Campbell Scientific CSIM11 pH and ORP Sensors - Manual (Page 32)

Appendix C. Detailed Calibration Procedure and Manufacturer Tips
7. Remove and rinse the electrode, then place it in the 10.00 buffer.
8. In the datalogger real time monitor mode, the electrode should read
10.00 pH, ±0.2, depending on temperature.
9. If the readings in Steps 6 and 8 show a slope error (error proportional to
the difference between the buffer pH and 7), then the probe has a slight
slope error, which should be easy to correct by adjusting the multiplier in
VoltDiff() or Volt (Diff) (P2).
C.2 Tips and Techniques
Stirring the buffers and samples improves the stability and speed of response of
the measurement.
Rinse the electrode with distilled water between samples and lightly blot the
water on a paper towel before immersing it in the next sample. Never wipe the
pH bulb since dust may scratch the delicate gel layer impairing response.
Rinsing the electrode with a small amount of the sample before immersing it
will eliminate any contamination of the sample.
The simplest form of temperature compensation compensates for the change in
the electrode’s slope due to temperature, in accordance with the Nernst factor.
It does not compensate for changes in the actual pH of the sample that occurs
with a change in temperature. The pH of a sample at 25 °C is most likely
different than the pH of that same sample at 75 °C.
The temperature of the electrode, the sample, and the calibration buffers should
be the same. Allow the electrode to come to temperature equilibrium with the
sample before recording the measurement value. Measurements made more
than 20 °C from the calibration temperature should include a one-point
calibration at that temperature for maximum accuracy.
Low ionic strength samples, highly viscous samples, and salt brines slow the
speed of response of the electrode. While the electrode will be stable to a
change in pH buffers after 10 to 15 seconds, it may take up to five minutes for
the electrode to stabilize in a difficult sample.
C-2