As the temperature of a solution changes, the actual pH changes. This is not an error of the probe or meter being used, but is the actual pH of the solution at that particular temperature. The temperature effect on the pH value is 0.003 pH units per oC away from 25oC, per pH units away from pH 7. This effect can be either negative or positive, depending on if the temperature is above or below 25oC, and if the pH is above or below pH 7. At 25oC and pH 7, there is no change in the pH value.
The chart below shows how the actual pH changes with temperature and pH, which allows you to correct the pH reading to 25oC. As an example, if a sample measured pH 5 at a temperature of 5oC, the chart indicates this a negative effect, therefore the sample pH would be 5 – 0.12 = 4.88pH corrected to 25oC:
MANTECH software accounts for temperature by recording the temperature during calibration via a thermistor probe, then it corrects the pH reading at the time of measurement to account for the difference in measured temperature of the sample vs. the calibration buffers. The reported pH value is the corrected value @ the sample temperature. The equation below shows how the PC-Titrate software calculates pH:
EMeas = Voltage measure by electrode at the time of titration (mV)
EInt = Voltage of the Intercept value calculated from the calibration equation. (mV)
Slope = slope of the line calculated from the calibration equation. (mV)
TMeas = temperature measured at the time of pH measurement (K degrees)
TCal = temperature taken at the time of last calibration. (K degrees)
MANTECH also offers the option to report pH values corrected to a set temperature, such as 25°C. This is implemented in the software as an additional step after the sample pH is recorded. MANTECH offers this as a feature for new MT-Series systems, or as an upgrade to existing MANTECH systems.
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