44
A
dvanced
S
ensor
T
echnologies,
I
nc. U.S.A.
Website:
www.astisensor.com
IOTRON
TM
Trademark of ASTI
IOTRON
TM
pH / ORP / ISE / DO / Conductivity Measurement Products Lines
APPENDIX “A”
Temperature Considerations for Calibrating pH Sensors with pH Buffers – Part 1 of 2
Exact pH Values of the NIST Traceable pH buffers at Various Temperatures
Nominal pH Buffer Designation @ 25°C Shown in Gray at Top of Column
Temp °C
1.68
4.00
6.86
7.00
9.18
10.01
12.45
0
1.67
4.01
6.98
7.11
9.46
10.32
13.42
5
1.67
4.00
6.95
7.08
9.39
10.25
13.21
10
1.67
4.00
6.92
7.06
9.33
10.18
13.00
15
1.67
4.00
6.90
7.03
9.28
10.12
12.81
20
1.68
4.00
6.88
7.01
9.23
10.06
12.63
25
1.68
4.00
6.86
7.00
9.18
10.01
12.45
30
1.68
4.01
6.85
6.98
9.14
9.97
12.29
35
1.69
4.02
6.84
6.98
9.10
9.93
12.13
40
1.69
4.03
6.84
6.97
9.07
9.89
11.98
45
1.70
4.04
6.83
6.97
9.04
9.86
11.84
50
1.71
4.06
6.83
6.97
9.02
9.83
11.71
55
1.72
4.07
6.83
6.97
8.99
9.80
11.57
60
1.72
4.09
6.84
6.98
8.97
9.78
11.45
NIST traceable pH buffers are the most commonly used methods for calibration of pH sensors. On each pH buffer bottle
is written the exact pH value of the buffer at variety of temperature conditions. Listed above are exact pH values for the
most commonly used buffers between 0 and 60 °C. When using the ASTI HiQDT Touchscreen Controller for calibration
of your IOTRON™ series Smart Digital HiQDT type RS-485 MODBUS RTU pH sensors use the autobuffer calibration
mode if using the pH buffers detailed above. For any other pH buffers you will need to obtained the exact pH value for
the current temperature condition. This information is typically provided on the label of the pH buffer.
NOTE: ASTI HiQDT touchscreen controller software automatically corrects for temperature induced change to buffer
to compute the exact pH value of buffer automatically when calibrations are performed with autobuffer calibration
mode. Exact pH value of the buffer at the current temperature obtained from the connected HiQDT pH sensor is used
for calibration. This may differ from the nominal value of the buffer at the reference 25 degree Celsius condition.
To use any pH buffer besides 1.68, 4.00, 6.86, 7.00, 9.18, 10.01 or 12.45 you will need to account for the temperature
induced shift of the pH value for the buffer in both the Windows software as well as any other devices used to perform
calibrations of the HiQDT pH sensors. There are no reliable pH buffers below 1.69 and above 12.45 and so specialized
and custom calibration schemes needed to be used for these situations. Contact factory for assistance in such cases.
Inquire to the factory if you plan to measure consistently below pH=1.0 or above pH=13.0 for special assistance. As can
be seen from mere inspection the temperature dependence of high pH buffers is much more significant than for low pH
buffers. Similarly for process solutions with high pH the temperature induced pH dependence may be quite significant
and should be considered when trying to control such systems with fluctuating temperature. Process solutions with
relatively weak ionic strength (low conductivity) are also rather prone to higher temperature induced pH shifts whereas
process solution with relativley high ionic strength (high conductivity) are less prone to temperature induced pH shifts.