What is a good pH electrode slope?

Close to 100 % of the Nernst value, and meters and makers commonly flag a slope that falls outside roughly 95 to 105 %. The limit that matters is the one in your procedure or the electrode's manual, which is why it is an input here.

What is pH electrode offset?

The millivolt reading the electrode gives at pH 7, ideally 0 mV. A large offset points to a contaminated or ageing reference junction, and many procedures set a limit of a few tens of millivolts.

Why is the theoretical slope 59.16 mV per pH?

It comes from the Nernst equation, ln(10)·R·T/F, evaluated at 25 °C. At 37 °C it is 61.5 mV/pH and at 0 °C 54.2 mV/pH, which is why the buffer temperature matters.

Usually an ageing or coated glass membrane, a clogged reference junction, or a poor buffer. If the segments disagree, with a good slope between two buffers and a poor one with the third, suspect that buffer rather than the electrode.

pH electrode slope and offset calculator

Enter the millivolt readings your pH electrode gives in two or three buffers and get the slope as a percentage of the ideal Nernst value, the offset at pH 7, and a check against your own limits.

The buffer readings

Buffer 1

Buffer pHElectrode readingmV

Buffer 2

Buffer pHElectrode readingmV

Buffer 3 (optional)

Buffer pHElectrode readingmV

Buffer temperature°CYour slope limit, lowFrom your SOP or the electrode maker.%Your slope limit, high%

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Result

96.5 %Within your limits

Measured slope: 57.08 mV/pH
Ideal at 25 °C: 59.16 mV/pH
Offset at pH 7: 0.82 mV

Slope between each pair: pH 4.01–7: 95.1 % · pH 7–10.01: 97.8 %. A big difference between segments points to a bad buffer, not to a bad electrode.

This reports the slope and offset; it does not decide what is acceptable. Limits come from the electrode maker and your procedure, which is why they are inputs here rather than built in. Use fresh buffers at a measured temperature, because the buffers themselves change pH with temperature.

How the calculation works

01An ideal glass electrode changes by S = ln(10)·R·T/F ≈ 0.1984·T mV per pH unit, with T in kelvin: 59.16 mV/pH at 25 °C, rising with temperature.
02The measured slope is the magnitude of (E_last − E_first)/(pH_last − pH_first), and the slope percentage is that divided by the ideal value at the buffer temperature. Each adjacent pair of buffers is also reported.
03The offset is the electrode output extrapolated to pH 7, where an ideal electrode reads 0 mV.

Limitations

It reports slope and offset and does not decide acceptability. Limits come from the electrode maker and your procedure, so they are inputs.
Buffers change pH with temperature. Enter the actual pH of each buffer at the temperature used, from its certificate.
A glass electrode departs from linearity at extremes (alkaline error above about pH 12, acid error below about pH 1), so the slope here applies to the buffers used.

Frequently asked questions

What is a good pH electrode slope?: Close to 100 % of the Nernst value, and meters and makers commonly flag a slope that falls outside roughly 95 to 105 %. The limit that matters is the one in your procedure or the electrode's manual, which is why it is an input here.
What is pH electrode offset?: The millivolt reading the electrode gives at pH 7, ideally 0 mV. A large offset points to a contaminated or ageing reference junction, and many procedures set a limit of a few tens of millivolts.
Why is the theoretical slope 59.16 mV per pH?: It comes from the Nernst equation, ln(10)·R·T/F, evaluated at 25 °C. At 37 °C it is 61.5 mV/pH and at 0 °C 54.2 mV/pH, which is why the buffer temperature matters.
Why is my slope low?: Usually an ageing or coated glass membrane, a clogged reference junction, or a poor buffer. If the segments disagree, with a good slope between two buffers and a poor one with the third, suspect that buffer rather than the electrode.

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Calibration by Parameter

pH meter calibration: buffers, slope and why temperature is not optional

Automatic temperature compensation corrects the electrode. It does nothing about the buffer's own pH changing with temperature, which is the quiet error.

2026-09-30 · 3 min read

Calibration by Parameter