Brewer Encyclopedia

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 American Society of Brewing Chemists defines titratable acidity as, “the number of acidic protons that the organic acid potentially can donate. It is experimentally determined by measuring the amount of sodium hydroxide required to raise the pH to 8.2, where neutralization of the acids occurs” (ASBC Beer-8).
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Titratable acidity measurements are used to determine the concentration of acidic compounds in solution, including organic acids and free protons. In contrast, pH measurements can only be used to determine the concentration of free protons in solution and cannot be used to determine the concentration of organic acids. This is important because organic acids are frequently the compounds responsible for providing flavor traits. What does this mean? To put it simply: titratable acidity measurements will generally correlate with taste perception of acidity, while pH measurements will not. 

An acid is a molecule that can donate a proton or accept an electron pair in reactions. There are strong acids (such as inorganic acids like hydrochloric acid or phosphoric acid) and weak acids (such as organic acids like acetic acid and lactic acid).