Emerson 396RVP instruction manual ORP of Saturated Quinhydrone Solution

MODEL 396R pH/ORP	SECTION 5.0
	START UP AND CALIBRATION

5.3396R ORP CALIBRATION. An ORP loop is best calibrated using an ORP standard solution. Most indus- trial 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 potential is absolute because it is the result of the oxi- dation-reduction equilibrium. However, the actual measured potential is dependent on many factors, including the condition of the surface of the ORP plat- inum electrode. Therefore, the sensor should be allowed 1-2 hours to become “conditioned” to the stream when first set-up or after being cleaned.

5.3.1 ORP Calibration Procedure

1.Make a temporary electrical connection between the sensor and the instrument.

2.Obtain a ORP standard solution (PN R508-8oz) or one can 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, only 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 in Table 5-1. The result- ing potentials, measured with a clean platinum electrode and saturated KCl/AgCl reference elec- trode, should be within ±20 millivolts of the value shown in Table 5-1. Solution temperature must be noted to insure 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.

TABLE 5-1

ORP of Saturated Quinhydrone Solution

		pH 4			pH 7
TEMPERATURE °C	20	25	30	20	25	30
Millivolt Potential	268	264	260	94	87	80

31