Inputs
Molecular SO2 depends strongly on pH. Small pH differences can change the target free SO2 a lot.
Winemaking
Molecular SO2 Calculator
Estimate how much free SO2 is needed to reach a molecular SO2 target at a given pH.
Use pH to make sulfite targets realistic
The same free SO2 level can mean very different protection at different pH levels. This calculator converts a molecular SO2 target into a free SO2 planning target and optional KMS addition.
- Measure pH close to the time you make the SO2 decision.
- Use current free SO2 from a reliable test, not an old estimate.
- Retest after dosing because some added SO2 can bind.
Molecular SO2 notes
The calculation uses pKa 1.81 for sulfurous acid. It is a planning model, not a substitute for measured free and total SO2.
White, red, sweet, dry, high-pH, and low-pH wines may call for different practical targets. Check local regulations and style requirements.
Why pH dominates the number
As pH rises, a smaller fraction of free SO2 is present as molecular SO2. That is why high-pH wines often need much higher free SO2 to reach the same molecular target.
If the calculated free SO2 target looks too high for the style, the best answer may be an acid, stability, microbial, or bottling decision rather than blindly adding sulfite.
Before using the result on a full batch
Use this as planning arithmetic. Measure carefully, write down the assumptions, and make the smallest practical trial before a full cellar adjustment.
Best use
Estimating a pH-based free SO2 target before calculating a measured addition.
Do not skip
Current pH, current free SO2, legal limits, product strength, and a post-addition retest.
If the number looks surprisingly large or small, recheck every unit before acting. Most cellar mistakes start as unit mistakes.
How It Works
This calculator estimates free SO2 needed for a molecular SO2 target at a given pH.
- Enter wine pH, volume, current free SO2, and target molecular SO2.
- The tool estimates molecular fraction and required free SO2.
- Review the free SO2 increase and KMS addition estimate.
FAQ
Why does pH matter so much?
At higher pH, a smaller fraction of free SO2 is present as molecular SO2.
Is 0.8 mg/L always the right target?
No. It is a common planning value, but style, microbial risk, sweetness, pH, and local rules matter.
Should I use this instead of the sulfite addition calculator?
Use this first to choose a pH-based free SO2 target, then use addition math and retesting to plan the dose.