GIFSQ Tools · Module 2 — Food Chemistry & Physicochemical Testing · Tool 21 of 500
Log a pH reading with its temperature and calibration buffers. The tool checks the reading against the calibration range and checks the electrode slope against the theoretical Nernst slope.
Try sample values — fills the form with the worked example below.
Reading log
Saved on this device only. Use "Add current reading to log" after a calculation.
| Date / time | pH | Temp (°C) | Slope % | Range | Slope OK |
|---|
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Theoretical slope (mV/pH)
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Electrode slope (mV/pH)
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Reading vs calibration range
Equation: Theoretical slope S = 0.198412 × (273.15 + T) mV per pH unit, where T is the temperature in °C. Electrode slope = slope % × S ÷ 100.
Method: AOAC 981.12 (pH of acidified foods) and AOAC 943.02 — glass electrode, two-point calibration with standard buffers that bracket the sample, reading taken at the stated temperature.
Assumptions: the electrode follows the Nernst response; the buffers are fresh and at their stated pH; the sample and the buffers are at the stated temperature.
Limitations: a passing slope does not prove accuracy — a drifting electrode can pass the slope check and still read wrong, so check the reading against a known buffer as well. Junction error, sodium error at high pH, and protein or fat fouling are not detected by this check.
Rounding: pH to 2 decimal places; slope to 2 decimal places. The accepted slope range is 95–105 % of theoretical.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: AOAC 981.12 (pH of acidified foods) and AOAC 943.02 — glass electrode, two-point calibration with standard buffers that bracket the sample, reading taken at the stated temperature.
Assumptions: the electrode follows the Nernst response; the buffers are fresh and at their stated pH; the sample and the buffers are at the stated temperature.
Limitations: a passing slope does not prove accuracy — a drifting electrode can pass the slope check and still read wrong, so check the reading against a known buffer as well. Junction error, sodium error at high pH, and protein or fat fouling are not detected by this check.
Rounding: pH to 2 decimal places; slope to 2 decimal places. The accepted slope range is 95–105 % of theoretical.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: pH 4.02 at 25 °C, calibrated with pH 4.00 and 7.00 buffers, electrode slope 98.5 %. Theoretical slope = 0.198412 × 298.15 = 59.16 mV/pH; electrode slope = 0.985 × 59.16 = 58.27 mV/pH; the reading lies inside the 4.00–7.00 calibration range and the slope is inside 95–105 %.
Enter values above to see results.
Typical food pH: milk 6.5–6.7, yogurt 4.0–4.5, orange juice 3.3–4.2, pickles 3.0–3.5, meat 5.4–6.2. Calibrate daily, and re-check with a buffer after every 10 readings. Tool 21 of 500 — GIFSQ
