Virtual pH-Meter Calibration & Measurement Bench

A glass pH electrode is a Nernstian sensor: it turns pH into a voltage of about −59 mV per pH unit at 25 °C. Before it can read pH it must be calibrated (its true slope and offset measured against standards). Temperature, miscalibration and a tired or dirty electrode all pull the reading away from the truth — calibrate below and find out.

1 · Calibrate (read standards)

2 · Measure unknowns

Bench settings

Electrode condition (hidden true characteristics)

Temperature solution & electrode

25 °C

ATC — automatic temperature compensation

When OFF, the meter keeps the slope it fitted during calibration. Measuring at a different temperature then introduces error.

Step 1 — read the standards

Rinse, immerse in each standard buffer, let it settle, record.

No standards read yet.

Step 2 — measure an unknown

Unknown sample

Calibrate first to unlock measurement.

Calibration

Read at least two standards (pH 7 and pH 4 or 10), then press Calibrate. A two-point calibration gives both slope and offset; a single point gives offset only.

Fitted slope

— mV/pH

Slope efficiency

— %

Offset (asymmetry)

— mV

Ideal slope @ temp

— mV/pH

Calibrated at

—

The calibration line maps pH to millivolts. Its steepness is the slope; a healthy electrode is close to the ideal ≈−59 mV/pH.

Standard readings Fitted line

Measurement

Reported pH

— · —

raw: — mV

temp: 25 °C · ATC on

What the bench is teaching you

The Nernst slope (≈59 mV/pH)

A pH electrode develops a potential that is linear in pH. The ideal slope is S = 2.303·R·T / F, which at 25 °C is 59.16 mV per pH unit. Because the term contains absolute temperature, the ideal slope itself grows with temperature — that is why temperature matters.

Why old electrodes read low slope

Stored dry, fouled, or simply aged, the glass membrane responds less fully to pH. The measured slope falls below ideal — its efficiency (% of ideal) drops. Below ~95 % the electrode should be cleaned, reconditioned or replaced; calibration cannot fully rescue a tired sensor.

Temperature vs ATC

Calibrate at one temperature, measure at another, and the true mV-per-pH relationship has shifted. With ATC on the meter recomputes the slope for the new temperature; with it off it keeps the old slope and the reading drifts. Try switching ATC off, then move the temperature slider before measuring.

Simplified teaching model. The electrode is treated as perfectly linear about the pH 7 isopotential point with a fixed efficiency and offset; activity coefficients, junction potentials, drift and real measurement noise are reduced to a small deterministic wobble. Standard-buffer pH values are taken at their nominal labels. Use a real, properly maintained meter for laboratory work.