Yokogawa EXA PH202 2-Wire pH/ORP Analyzer - Manual (Page 107)

IM 12B0702-01E
11-10 Appendix
11-5. Calibration in ORP Mode
For an ORP meter, unlike the case for a pH meter, there is no predefined "standard solution" that
you must use for calibration. For an ORP meter, normal maintenance is limited to a check of the
electrodes. Electrode checks and calibration are performed in the following circumstances:
Electrode Check
* When you change the ORP sensor, or when you stop continuous operation.
* When you wash the sensor tip or liquid path of the ORP sensor.
Calibration
* When the electrode potential is out of limits.
* When you want two or more ORP meters to read as near as possible to the same value.
Note: There are two ways of calibrating the PH202 in ORP mode, but both are one-point calibration
(zero shift).
1. Manual calibration (MAN.CAL) and
2. Calibration with sample (SAMPLE).
If you use the FU20 combination pH and ORP sensor, then you can calibrate ORP using the “2nd
parameter” setting as per the section 6-5-3. Manual Calibration.
11-5-1 Electrode Check
To determine whether the ORP electrode is normal or not, a solution of known ORP is measured to
check if the measured value is within tolerance (see Fig. 11-3). This is done in measurement mode.
Electrode test solution
A solution of known ORP (Oxygen Reduction Potential) is used. Usually quinhydrone or ferric chloride
solution is used as the test solution.
Yokogawa sells the following chemicals for making test solution. The procedure for mixing solution is
described below.
Quinhydrone salts (P/N K9024EC) and Ferrous & Ferric salts (P/N K9024ED)
Procedure for Electrode Test
You determine whether or not the ORP sensor is normal by measuring the ORP of a test solution of
known ORP and determining if the measured value is within tolerance.
Before starting normal operation, when you need to check if the ORP sensor is normal or not, use the
procedure below:
A solution of known ORP is measured to check if the measured value is within tolerance.
Here we describe how to mix test solutions using the quinhydrone salts supplied with the OR8AX
accessory kit, or the ferrous salts that are sold separately by Yokogawa.
(1) Mix test solution
<To mix quinhydrone test solution>
Pour one packet of quinhydrone into a wide-mouth (250 ml) beaker and dissolve in pure water to
make 250 ml of solution. If the pure water is cold, the powder will not completely dissolve and some
may float on the surface, but this will not cause any problems in practice.
<To mix ferrous & ferric chloride test solution>
Pour one packet of the light purple or light green powder to a wide-mouth (250 ml) beaker and add
a 2 mol/l solution of sulfuric acid to make 250 ml of solution. If you want to start with concentrated
sulfuric acid, mix one packet of the powder with approximately 150 ml of pure water in a wide-mouth
beaker then, stirring vigorously, add 14 ml of concentrated sulfuric acid. Top up with pure water to
make 250 ml.
(2) Transfer about 50-100 ml of test solution to a clean 200 ml beaker.
(3) Remove the ORP sensor from its holder. If an immersion holder is used, first remove the holder then
remove the (sensor) protector from the holder.
(4) Wash off any of the measured solution adhering to the sensor using tap water, then wipe off any
moisture droplets adhering to the sensor.
(5) Immerse the sensor tip in the test liquid (see (2.) above), wait for the ORP reading to stabilize (this
typically takes 5 to 10 minutes).