This paper presents an experimental characterisation of the pH dependence of drift on ISFET sensors. Experiments are run on an array of over 4000 sensors fabricated in commercial CMOS technology with pH buffer solutions of known pH. A mathematical model is built and the fitted coefficients are compared between experiments where the pH is constant and where pH changes. An exponential and a linear model are compared, as well as different metrics for coefficient comparison. It is shown that a dependence can be found between drift and pH variation when drift rate exceeds 15uV/s. This serves as a first step towards the development of a new metric to aid classification of nucleic acid amplification experiments.