What this calculator does
Free SO₂ is the figure a winery measures, but molecular SO₂ is the fraction that actually does the antimicrobial work, and it depends heavily on pH. At pH 3.4, 30 mg/L of free SO₂ gives 0.75 mg/L molecular, which sits in the usual protective range.
The pH sensitivity is dramatic and is the thing to understand. The same 30 mg/L of free SO₂ gives 1.82 mg/L molecular at pH 3.0 and only 0.30 at pH 3.8. A high-pH wine needs several times the free SO₂ of a low-pH one for the same protection, which is why pH measurement matters as much as the sulphite addition itself.
The formula
Molecular SO₂ is the free SO₂ divided by one plus ten raised to the power of the pH minus 1.81, where 1.81 is the pKa of sulphur dioxide in wine. Because the relationship is logarithmic in pH, a change of a few tenths shifts the molecular fraction by a factor of two or more.
| Term | Meaning |
|---|---|
| Free SO₂ | Sulphur dioxide not bound to other wine components, which is what a standard laboratory measurement reports. |
| Molecular SO₂ | The undissociated fraction of free SO₂, which is the antimicrobially active form. |
| pKa 1.81 | The dissociation constant of SO₂ in wine, which sets how the fraction varies with pH. |
| 0.5 to 0.8 mg/L | The molecular SO₂ range commonly targeted for reliable microbial protection. |
The inputs explained
| Field | What to enter |
|---|---|
| Free SO2 (mg/L (ppm)) | Free SO₂ in mg/L, from a laboratory or titration measurement. |
| Wine pH | Wine pH. This is the dominant variable, and an estimate rather than a measurement will give a misleading result. |
When to use it
Setting a sulphite addition
Deciding how much SO₂ to add starts from the molecular target and works back through the pH to the free SO₂ required, which is why the calculation runs in both directions in practice.
Comparing two wines
Two wines with identical free SO₂ can be protected very differently if their pH differs, and the molecular figure is the one that tells you which is actually at risk.
Diagnosing a spoilage problem
A wine spoiling despite apparently adequate free SO₂ is often a high-pH wine where the molecular fraction was well below what was assumed.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does pH change molecular SO2?
The same free SO₂ across a range of wine pH values.
| Wine pH | Molecular SO2 | Share of free SO2 that is molecular | Common target |
|---|---|---|---|
| 3.0 | 1.82 mg/L | 6.06% | 0.5-0.8 mg/L molecular SO2 for reliable microbial protection in most wines |
| 3.2 | 1.17 mg/L | 3.91% | 0.5-0.8 mg/L molecular SO2 for reliable microbial protection in most wines |
| 3.4 | 0.75 mg/L | 2.51% | 0.5-0.8 mg/L molecular SO2 for reliable microbial protection in most wines |
| 3.6 | 0.48 mg/L | 1.60% | 0.5-0.8 mg/L molecular SO2 for reliable microbial protection in most wines |
| 3.8 | 0.30 mg/L | 1.01% | 0.5-0.8 mg/L molecular SO2 for reliable microbial protection in most wines |
Questions
Why does pH matter so much?
Because SO₂ exists in equilibrium between molecular and bisulphite forms, and the balance depends on pH logarithmically. Only the molecular form is antimicrobially active, so at higher pH most of the free SO₂ is present in a form that does little. A 0.4 unit rise in pH roughly halves the molecular fraction.
What molecular SO2 should I target?
0.5 to 0.8 mg/L is the range commonly cited for reliable protection against spoilage organisms including Brettanomyces. Sweet wines and those intended for long ageing are often taken to the upper end, and the trade-off is that too much becomes perceptible on the nose.
How do I reach a target in a high-pH wine?
It takes considerably more free SO₂, and sometimes an impractical amount. Acid addition to lower the pH is the other lever, and it is often the better one, since it improves the effectiveness of every subsequent SO₂ addition rather than just the current one.
What is the difference between free, bound and total SO2?
Free SO₂ is unbound and available; bound SO₂ has reacted with acetaldehyde and other compounds and is largely inactive; total is the sum. Only free SO₂ enters this calculation, and only part of that is molecular, so total SO₂ figures say very little about protection.
For alcohol content from gravity readings, see the ABV from gravity calculator. For priming sugar in beer, see the priming sugar calculator.