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Ion sensitive field effect transistors (ISFETs) are pH sensors that have generally been used in the analogue domain. It is possible to use them in the digital domain for chemical thresholding applications. To facilitate small pH change detection where a simple switch suffers from ripple and distorted output, a chemical switch with input noise immunity is needed. Reported are first measurements of an ISFET-based chemical Schmitt trigger. It provides a more stable output with better noise immunity. It can be used in applications where the digital detection of small pH changes is required.