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2. Measure and record the pH of the process solution sample with another temperature compen-
sated, calibrated pH instrument. For best results, standardization should be performed at the
process temperature.
3. Adjust the analyzer/transmitter value to the standardized value.
4.2 396P and 396PVP ORP Sensors
4.2.1 Sensor preparation
Most industrial applications have a number of ORP reactions occurring in sequence or simultane-
ously. There can be several components that are oxidized or reduced by the reagents that are
used. Theoretically, the ORP potential is absolute because it is the result of the oxidation-reduc-
tion equilibrium. However, the actual measured potential 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.
4.2.2 ORP Calibration
1. Make a temporary electrical connection between the sensor and the instrument.
2. Obtain an ORP standard solution, or 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 therefore a few crystals will be required. (Refer to Section 4.3. for an alternate ORP
standard solution).
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 in Table 5-1
(below) or on the label of the standard solution. 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 saturated quinhydrone solution is not stable over long periods of
time. Therefore, these standards should be made up fresh each time they are used.
5. Remove the sensor from the buffer, rinse and install in the process.
36 Start-Up and Calibration
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)
Section 4: Start-Up & Calibration 396P + 396PVP Sensors Instruction Manual
August 2013 LIQ_MAN_396P_396PVP