If a pH sensor is immersed into a liquid buffer solution, it does not show the correct pH value of that buffer solution instantaneously. A response time of approximately 30 seconds is normal in order to reach the buffer value with a tolerance of 0.01 pH units (0.6 mV). For this reason it is important to wait long enough during the calibration cycle for the pH reading on the transmitter or analyzer to become stable. Only then with a stable measurement, can the indicated value be adjusted to the buffer value. A premature interruption of the sensor adaptation to the buffer value is often a major source for incorrect pH measurements. Patience is a virtue in pH calibration!
The response time of an electrode assembly may be especially slow if the sensor temperature and the temperature of the buffer solution differ by more than 10°C. In cases where buffer solutions have to be heated, the user should delay the calibration procedure until the electrode reaches the temperature of the heated buffer solution. This ensures that proper heat transfer through the pH sensor body occurs and the internal temperature element can output the correct value.
A further point to consider is the storage solution used with pH sensors. When not in use, avoid storing the pH electrode in distilled water, as this will slow down the response time of that electrode assembly considerably. It has been found that a 3M KCl solution is the best storage liquid for glass electrode assemblies. Please refer to this separate article on pH sensor storage and regeneration.
Modern microprocessor based pH meters/transmitters include menu based software for early detection of a stable pH calibration value (auto-calibration). Hamilton verification measurements of many manufacturers have shown that the auto-calibration values do not differ more than 0.01 pH units from the actual value found between new electrodes and tested buffer solutions thus are highly reliable.