AP200 series
Rugged pH/Redox (ORP) sensor systems with rapid temperature response for critical processes
4 Calibration
IM/AP200 Rev. S
13
4 Calibration
4.1 Method
When the electrode system has been correctly connected and
all electrical connections made to the associated pH analyzer,
the system is ready for calibration by immersing the sensor
(using suitably sized beakers) either:
1. in a calibration solution (buffer) of known pH value for a
single-point calibration,
or
2. in two separate calibration solutions of known pH values
for a two-point calibration.
For sensors already in use:
1. Remove the electrode from the process or sample.
2. Wash the visible electrode surface with demineralized
water.
3. Proceed as described in the paragraph above.
To have agreement with a measured sample, there may be
times when a process calibration is necessary.
1. Perform a buffer calibration.
2. Ensure that the electrode is returned to the process for at
least 10 minutes before performing a process calibration.
3. To minimize solution temperature effects, measure the
sample at the same temperature as the process.
Refer to the instruction manual for the pH analyzer for full details
of the calibration procedures.
4.2 Buffer Solutions
Recommended buffer solutions are shown in Table 4.1.
Table 4.2 shows the change in pH value that occurs with a
change in the temperature of the recommended 4, 7, and 9 pH
buffer solutions.
4.3 Redox (ORP Sensor)
When the sensor has been correctly connected and all electrical
connections have been made to the associated Redox (ORP)
analyzer, it is ready for calibrating. Follow the calibration
procedure in the analyzer User Guide.
For sensors that are connected to analyzers that do not have
Redox (ORP) sensor calibration capabilities, it is possible to
check the response as follows:
1. Prepare standard 4 and 7 pH buffer solutions. Add one
gram (heaped spatula) of analar quinhydrone to 100 ml
(3.5 Flu oz.) of each buffer solution. Let them stand for 30
minutes.
2. Immerse the sensor in each solution in turn and note the
mV value when stable.
The values obtained should be within ±15 mV of the following:
Warning.
Close all isolating valves before removing an
electrode from a flow line.
Buffer Solution
Part No.
4pH 0.05M potassium hydrogen phthalate
0400 110
7pH Disodium hydrogen phosphate/monopotassium
dihydrogen phosphate mix
0400 120
9pH 0.05M borax
0400 130
Table 4.1 Recommended Buffer Solutions
Note.
Ensure that the visible surface of the electrode has
been cleaned using demineralized water.
When moving from one buffer solution to the next,
wash the electrode using demineralized water and dry
it carefully using a soft tissue.
°C
pH
0
10
20
25
30
40
50
60
70
80
90
4
4.00 4.00 4.00 4.01 4.01 4.03 4.05 4.08 4.12 4.16 4.21
7
7.11 7.06 7.01 7.00 6.98 6.97 6.97 6.97 6.99 7.03 7.08
9
9.48 9.35 9.23 9.18 9.13 9.05 8.98 8.93 8.90 8.88 8.84
Table 4.2 Buffer pH Value / Temperature (°C)
pH Buffer
mV
4
+259
7
+82
