info on measuring pH in DI water

[DJ88]

I haven't quite figured it all out.. all I know is that everytime I read about the H+ wih reference to a pH probe I keep getting the same thing. It measures the activity of the H+ ions or lack there of. It does make sense to me WRT the potential created between the two probes. Luckily I just did a lab in my physics course which created a similar voltage potential to measure heat transfer.