Sulfur dioxide protects wine from spoiling; it acts as an antioxidant, a bactericide, and a preservative. Potassium or sodium metabisulfite, $(\mathrm{K}_{2} \mathrm{S}_{2} \mathrm{O}_{5}$ or $\mathrm{NaS}_{2} \mathrm{O}_{5})$ is a good source of sulfur dioxide. $S O_{2}$ exists in a pH-dependent equilibrium system:
$$\begin{array}{c}{\mathrm{SO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{HSO}_{3}^{-} \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{SO}_{3}^{2-}} \\ {\mathrm{p} K_{\mathrm{a}}=1.81 \quad \mathrm{p} K_{\mathrm{a}}=7.2}\end{array}$$
In this equilibrium system, SO$_{2}$ is called "molecular SO$_{2}$" In the $\mathrm{HSO}_{3}^{-}$ form, it is called "free SO $_{2}$". Only molecular $\mathrm{SO}_{2}$ is active as a preservative. The amount of molecular $\mathrm{SO}_{2}$ that is available from a given amount of free $\mathrm{SO}_{2}$ is highly pH-dependent, which is one reason that winemakers carefully measure and adjust pH. The recommended amount of molecular SO $_{2}$ in white wine is $0.8 \mathrm{ppm},$ and for red wine it is 0.5 $\mathrm{ppm}$ . The bisulfite ion $\left(\mathrm{HSO}_{3}^{-}\right)$ or "free $\mathrm{SO}_{2}$" is measured in milligrams $S \mathrm{O}_{2}$ produced per $\mathrm{L},$ or $\mathrm{mg} / \mathrm{L}$ is approximately equivalent to 1 ppm. Figure a shows the recommended amount of free $S O_{2}$ required to obtain the correct amount of molecular SO$_{2}$ concentration as a function of pH in white wine and red wine.