Most pH meters use redox reactions as the mechanism for measuring [H+]. However, this is WAY beyond the scope of required MCAT knowledge. If it does show up on the MCAT it will be fully described.
It is useless to try and prepare for every possible passage they might throw at you. Science has advanced to such a degree that even a PhD might be ignorant of a certain aspect of his field. Just know the basics REAL WELL.