pH is technically defined as:
pH = -log[H+]
pH monitors the concentration of free protons in solution. It is frequently used to determine whether an environment is "acidic" (pH < 7) or "basic" (pH >7). The pH scale ranges from 0-14, where 0 is the most acidic and 14 is the most basic. pH is important to measure in beer and kombucha because it can provide insight into microbe health and fermentation progress.
There are two main ways of measuring pH: colorimetric and potentiometric measurements. Indicator molecules are used in the colorimetric measurements. The color of the indicator molecule is affected by the pH of the environment (phenolphthalein is a good example). pH meters are frequently used to perform potentiometric measurements.
The Brewer Encyclopedia is our attempt to bring our working understanding and definitions for brewing terms to you. It resulted because we found ourselves building a list of useful terms that we could refer back to on short notice. Eventually the list got long enough we figured we'd type it up. Original sources are cited at the end of each entry so that you can refer to them.