Temperature Calibration - GF Signet 276 Differential Series Operating Instructions Manual

pH System Calibration
There are two functions in a pH electrode that require the system to be calibrated:

Temperature Calibration

• The temperature output of the electrode (measured by a 3KΩ 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.
°C pH 2
15 0.15
25
35 0.15
45
55
0.45
Table 1
As the pH value moves away from neutral (7 pH) or the temperature moves away from 25 ºC, the electrochemical output is aff ected.
* Example: At pH 5 the mV output of the electrode is not aff ected 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.
Off set and Slope Calibration
The pH electrode calibration is an important function that must be done routinely to standardize the electrode due to its ever-changing
characteristics. The electrode contains a gel that depleates over time, so the instrument must be readjusted periodically to maintain
system accuracy. There are many infl uences, including aging, temperature, coatings and chemicals used that will aff ect the probe
characteristics. After cleaning and conditioning, Signet pH electrodes may be calibrated in two pH buff er solutions of diff erent pH
values (a two point calibration).
The pH electrode is calibrated by placing it in a buff er solution of known pH and measuring the cell potential.
This is a linear function of pH in the range of pH 2-11, therefore two calibration points are needed. Most
common calibration buff ers are pH 4.01 and pH 7.00 (@ 25 °C). If the pH measurements are made in an acid
to neutral range, we recommend using buff er pH 4.01 and buff er pH 7.00 for calibration. If an alkaline sample
is measured, we recommend using buff er 7.00 and buff er 10.01 for calibration.
The response of the glass electrode is characterized by two parameters, off set and slope. The theoretical
output of a pH electrode at pH 7 is 0 mV. The off set, or zero pH is the deviation of the pH from the nominal
value. In the Signet transmitters, the off set calibration is performed under STANDARD.
The glass electrode slope is the mV developed per each pH unit (mV/pH). At 25 °C, the theoretical slope is
59.16 mV/pH. So, at pH 4.01, +177 mV will be generated, while at pH 10.01 (with a lower H+ concentration),
a potential of -177 mV will be generated, (see Table 2). A new electrode however, will generate 0 15 mV
in pH 7.00 and will have a slope effi ciency between 93.2 and 103%. In the Signet transmitters, manual slope
calibration is performed under SLOPE.
pH error due to temperature changes in fl uid
pH 3 pH 4 pH 5
0.12 0.09 **0.06
0 0 0 * 0
0.12 0.09 0.06
0.3 0.24 0.18 0.12
0.36 0.27 0.18
pH 6 pH 7 pH 8 pH 9
0.03 0 0.03 0.06
0 0 0 0
0.03 0 0.03 0.06
0.06 0 0.06 0.12
0.09 0 0.09 0.18
Signet 275X, 276X, 2777X Series pH and ORP Electrodes
pH 10 pH 11 pH 12
0.09 0.12 0.15
0 0 0
0.09 0.12 0.15
0.18 0.24 0.3
0.27 0.36 0.45
Theoretical mV
Values @ 25 °C
pH
2
3 +236.64
4 +177.48
5 +118.32
6
7
8
9 –118.32
10 –177.48
11 –236.64
12
mV
+295.8
+59.16
0
–59.16
–295.8
Table 2
7