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).
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.