How this calculator works
Sulfur dioxide added to wine exists in several forms. Some is bound to other compounds, some is free, and a small part of the free portion is molecular SO2, which is the form that inhibits wild yeast and bacteria. The split depends on pH: as pH rises, a smaller share of the free SO2 is molecular, so more free SO2 is needed for the same protection. The usual target is 0.5 to 0.8 mg/L of molecular SO2, with 0.8 mg/L used when the wine is at higher risk. The relation here is the standard acid-base equilibrium, and it says nothing about how much total SO2 may lawfully be added.
Formula
molecular SO2 = free SO2 / (1 + 10^(pH - 1.81)). free SO2 needed = target molecular SO2 x (1 + 10^(pH - 1.81)). The constant 1.81 is the negative logarithm of the dissociation constant used for the sulfite equilibrium.
Worked example
A wine at pH 3.5 with 30 mg/L of free SO2 has a molecular fraction of 30 / (1 + 10^(1.69)) = 30 / 49.98 = 0.600 mg/L. To reach 0.8 mg/L at the same pH you would need 0.8 x 49.98 = 40.0 mg/L of free SO2. At pH 3.2 the factor falls to 25.55, so 0.8 mg/L needs only 20.4 mg/L of free SO2.
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Frequently asked questions
What molecular SO2 should I aim for?
Most winemaking guidance uses 0.5 to 0.8 mg/L of molecular SO2. The lower end suits clean, dry, low-risk wine; the upper end is for wine with residual sugar, high pH or a history of spoilage. Local rules may limit total SO2 additions, so check them before adding.
Why does pH matter more than the free SO2 number?
The molecular fraction changes by a factor of ten for every pH unit. A wine at pH 3.2 needs far less free SO2 than one at pH 3.8 to reach the same protection. That is why two wines with the same free SO2 analysis can behave very differently.
Is this the same as the total SO2 on a lab report?
No. A lab report may show free SO2 and total SO2. This calculator uses free SO2, because that is the pool from which molecular SO2 comes. Bound SO2 is already attached to other compounds and does not contribute directly.
Can I use this for cider or beer?
The equilibrium applies to any aqueous sulfite solution, but the target molecular levels and legal limits differ by product. Use the calculator as a guide for wine and check the rules for the beverage you are making.