46
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 “C”
HiQDT-pH “ Buffer Choices for Autocalibration
AVAILABLE pH BUFFERS FOR AUTOCALIBRATION MODE:
Asymmetric Potential (A.P):
7.00 or 6.86
Acid Slope:
4.00 of 1.68
Alkaline Slope:
10.00 or 9.18 or 12.45
CALIBRATION SCHEME # 1 – Typical for most installations in the USA
Asymmetric Potential (A.P):
7.00
Acid Slope:
4.00
Alkaline Slope:
10.00
This scheme is the most common pH buffer scheme for most customers in the USA. The 10.01 pH buffer must
be used carefully since it is more prone to shifting substantially more than the very stable 4.00 or even the 7.00
pH buffer. Intrusion of carbon dioxide into the 10.01 pH buffer from the atmosphere is the main culprit
creating an erroneous non-temperature induced shift in pH by exceeding the buffer capacity. Care should be
taken that the pH10 buffer is fresh to ensure relaible alkaline slope calibration results.
CALIBRATION SCHEME # 2 – Typical for most installations in Europe
Asymmetric Potential (A.P):
6.86
Acid Slope:
4.00
Alkaline Slope:
9.18
Typical values for most European pH installations are 4.00, 6.86 and 9.18 pH buffers. This is the best practice
pH buffer scheme for most pH measurements that do not commonly go much below pH 4.00 and or else much
above pH 9.20. The 6.86 & 9.18 pH buffers are most stable than the 7.00 & 10.01 pH buffer counterparts but are
still more prone to shifting then the very stable 4.00 pH buffer. Care should be taken that the pH 9.18 buffer is
fresh to ensure best alkaline slope calibration results
CALIBRATION SCHEME # 3 – For batch style installations where pH can vary quite considerably
Asymmetric Potential (A.P):
1.68
Acid Slope:
6.86
Alkaline Slope:
12.45
This pH buffer calibration scheme is typical for batch type process applications that often go below pH2 and
above pH12. The 1.69 and 6.86 pH buffers are quite stable but the 12.45 pH buffer shifts in value quite easily.
Great care should be taken when using the 12.45 buffer to ensure accurate results. In particular this buffer
should always be in code, well stored in a cool dry place and not exposed to light or air. Make sure that the
12.45 pH buffer is always fresh to ensure reliable alkaline slope calibrations results.