Myron L 3P - Manual (Page 13)

Like acidity and alkalinity, the increase of one is at the expense of the
other, so a single voltage is called the Oxidation-Reduction Potential,
with a positive voltage showing a solution wants to steal electrons
(oxidizing agent). Chlorinated water will show a positive ORP value, for
instance.
2. ORP Units
ORP is measured in millivolts, 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 does indicate
concentration. Also, as with pH, a very dilute solution will take time to
accumulate a readable charge.
3. The ORP Sensor
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 a reference
junction - same as the pH sensor uses.
4. The Myron L ORP Sensor
Figure 10 pg. 21 shows the platinum button in a glass sleeve. The same
reference is used for both the pH and the ORP sensors. Both pH and
ORP read out 0 for a neutral solution. Calibration at zero compensates for
error in the reference junction.
A zero calibration solution for ORP is not practical, so the Ultrameter uses
the offset value determined during calibration to 7 in pH calibration (pH 7
= 0 mV). Sensitivity of the ORP surface is fixed, so there is no gain
adjustment either.
5. Sources of Error
The basics are presented in Cleaning pH/ORP Sensors, pg. 16 because
sources of error are much the same as for pH. The junction side is the
same, and though the platinum surface will not break like the glass pH
surface, its protective glass sleeve can be broken. A surface film will slow
the response time and diminish sensitivity. It can be cleaned off with
detergent or acid, as with the pH glass.
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XIV. GLOSSARY
Logarithm - An arithmetic function. pH Units, pg. 20.
ORP - Oxidation-Reduction Potential or REDOX, See ORP/
Oxidation-Reduction Potential/REDOX, pg. 21.
For details on specific areas of interest refer to Table of Contents.
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