Molecular SO2 Calculator: Free SO2, pH, and Target
Molecular SO2 Calculator
Current Molecular SO2 (mg/L)
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Free SO2 Needed to Meet Target Molecular SO2 (mg/L)
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Free SO2 to Add (mg/L)
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Molecular SO2 = free SO2 / (1 + 10(pH − pKa)). pKa = 1.81 (20 °C). The addition shown is free SO2 only. Some of any addition will bind, so confirm free SO2 by analysis after adding.
Frequently asked questions
What is molecular SO2?
Molecular SO2 is the portion of free SO2 present as undissociated SO2. It is the form that inhibits yeast and bacteria. At typical wine and cider pH, most free SO2 is present as bisulfite (HSO3−), and only a small percentage is molecular.
How is molecular SO2 calculated?
Molecular SO2 = free SO2 / (1 + 10(pH − pKa)). This calculator uses a pKa of 1.81. For example, 30 mg/L free SO2 at pH 3.40 gives about 0.75 mg/L molecular SO2.
What molecular SO2 should I target?
0.8 mg/L is the common target for white wine, rosé, and cider. 0.5 mg/L is common for red wine, which gets some added protection from phenolics. Wines with residual sugar or without sterile filtration may need additional protection.
Why does pH have such a large effect?
The molecular fraction drops by roughly half for every 0.3 pH unit increase. To hold 0.8 mg/L molecular SO2, you need about 20 mg/L free SO2 at pH 3.20, about 40 mg/L at pH 3.50, and about 79 mg/L at pH 3.80.
Why does this calculator use a pKa of 1.81?
1.81 is the pKa of SO2 in water at 20 °C and is the standard value used in winemaking calculations. Ethanol and temperature raise the effective pKa in wine, so the true molecular fraction is usually somewhat higher than the 1.81 result. Using 1.81 is the conservative choice.
Can I use this for cider and mead?
Yes. The equilibrium is the same in any aqueous beverage. Use an accurate pH and a free SO2 value measured by aeration-oxidation or an equivalent method.