When measuring the pH of a substance, you are measuring the concentration of hydrogen ions (H+).

Here's why:

* pH Scale: The pH scale is a logarithmic scale that measures the acidity or alkalinity of a solution.

* Hydrogen Ions: The concentration of hydrogen ions (H+) in a solution directly determines its acidity.

* Low pH: A solution with a high concentration of H+ ions has a low pH, indicating it is acidic.

* High pH: A solution with a low concentration of H+ ions has a high pH, indicating it is alkaline (basic).

So, when you measure the pH of a substance, you are essentially determining the amount of hydrogen ions present in that substance.