Knowing both the amount of free and total sulfites is very important because only the free forms of sulfites are available for providing a preservative role in wine. This is often expressed as ppm free SO2/ppm total SO2 to denote which number is free and which is total; these numbers can readily be determined by chemical analysis.

Sulfur dioxide concentration is measured in parts permillion (ppm) or as milligrams per liter (mg/L). A wine with 19 ppm free SO2 and 51 ppm total SO2 would be noted with 19/51. The amount of bound sulfites would be equal to the total SO2 minus the free SO2, (51 - 19 = 32). For dry table wines the level of free sulfur is usually somewhere around 40% to 75% of the level of total SO2.

Although the sulfite ion (SO3=) can bind with oxygen, there is almost no sulfite ion present in solution at the pH range found in wine (see Figure I). Rather sulfur dioxide prevents oxidation by binding with the precursors involved in oxidative reactions preventing them from reacting with oxygen or by binding with compounds already oxidized to reverse oxygen’s effect. Sulfur dioxide also acts by reducing the activity of the degenerative enzyme tyrosinase (polyphenol oxidase), which is present in juice.

Sulfur dioxide itself is a gas that readily dissolves in water. Once dissolved, it reacts with water to form new compounds called sulfites. See following reaction:
H2O + SO2 ↔ H+ + HSO3– ↔ 2H+ + SO3=

In solution with H2O (water), SO2 is called molecular SO2, HSO3– is called bisulfite and SO3= is called sulfite. The negative signs (– and =) denote the negative charge of the bisulfite and sulfite ions (molecules with a charge are called ions). The double arrows (↔) of the equation denote that the reaction is at equilibrium.

At equilibrium, the rate at which bisulfite ions become sulfite is the same as the rate sulfite ions become bisulfite. The reaction between the different types of sulfite is going both ways at a steady state so the concentration of the sulfite compounds remains steady.

While the concentration of the different forms of sulfites may be steady, it does not mean there are equal amounts of the compounds in solution; the acidity or pH of the water has a huge affect on their concentration. The more acidic or the lower the pH of the water, the more heavily the reaction is weighted to the molecular SO2 side. The more basic or higher

the pH is, the more sulfite that is present as illustrated in the graph (Figure I). dissolves in wine? Since wine is more than 80% water, the reaction, or disassociation, of molecular SO2 into sulfites happens in much the same manner. Since wine is acidic (between 3 and 4 pH), molecular SO2 and bisulfite make up the vast majority (99.99% at pH 3.4) of the sulfite compounds present in wine.

Sulfites will also react with other chemical constituents found in wine such as sugars, acetaldehyde, and phenolic compounds.When a sulfite reacts with another molecule and becomes part of its structure it no longer takes part in the equilibrium reaction and it is called bound. Sulfites that still are part of the equilibrium reaction are called free. The combined amounts of free and bound sulfites are called Total SO2.

Themore compounds that are available in awine for sulfites to bind to, the higher the ratio of bound to total sulfites there will be. Therefore, wines that are sweet or have high solids because they have not been settled or filtered will have a lower ratio of free to total SO2.