pH Electrode Slope and Offset Calculator
Check a pH electrode from a two point buffer calibration. Calculate slope (as a percentage of the Nernst value) and offset, then convert a sample reading in mV to pH.
- Slope = (E1 − E2) ÷ (pH2 − pH1) = 57.86 mV/pH
- Nernst slope = 0.19842 × T(K) = 59.16 mV/pH → 97.8 %
- Sample pH = pH1 + (E1 − Es) ÷ slope = 5.88
Nernst Equation for pH
SNernst = 2.303 RT ÷ F = 0.19842 × T (mV/pH, T in K)
At 25 °C the theoretical slope is 59.16 mV/pH. Real electrodes typically show 95 to 102 % when new, falling as they age.
Worked example
Buffers pH 7.00 at −5 mV and pH 4.01 at +168 mV give a slope of 57.9 mV/pH (97.8 %) and an offset of −5 mV. A sample at +60 mV is pH 5.88.
Frequently Asked Questions
What is a good pH electrode slope?
Between about 95 and 105 % of the Nernst value. Below about 90 % the electrode should be replaced.
What is pH electrode offset?
The mV reading in pH 7 buffer, ideally 0 mV. Large offsets indicate a contaminated junction or aged reference.
Which buffers should I use?
Two buffers that bracket the expected sample pH, usually pH 7 plus pH 4 or pH 10.
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