A pH probe is a millivolt source
The glass membrane develops a potential proportional to hydrogen ion activity, and the analyser simply converts millivolts to pH. The relationship comes from the Nernst equation:
At 25 °C that gives the familiar 59.16 mV per pH unit. A perfect electrode reads 0 mV at pH 7, +177 mV at pH 4 and −177 mV at pH 10.
Slope temperature dependence
| Temperature | Theoretical slope |
|---|---|
| 0 °C | 54.20 mV/pH |
| 10 °C | 56.18 mV/pH |
| 25 °C | 59.16 mV/pH |
| 40 °C | 62.13 mV/pH |
| 50 °C | 64.12 mV/pH |
| 80 °C | 70.07 mV/pH |
This matters when judging a probe: a slope of 60 mV/pH is 101% at 25 °C but only 94% at 40 °C. Comparing against a fixed 59.16 rather than the slope for the actual calibration temperature is a common way to misjudge a perfectly good electrode — or pass a failing one.
Worked example
Two-point calibration at 25 °C in pH 4.01 and pH 10.01 buffers:
- Readings +171 mV and −171 mV → span 342 mV over 6.00 pH units
- Slope = 342 ÷ 6.00 = 57.0 mV/pH
- Efficiency = 57.0 ÷ 59.16 = 96.4% — healthy
- Offset at pH 7 = 0 mV — no reference junction problem
Reading the diagnosis
Slope and offset fail in different ways and point at different causes:
- Slope falling, offset normal — the glass membrane is ageing or coated. Clean it; if the slope does not recover, the electrode is near end of life.
- Offset drifting, slope normal — the reference junction is contaminated or the reference electrolyte is depleted. Often recoverable.
- Both degraded — replace the electrode.
- Sluggish response but good numbers — a coated bulb. The numbers can look acceptable while the probe takes minutes to settle, which makes control useless.
The trend matters more than the value. An electrode at 94% that was 97% last month is failing faster than one that has sat at 93% for a year. Recording slope and offset every calibration turns pH maintenance from reactive to predictable — the same as-found discipline that applies to any instrument, discussed in our calibration procedure guide.
Field notes
- Buffers expire and absorb CO₂. An alkaline buffer left open drifts downward within weeks, and a bad buffer produces a bad calibration that looks perfectly successful.
- Bracket the process. Calibrate with buffers either side of the operating pH; extrapolating far outside the calibrated range compounds any slope error.
- Temperature compensation has two parts — the Nernst slope correction, which the analyser handles automatically, and the solution's own pH shift with temperature, which it cannot.
- The output is still a 4-20 mA loop. A perfect electrode behind a mis-ranged transmitter still reads wrong — check the scaling with the 4-20 mA converter.
Frequently asked questions
What is the theoretical pH electrode slope?
59.16 mV per pH unit at 25 °C, from the Nernst equation. It is temperature dependent: 54.20 mV/pH at 0 °C, 59.16 at 25 °C and 64.12 at 50 °C. A probe is judged against the slope for its calibration temperature, not against 59.16 universally.
What slope percentage means a pH probe should be replaced?
Above 95 percent is healthy and 92 to 95 percent is serviceable. Below about 90 percent the probe is near end of life, and below 85 percent most procedures require replacement. A rapidly falling slope between calibrations matters more than any single reading.
What is the zero point or offset?
The millivolt output at pH 7, which should be close to 0 mV. Offsets beyond roughly ±30 mV suggest a contaminated reference junction, a fouled bulb or a failing electrode, even when the slope still looks acceptable.
Why does temperature compensation matter for pH?
Two separate effects exist. The Nernst slope itself changes with temperature, which analysers correct automatically from a temperature input. The chemistry of the solution also shifts pH with temperature, and that is process-specific — no analyser can correct it without knowing the solution.
Provided for reference and education. Follow the analyser and electrode manufacturer's procedures and acceptance criteria. See our disclaimer.