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Calculating Molecular SO2


Bob Becker — Pleasant Hill, California asks,

Can you explain how to figure Molecular SO2? Here is my situation: I have a 6-gallon (23-L) carboy of Merlot with a pH of 3.6. The free SO2 is at 12 PPM. I’d like to bring the SO2 up to 40 PPM but I can’t figure out how much additional SO2 to add. The online calculator asks for molecular SO2 measurements, which I don’t understand.

Sulfur dioxide, or SO2, has many benefits in winemaking and has been added to wine for centuries to act as a preservative. When you add SO2 to your wine, part of it will bind up with other wine components like acetaldehydes, polyphenols, and weakly, to sugars (contributing to “total SO2”) and some will remain free in solution to interact with oxygen, microbial membranes, etc. to help protect your wine. The “free SO2” can exist in three different forms: Bisulfite ion, sulfite ion, and molecular SO2. Additionally, free SO2 swimming around in your wine exists in two forms — molecular and bisulfite. It’s only the molecular SO2 that’s the most active operator here against microbes and the amount of free SO2 that exists as molecular can change widely based on the pH of your wine. Most experts agree that a molecular SO2 of 0.5 mg/L (or ppm) for red wines and 0.80 mg/L (or ppm) for whites is sufficient for antioxidant and antimicrobial protection. With molecular levels in that range, in my experience, it’s likely you’ll have enough bisulfite in the
Response by Alison Crowe.