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MODEL 399VP pH/ORP SENSOR SECTION 4.0
START-UP AND CALIBRATION
4.3 ORP CALIBRATION
Most industrial applications have a number of ORP reactions occurring in sequence or simultaneously. There can
be several components that are oxidized or reduced by the reagents that are used. Theoretically, the ORP poten-
tial is absolute because it is the result of the oxidation-reduction equilibrium. However, the actual measured poten-
tial is dependent on many factors, including the condition of the surface of the ORP platinum electrode. Therefore,
the sensor should be allowed 1-2 hours to become "conditioned" to the stream when first set up or after being
cleaned.
CAUTION
The solution used during the following check is an acid and should be handled with care. Follow the direc-
tions of the acid manufacturer. Wear the proper protective equipment. Do not let the solution come in con-
tact with skin or clothing. If contact with skin is made, immediately rinse with clean water.
1. Make a temporary electrical connection between the sensor and the instrument.
2. Obtain an ORP standard solution (PN R508-16OZ). A standard solution can also be made quite simply by
adding a few crystals of quinhydrone to either pH 4 or pH 7 buffer. Quinhydrone is only slightly soluble there-
fore a few crystals will be required.
3. Immerse the sensor in the standard solution. Allow 1-2 minutes for the ORP sensor to stabilize.
4. Adjust the standardize control of the instrument to the solution value shown on the label of the PN R508-16OZ
standard solution or, if using a quinhydrone solution, in Table 4-1. The resulting potentials, measured with a
clean platinum electrode and saturated KCl/AgCl reference electrode, should be within ±20 millivolts of the
value. Solution temperature must be noted to ensure accurate interpretation of results. The ORP value of sat-
urated quinhydrone solution is not stable over long periods of time. Therefore, these standards should be
made fresh each time they are used.
5. Remove the sensor from the buffer, rinse, and install in the process.
pH 4 Solution pH 7 Solution
Temp
°°
C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80
TABLE 4-1. ORP of Saturated Quinhydrone Solution (In Millivolts)