An electrochemical cell which is the basis of practical pH measurement consisting of a hydrogen ion-responsive electrode (hydrogen gas, or glass) and a reference electrode immersed in the test solution.
See: pH, reference electrode, test solution (in analysis)
Source:
PAC, 1985, 57, 531. 'Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)' on page 540 (https://doi.org/10.1351/pac198557030531)