The pH glass in all electrodes follows the Nernst Equation in the Voltage output versus pH and temperature. The electrical potential difference between the internal and external pH glass surfaces is given by:
The most common cause of error in pH measurements is temperature.
There are four errors inherent in pH measurements:
The pH glass in all electrodes follows the Nernst Equation in the Voltage output versus pH
and temperature. The electrical potential difference between the internal and external pH
glass surfaces is given by:
Eo-Ei=0.1984(T+273.16)(7-pH)
The above graph shows the thermal slopes at various temperatures. At 25 degrees C, pH electrodes generate 59.16 mV/pH unit away from 7 pH. At 100 degrees C, the slope is 74 mV/pH. This thermal effect is also shown in a table. This is the only thermal error which is compensated in meters and controllers with Automatic Temperature Compensation.This thermal error is caused by the thermal lag of one part of the electrode measuring system relative to the other, e.g. the reference gel or fill solution reaching the sample temperature before the pH internal solution. HANNA has address this problem in its Automatic Temperature Compensation pH Electrodes.
Most internal reference solutions are composed of buffers. As the Buffer Thermal Tables show, the pH of the buffers vary with temperature. Since the internal solution is used as a reference in the electrochemical measurement of pH, any variation in its pH will affect the measured pH of the sample.
Since pH electrodes measures the activity of the Hydrogen ions and ions become more active as the temperature increases, the pH of the sample will change with temperature. Thus, when taking a sample from a process into the laboratory for pH verification, you will need to reproduce the temperature of the process before getting a comparable reading.