also a possible problem when checking strong oxidizers or reducers, and rinsing well will help greatly. Although a better indicator of bactericidal activity, ORP cannot be used as a direct indicator of the residual of an  oxidizer  due  to the  effect  of pH and temperature on the reading. ORP can be correlated to a system by checking the oxidizer or reducer in a steady state system with a wet test, and measuring pH. If the  system stays within the  confines of this steady state parameter (usually maintained by in- line or continuous control), a good correlation can be made. The best recommendation for ORP is to use wet tests, and over three test periods correlate the ORP values to those test parameters.
FREE  CHLORINE  CONVERSION  USING ORP
The most ubiquitous and cost-effective sanitizing agent used in disinfection systems is chlorine. When chlorine is used as the sanitizer, free chlorine measurements are required to ensure residual levels high enough for ongoing bactericidal activity. Myron L digital handheld instrumentation accurately converts ORP measurements to free chlorine based on the understanding of the concentrations of the forms of free chlorine at a given pH and temperature. The conversion is accurate when chlorine is the only oxidizing/reducing agent in solution and pH is stable between 5 and 9. This pH range fits most applications because pH is usually maintained such that the most effective form of free chlorine, hypochlorous acid, exists in the greatest concentration with respect to other variables such as human tolerance.
MYRON L INSTRUMENTS
Myron L Company offers  a variety  of handheld instruments and in-line Monitor/controllers that may be used to measure, monitor and/or control ORP. The following table lists some of the model numbers for measuring, monitoring, and/or controlling ORP. The Ultrameter II™ 6Psi, PoolPro™ and D-6 Digital Dialysate Meter™ are multi-parameter handheld instruments with ORP and free chlorine measuring capabilities. These instruments also have the capability to measure conductivity, TDS, resistivity, pH, mineral/salt concentration and  temperature, making  them  the preferred instruments for all water treatment professionals. The 720 Series II Monitor/ controllers are an excellent choice for continuous in- line measurements. For additional information, contact your local distributor  or  the  Myron  L Company directly.

 

Test HANDHELD INSTRUMENTS MONITOR /CONTROLLERS
Model Display Range Model Display Range
ORP UM 6PIIsi Digital ±999 mV 726II Analog ± 2000 mV
PS6si Digital ±999 mV 727II Analog ± 2000 mV
D-6 Digital ±999 mV 728II Digital ± 1999 mV
729II Digital ± 1999 mV
FreeChlorine UM 6PIIsi Digital 0.20-9.99 ppm
PS6SI Digital 0.20-9.99 ppm
D-6 Digital 0.20-9.99 ppm