Why free SO₂ alone isn't enough
When you measure free SO₂ in a wine you are measuring a mix: part of it is molecular SO₂ (dissolved gas) and the rest is bisulfite and sulfite. Only the molecular form has the strong antimicrobial effect you want when you sulfite; the rest protects far less.
pH decides the split: the lower the pH, the larger the molecular share of free SO₂. That is why 30 mg/L of free SO₂ can be more than enough in a wine at pH 3.1 (1.46 mg/L molecular) and fall short in one at pH 3.7 (0.38 mg/L).
The formula
Molecular SO₂ (mg/L) = free SO₂ ÷ (1 + 10^(pH − 1.81))
1.81 is the equilibrium constant the laboratory uses. Its exact value shifts a little with temperature and alcohol, so treat it as a very good approximation, not as an analytical lab result.
An example: a wine with 35 mg/L of free SO₂ at pH 3.40 has 35 ÷ (1 + 10^1.59) = 35 ÷ 39.9 ≈ 0.88 mg/L of molecular SO₂. The same free SO₂ at pH 3.80 gives 35 ÷ 98.7 ≈ 0.35 mg/L. The number on the analysis is the same; the protection is not.
Table: how much free SO₂ you need by pH
| pH | Free for 0.5 mg/L | Free for 0.8 mg/L |
|---|---|---|
| 3.0 | 8.2 | 13.2 |
| 3.2 | 12.8 | 20.4 |
| 3.4 | 20.0 | 31.9 |
| 3.5 | 25.0 | 40.0 |
| 3.6 | 31.3 | 50.1 |
| 3.8 | 49.4 | 79.0 |
Values in mg/L of free SO₂, calculated with the laboratory's own formula.
Rather not work it out by hand? The free molecular SO₂ calculator gives you the free SO₂ you need and the grams of metabisulfite for your volume.
What value to aim for
The general reference is to keep molecular SO₂ between 0.5 and 0.8 mg/L. LagarLab uses that window: inside it the tank is fine; between 0.3 and 0.5 it is at risk; below 0.3 it is critical. Above 0.8 the laboratory also flags it as risk, because an excess can give a sharp sulfur smell and become toxic to yeast.
It is a reference, not a universal rule. Many wineries aim for the low end in dry reds and the high end in whites or wines with residual sugar, and each adjusts to its own style and microbiological control.
How to correct it in practice
- Measure pH and free SO₂. Both numbers matter equally: with only one of them you cannot calculate anything.
- Find your pH in the table (or apply the formula) to get the free SO₂ you need as a target.
- Subtract the current free SO₂ from the target. That difference is what you need to add.
- Not everything you add stays free. In the laboratory, 15% of the SO₂ you add binds instantly and the free fraction keeps dropping over the following days, down to a floor of 30% of what was added. To raise free SO₂ by 10 mg/L the same day, add about 12 mg/L of SO₂ (10 ÷ 0.85).
- Convert to product. 1 g/hL of potassium metabisulfite adds about 5.8 mg/L of SO₂.
A note on pH: lowering pH also raises the molecular fraction, but it changes acidity and taste. If you are going to adjust it, do it before deciding the final SO₂ dose. See how to calculate the dose in the tartaric acid guide.
Why this is practiced in LagarLab: the laboratory calculates the molecular SO₂ of each tank with this formula, from the tank's real pH and free SO₂. You can add SO₂ today and watch how it evolves, or compare the same SO₂ in two wines with different pH.
What if you tried it on your own tank?
In LagarLab you add SO₂ to a tank, change the pH and watch molecular SO₂ move, without risking a real batch.
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Frequently Asked Questions
What is molecular SO2 and how is it different from free SO2?
Free SO2 is all the SO2 that is not combined with other compounds in the wine. Only part of it is molecular SO2, the dissolved gas that has the strong antimicrobial effect. The rest is bisulfite and sulfite. The split depends on pH.
How much free SO2 do I need at pH 3.5?
To reach 0.5 mg/L of molecular SO2 you need about 25 mg/L of free SO2, and to reach 0.8 mg/L about 40 mg/L, according to the laboratory's formula.
Why does a high pH require more SO2?
Because at high pH a smaller share of free SO2 is molecular. The same free SO2 reading protects a pH 3.8 wine less than a pH 3.2 wine, so you need more free SO2 to reach the same molecular SO2.
Is 0.8 mg/L of molecular SO2 a maximum?
It is the upper end of the reference window LagarLab uses (0.5 to 0.8 mg/L). Above it, the laboratory flags risk because an excess can give a sharp smell and be toxic to yeast. The real target depends on the wine style and on each winery.
Does all the SO2 I add stay free?
No. In the laboratory's model, 15% binds instantly and the free fraction keeps dropping over the following days down to a floor of 30% of what was added, because SO2 binds to acetaldehyde and sugars. That is why it pays to measure free SO2 after adding, not just calculate the dose.