Why Do I Always Have to Calibrate in pH 7.00 and Then at Least One Other Buffer?

pH is considered an inferred value. This means that raw data (in this case potential as mV and temperature) are used to calculate pH with a known algorithm called the Nernst Equation.

It's important to quantify the following two correction factors before calculating pH:

  • Offset takes into account variances in the observed potential as a result of the electrode condition, as well as any manufacturing tolerances of the pH-sensitive glass bulb or the junction. This value is the potential observed using pH 7.00 calibration buffer. With an ideal electrode at 25°C, the offset determined in pH 7.00 buffer should be 0 mV. However, this may never actually be observed. An acceptable range for the offset is ± 30 mV.
  • Slope takes into account variances in the quality of the calibration buffers used. This value is represented by the change in potential over a change in pH (mV/pH). When measuring calibration buffers at 25°C with an ideal electrode, the theoretical slope is -59.16 mV/pH. The HOBO MX2501 data logger takes into account the effect of temperature and reports slope as a percentage of the ideal slope. An acceptable range for the slope is 85-100%.
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Measurement and accuracy