SO₂ management from crush to bottle: free, molecular and the math behind every addition

Sulfur dioxide is the single most important additive in the cellar and the one most often mismanaged — usually because the wrong number is being watched. Free SO₂ is what the analysis reports, but molecular SO₂ is what actually protects the wine, and the two only line up once you bring pH into the picture. This is a practical walk through SO₂ from crush to bottle: the fractions, the targets, and the arithmetic behind every addition.

Bound, free and molecular — three different things

Total SO₂ is everything you added. Most of it binds almost immediately to acetaldehyde, sugars and pigments and becomes bound SO₂ — still counted, but no longer active. What is left is free SO₂, and only a small fraction of that free pool exists as molecular SO₂, the uncharged gas that is antimicrobial and antioxidant.

• Total SO₂ — free + bound. Capped by law (varies by wine type and market).
• Free SO₂ — the reserve available to protect the wine; what most labs report.
• Molecular SO₂ — the active fraction, set by free SO₂ and pH. This is the number that matters.

Why pH is half the equation

The share of free SO₂ that is molecular collapses as pH rises. At pH 3.0 roughly 6% of your free SO₂ is molecular; at pH 3.6 it is under 2%. So the same 30 mg/L of free SO₂ that comfortably protects a crisp white can leave a high-pH red effectively unprotected. A common protective target is around 0.5–0.8 mg/L molecular — which means the free SO₂ you need is not a fixed number, it is a function of pH.

The formula: molecular SO₂ = free SO₂ ÷ (1 + 10^(pH − 1.81)). Rearranged, free SO₂ needed = molecular target × (1 + 10^(pH − 1.81)). At pH 3.5, hitting 0.5 mg/L molecular needs ≈ 25 mg/L free; at pH 3.7 it needs ≈ 39 mg/L.

Targets through the wine’s life

• At crush — a modest dose (or none, for some reds and natural-leaning wines) to slow oxidation and wild flora while yeast establishes. Never sulfite hard before a wanted malolactic.
• After malolactic fermentation — the first real free-SO₂ target, once the bacteria have finished, to lock in stability.
• During ageing — top up to hold the molecular target as SO₂ binds and evaporates; check periodically, not once.
• At bottling — the most important adjustment: set free SO₂ to hold the molecular target through the wine’s market life, allowing for the drop after filtration and filling.

Turning a target into grams

Once you know the free SO₂ you want to add, the product quantity depends on its strength: potassium metabisulfite is ~57% SO₂, sodium metabisulfite ~67%, and a percentage solution is grams of SO₂ per 100 mL. For a tank, multiply the free-SO₂ increase (mg/L) by the volume (L) to get milligrams of pure SO₂, then divide by the product fraction. The public calculator does this — and the pH-to-molecular conversion — in one step.

A number is only useful if you can find it again. In GrapeFlow every SO₂ addition is logged against the lot with its date, so the ageing top-ups and the bottling adjustment build a record that is ready for an audit or a recall.

These are planning guidelines. Always confirm with your own analysis, legal total-SO₂ limits for your wine and market, and bench trials before dosing a tank.