pH

Definition

The pH is, by definition, the negative of the decimal logarithm of the H+ concentration:

or [H+] = 10-pH.

The pH is an indication of acidity (see pH and acidity in Syntheses for more details). A  pH lower than 7 is said acid, a pH higher than 7 is basic and a pH of 7 is called neutral (see figure 4 for examples).

Reactions that depend on pH

The law of mass action can be applied to the reaction involving active SO₂ and HSO₃^-

where the constant of reaction K = 1,7*10-2. We can write that  . If we want to know how much SO₂ there is compared to HSO₃^-:  . Using the definitions of pH and pK: [formule]

The amount of SO₂ present in the must will therefore depend on the pH of the must (figure 9). When the pH is equal to the pK (= 1,77), there is as much SO₂ as HSO₃^-. But if the pH is higher than pK (the pH of the must or of the mead is between 3 and 4), there is less SO₂. If the pH is 3,5 for example,  . See also sulfite in Ingredients.