For example, at high voltage gradient, 14 V/cm, the heating rate was high (2.031C/s), therefore the residence time for the sample to heat up from 26 to 96°C was short, thus the change of the pH was limited (1.2%) because the reaction time was short. In comparison, at low voltage gradient, 30V/ cm, the heating rate was 0.325°C/s, which means high residence time at which the change in the pH was maximum (8.7%) because of the longer reaction time.