Two-point calibration adjusts a pH meter using two reference buffers. The first, almost always pH 7.00, sets the offset (the zero point, where an ideal electrode outputs 0 mV). The second, usually pH 4.01 for acidic work, sets the slope, the millivolts per pH unit. An ideal electrode gives about 59.16 mV per pH unit at 25 °C. One-point calibration fixes only the offset and can mislead on worn probes.
In practice
For grow solutions in the 5–7 range, calibrate at 7.00 then 4.01. Steps: rinse with distilled water, place in pH 7, let the reading settle, confirm; rinse; place in pH 4, settle, confirm. Many meters then report slope as a percentage. A slope of roughly 95–105% means the electrode is healthy; below about 90% suggests cleaning or replacing it.
Calibrate weekly for regular hydro use, and always after storage, cleaning, or a reading that looks wrong. A three-point calibration (adding pH 10) helps only if you measure alkaline samples.
Units and formula
Nernst slope ≈ 59.16 mV per pH unit at 25 °C; slope % = measured slope ÷ theoretical slope × 100.
Don't confuse with
- EC calibration: usually one-point, with a standard such as 1.413 mS/cm.
- Temperature compensation: corrects for temperature, not for electrode ageing.
