Ph system calibration orp system calibration – GF Signet 2724-2726 pH_ORP Electrodes User Manual
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Signet 2724-2726 and 2734-2736 Series pH and ORP Electrodes
Electrode slope is the
ratio of mV to pH units.
At 25 °C the theoretical
slope is 59.16 mV per pH.
• The mV output from the electrode is created by the interaction of the electrode and the fl uid.
• The electrode contains a gel that depletes over time, so the instrument must be readjusted periodically to
maintain system accuracy.
• The need for recalibration varies with each application, but the life of the electrode is usually consistent.
• Keep a maintenance log to establish a depletion trend in new systems.
• The mV calibration is a two-point procedure.
• Signet offers pH buffer solutions prepared specifi cally for this purpose.
• pH buffer solutions can be used for calibrating more than one sensor within a day provided that the solutions
are protected from debris and are not diluted by rinse water from the calibration procedure.
• Use clean water to rinse buffer solutions from the electrode.
• Dispose of all buffer solutions at the end of the day.
• If the pH sensor will not calibrate within acceptable limits, clean the electrode and recalibrate.
• If the calibration results remain outside of acceptable limits, the sensor is depleted and must be replaced.
• Follow the guidelines of local waste disposal regulations when discarding buffer solutions and spent electrodes.
• ORP measurements are relative values, and single-point adjustments are suffi cient for most applications.
• Calibration should be done using ORP test solutions such as Zobell's solution, Light's solution, or in pH buffers that have been
saturated with quinhydrone (Table 3). Quinhydrone is the oxidizer that is measured by the ORP electrode.
• Zobell's solution and Light's solution are not compatible with the AutoCal function in Signet pH instrumentation.
• A new ORP electrode measures the listed values ±15 mV.
• The ORP electrode is functional until the offset exceeds 50 mV.
• An electrode whose offset measures greater than 50 mV should be cleaned or replaced if necessary.
• ORP solutions made with quinhydrone are very unstable and may not read properly after being exposed to air for a prolonged time.
These solutions must be discarded after a few hours.
• Dispose of all calibration solutions in accordance with local, state and federal guidelines.
• Use clean water to rinse buffer solutions from the sensors.
Theoretical mV
Values @ 25 °C
pH
mV
2
+295.8
3
+236.64
4
+177.48
5
+118.32
6
+59.16
7
0
8
–59.16
9
–118.32
10
–177.48
11
–236.64
12
–295.8
pH System Calibration
ORP System Calibration
ORP electrodes do not incorporate a temperature sensor, so the only system calibration required is the electrochemical adjustment.
There are two functions in a pH electrode that require the system to be calibrated:
Electrochemical pH vs. mV Ratio
Temperature
Electrochemical ORP vs. mV Ratio
As the pH value moves away from neutral (7 pH) or the temperature moves away from 25 ºC,
the electrochemical output is affected.
*
Example: At pH 5 the mV output of the electrode is not affected if the temperature is at 25 °C.
**
The electrode output will be shifted by 0.06 pH units if the temperature is reduced to 15 °C.
pH error due to temperature changes in fl uid
°C
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
15
0.15
0.12
0.09
**0.06
0.03
0
0.03
0.06
0.09
0.12
0.15
25
0
0
0
* 0
0
0
0
0
0
0
0
35
0.15
0.12
0.09
0.06
0.03
0
0.03
0.06
0.09
0.12
0.15
45
0.3
0.24
0.18
0.12
0.06
0
0.06
0.12
0.18
0.24
0.3
55
0.45
0.36
0.27
0.18
0.09
0
0.09
0.18
0.27
0.36
0.45
• The temperature output of the electrode (from a 3K Balco or PT1000 RTD) must be calibrated only once.
When a new electrode is installed, it does not need to be repeated.
• Because the temperature measurement has a signifi cant infl uence on the electrochemical measurement,
the temperature output in new pH electrodes should always be calibrated before the pH/mV calibration.
NOTE:
All Signet transmitters and controllers incorporate automatic temperature compensation.
Calibration
Zobell's
solution
Light's
solution
4 pH buffer
w/quinhydrone*
7 pH buffer
w/quinhydrone*
ORP at 20 °C
268 mV
92 mV
ORP at 25 °C
228 mV
469 mV
263 mV
86 mV
ORP at 30 °C
258 mV
79 mV
*Saturate 50 mL of pH 4 or
pH 7 buffer with 1/8 g quinhydrone
Table 3:
ORP test solutions
Table 1
Table 2