Glass electrodes for measuring pH in solutions show relatively fast responses with slow drifts. To design compensators to remove the drifts that hinder fast measurements of pH, a dynamic model that consists of ordinary and partial differential equations is proposed. It can explain such two-time scale responses of glass electrodes. The fitting accuracies of the proposed model are experimentally evaluated in frequency-domain and time-domain. The frequency responses obtained from the square wave responses show the fitting abilities of the proposed model, and the step responses also support this. The step responses filtered based on the proposed model show that pH measurements can be made considerably faster. The proposed model can be used to improve the dynamics of glass pH electrodes by compensating dynamic elements causing slow drifts.