Molecular SO2 by pH: How Much Free SO2 You Actually Need
Only the molecular form of sulfur dioxide protects wine, and how much of your free SO2 exists in that form depends entirely on pH. Molecular SO2 equals free SO2 divided by one plus ten to the power of pH minus 1.81, where 1.81 is the pKa of the sulfur dioxide and bisulfite pair at 20 degrees Celsius. To hold the conventional target of 0.5 mg/L molecular SO2, a red wine at pH 3.4 needs about 20 mg/L free SO2, while the same wine at pH 3.8 needs about 49 mg/L. TTB caps total SO2 in wine at 350 parts per million, and the table below shows the free SO2 required at every pH from 2.90 to 4.00.
What is molecular SO2 and why does it matter?
Molecular SO2 is the only one of the three forms of sulfur dioxide in wine that actually kills spoilage organisms, and it is typically a very small fraction of the free SO2 you measure. Sulfur dioxide in solution exists as molecular SO2, bisulfite, and sulfite. At wine pH the bisulfite form dominates, and bisulfite does very little antimicrobial work.
This is why a free SO2 number on its own tells you almost nothing. Two wines can both show 30 mg/L free SO2 and have completely different levels of actual protection, because the split between those three forms is set by pH. The free SO2 assay measures the pool; the molecular calculation tells you how much of the pool is doing the job.
The relationship is governed by the pKa of the sulfur dioxide and bisulfite pair, 1.81 at 20 degrees Celsius:
molecular SO2 = free SO2 / (1 + 10(pH - 1.81))
Rearranged to answer the question a winemaker actually asks, which is how much free SO2 to carry:
free SO2 required = molecular target × (1 + 10(pH - 1.81))
Free SO2 required by pH: the full table
Values are milligrams per liter of free SO2, rounded to the nearest whole number, calculated at the pKa of 1.81. The 0.5 column is the conventional red wine target; the 0.8 column is the conventional white wine target.
| Wine pH | Free SO2 for 0.5 mg/L molecular (red) | Free SO2 for 0.8 mg/L molecular (white) |
|---|---|---|
| 2.90 | 7 | 11 |
| 2.95 | 7 | 12 |
| 3.00 | 8 | 13 |
| 3.05 | 9 | 15 |
| 3.10 | 10 | 16 |
| 3.15 | 11 | 18 |
| 3.20 | 13 | 20 |
| 3.25 | 14 | 23 |
| 3.30 | 16 | 26 |
| 3.35 | 18 | 29 |
| 3.40 | 20 | 32 |
| 3.45 | 22 | 36 |
| 3.50 | 25 | 40 |
| 3.55 | 28 | 45 |
| 3.60 | 31 | 50 |
| 3.65 | 35 | 56 |
| 3.70 | 39 | 63 |
| 3.75 | 44 | 70 |
| 3.80 | 49 | 79 |
| 3.85 | 55 | 89 |
| 3.90 | 62 | 99 |
| 3.95 | 70 | 111 |
| 4.00 | 78 | 125 |
Read down the table and the cost of high pH becomes obvious. A white wine at pH 3.2 needs 20 mg/L free SO2 to sit at 0.8 molecular. The same wine at pH 3.8 needs 79 mg/L, roughly four times the sulfur, to reach identical protection.
What molecular target should you use?
The conventional targets are 0.5 mg/L molecular SO2 for red wine and 0.8 mg/L for white wine. Those two numbers cover the large majority of dry table wine production.
The gap between them is not an accident of taste. At 0.8 mg/L molecular, malolactic bacteria are effectively inhibited, which is the point for a white wine going to bottle without malolactic fermentation. Reds are usually held at 0.5 specifically to leave that door open, then adjusted upward once malolactic is complete and the wine no longer needs to support bacterial activity.
Sweet and dessert wines are commonly carried higher, sometimes to 1.5 mg/L molecular, because residual sugar gives refermenting yeast and spoilage organisms a substrate that a dry wine does not. Wines heading for extended barrel aging, wines with elevated volatile acidity, and wines with a history of Brettanomyces are all arguments for the upper end of the appropriate range rather than the middle.
Why pH matters more than the dose
Because pH determines what fraction of your free SO2 is in the active form, and that fraction moves fast. At pH 3.0 roughly 6 percent of free SO2 is molecular. At pH 3.8 it is closer to 1 percent. Same chemistry, roughly a quarter of the protection per unit added.
The practical consequence is that acid adjustment is often the cheaper intervention. Dropping a wine from pH 3.8 to pH 3.6 with tartaric acid takes the free SO2 needed for 0.5 molecular from 49 mg/L down to 31 mg/L, a 37 percent reduction in sulfur for a modest addition of acid. On a high pH lot destined for long aging, that arithmetic usually favors the acid.
It also means pH has to be current. A free SO2 target calculated against a pH measured before malolactic fermentation is wrong afterward, because malolactic raises pH. Wineries that set a sulfur regime once at crush and hold it all year are, without realizing it, under-protecting exactly the lots whose pH drifted up.
What is the legal limit on SO2 in wine?
TTB caps total SO2 in wine at 350 parts per million. That is a ceiling on total SO2, free plus bound, not on free SO2 alone, which matters because the bound fraction in a heavily sulfured or oxidized wine can be several times the free reading.
A second threshold applies to labelling rather than production. Wine containing 10 parts per million or more of total SO2 has to carry a sulfite declaration on the label. In practice almost every commercial wine crosses that line, including wines with no sulfur added at all, because fermentation itself produces small quantities of SO2.
Neither limit is a practical constraint on normal dry table wine production. A red wine held at 0.5 molecular at pH 3.6 carries 31 mg/L free SO2, well under the ceiling even after bound SO2 is counted. The wines that get close are sweet whites and botrytized dessert wines, where high molecular targets meet high binding rates from residual sugar and oxidized compounds.
How do you convert a target into an addition?
Subtract your current free SO2 from your target free SO2, then convert the difference into grams of your chosen sulfite source. Potassium metabisulfite, the most common form, is roughly 57 percent SO2 by weight, so about 1.75 grams per hectoliter raises free SO2 by 10 mg/L in theory.
In theory is doing real work in that sentence. Some fraction of any addition binds immediately to acetaldehyde and other carbonyl compounds and never appears in the free pool at all. Binding is heaviest in young wine, in wine that has seen oxygen, and in wine made from botrytized fruit. A 20 mg/L addition that measures out at 14 mg/L of new free SO2 is unremarkable.
The discipline that fixes this is simple and rarely followed: measure free SO2 before the addition, calculate the dose, make the addition, then measure again after the wine has had time to equilibrate. The second measurement is the one that tells you whether the lot is protected. The calculation only tells you what you attempted.
Every one of those additions is also a record. Federal wine production regulations require the treating material, the quantity, the date, and the wine it was added to be documented, and additions records are among the first things examined in a TTB audit. The chemistry and the paperwork are generated by the same event, which is a good argument for capturing them in the same place at the same moment.
Frequently asked questions
What is the formula for molecular SO2?
Molecular SO2 equals free SO2 divided by the quantity one plus ten raised to the power of pH minus 1.81. The constant 1.81 is the pKa of the sulfur dioxide and bisulfite pair at 20 degrees Celsius. Rearranged, the free SO2 you need equals your molecular target multiplied by one plus ten to the power of pH minus 1.81.
What molecular SO2 should I target for red and white wine?
The conventional targets are 0.5 mg/L molecular SO2 for red wine and 0.8 mg/L for white wine. The lower red target is deliberate: it leaves room for malolactic fermentation, which 0.8 mg/L would inhibit. Sweet and dessert wines are often held higher because residual sugar gives spoilage organisms something to work with.
How much free SO2 do I need at pH 3.6?
About 31 mg/L free SO2 to reach 0.5 mg/L molecular, or about 50 mg/L to reach 0.8 mg/L molecular. Compare that to pH 3.2, where the same targets need only 13 and 20 mg/L. Four tenths of a pH unit more than doubles the sulfur requirement.
What is the legal limit for SO2 in wine in the United States?
TTB caps total SO2 in wine at 350 parts per million. Separately, any wine containing 10 parts per million or more of total SO2 must carry a sulfite declaration on the label. The 350 ppm figure is a total SO2 ceiling, not a free SO2 ceiling, so it includes the bound fraction as well.
Why does high pH wine need so much more sulfur dioxide?
Only the molecular form of SO2 is antimicrobial, and the fraction of free SO2 existing in that form is determined entirely by pH. At pH 3.0 roughly 6 percent of free SO2 is molecular; at pH 3.8 it is closer to 1 percent. The dose has to rise to compensate, which is why acid adjustment is often cheaper protection than more sulfur.
How much potassium metabisulfite raises free SO2 by a given amount?
Potassium metabisulfite is roughly 57 percent SO2 by weight, so about 1.75 grams per hectoliter raises free SO2 by 10 mg/L in theory. Real recovery is lower because some of the addition binds immediately to acetaldehyde and other carbonyls, particularly in young or oxidized wine. Always re-measure free SO2 after an addition rather than assuming the calculated figure landed.
Does molecular SO2 need to be recorded for TTB purposes?
The molecular figure itself is a cellar calculation, not a reportable number. What TTB requires is a record of the treating material added, the quantity, the date, and the lot it went into, kept as part of your wine production records. Molecular SO2 is how you decide the dose; the addition record is what documents it.
The number that matters is not the one on the lab sheet
Free SO2 is what the assay reports. Molecular SO2 is what protects the wine, and the two only line up if you know the current pH of the lot in front of you. A cellar that carries one blanket free SO2 target across every lot is simultaneously over-sulfuring its low pH wines and leaving its high pH wines exposed, and the lab sheet will look fine in both cases. That is the failure mode worth designing against: not a bad number, a right number applied to the wrong wine.
Solera keeps pH and free SO2 on the same lab record per lot in the Vintage and Lab module, so the molecular figure follows the wine as its chemistry moves, and every sulfur addition writes to an append-only additions log in the TTB Compliance module. The cellar decision and the compliance record come out of one entry instead of two systems that have to be reconciled later.
This page is informational and not legal or regulatory advice. Verify current SO2 limits and labelling requirements with TTB or your compliance counsel before relying on them.