Molecular SO₂ Calculator — Wine Sulfite Management
Only the molecular (undissociated H₂SO₃) form of SO₂ is antimicrobially active in wine. Enter your free SO₂ reading and wine pH to find the molecular SO₂ level, and calculate how much free SO₂ you need to hit a protective target (0.8 mg/L is the standard benchmark).
mg/L
mg/L
minimum protection (0.5–0.8 mg/L)
- 1
pH − pKa₁ exponent
3.5 − 1.81 = 1.69 - 2
Denominator (1 + 10^exponent)
1 + 10^1.69 = 49.9779Lower pH gives a smaller denominator, meaning more SO₂ stays in the protective molecular form. - 3
Molecular fraction
1 ÷ 49.9779 = 0.020009 - 4
Molecular SO₂
35 × 0.020009 = 0.70
How does this calculator work?
Molecular SO₂ (mg/L) = Free SO₂ / (1 + 10^(pH − 1.81)). Only this undissociated H₂SO₃ fraction protects wine against spoilage organisms. At typical wine pH 3.0–4.0, molecular SO₂ is only 1–6% of the free SO₂, so small pH changes require large adjustments to the free SO₂ level to maintain the standard 0.8 mg/L protective target.
Formula
How this is calculated
Sulfur dioxide added to wine distributes into three chemical forms: molecular SO₂ (H₂SO₃), bisulfite (HSO₃⁻) and sulfite (SO₃²⁻). Only the molecular form effectively inhibits spoilage yeasts (Brettanomyces) and bacteria (Acetobacter, Lactobacillus) and acts as an antioxidant. The distribution is governed by the acid–base equilibrium H₂SO₃ ⇌ H⁺ + HSO₃⁻ with pKa₁ = 1.81 at 20 °C (the most widely accepted winemaking value, derived from Ribereau-Gayon et al.).
The molecular fraction is therefore 1/(1 + 10^(pH − 1.81)). At wine pH 3.5 only about 2% of the free SO₂ is molecular; at pH 3.0 it rises to about 6%. This steep pH dependence means a wine at pH 3.8 needs roughly four times as much free SO₂ as the same wine at pH 3.2 to achieve the same molecular SO₂ level. The standard protective target is 0.5–0.8 mg/L molecular SO₂; commercial winemakers often aim for exactly 0.8 mg/L. Legal total SO₂ limits (e.g. EU: 150 mg/L red / 200 mg/L white for dry wines) and sensory thresholds for SO₂ off-flavours also set practical upper bounds on free SO₂.
The pKa₁ value of 1.81 is calibrated for ~20 °C and an ionic strength typical of wine. Temperature and ionic strength affect it slightly, but the variation is small enough that winemakers use the 1.81 constant universally.
Frequently asked questions
Lower pH means higher hydrogen-ion concentration, which pushes the bisulfite equilibrium back toward undissociated H₂SO₃. A larger fraction of your free SO₂ is therefore molecular, so you need less total free SO₂ to hit the 0.8 mg/L protective level.
Free SO₂ is the molecular + bisulfite fraction — all the SO₂ not yet bound to acetaldehyde, sugars or other wine components. It is measured by titration (Ripper method) or by aeration-oxidation. Total SO₂ includes bound fractions and is always higher.
It is the widely cited benchmark for microbial stability, particularly against Brettanomyces. Some winemakers use 0.5 mg/L as a minimum and up to 1.5 mg/L for wines prone to oxidation. Natural/low-intervention winemakers often work well below these levels. Always cross-check against your regional legal limits for total SO₂.
Also known as
TG we-Calculate Editorial Team. (2026). Molecular SO₂ Calculator — Wine Sulfite Management [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/molecular-so2-calculator
TG we-Calculate Editorial Team. "Molecular SO₂ Calculator — Wine Sulfite Management." TG we-Calculate. 2026. https://we-calculate.com/calculator/molecular-so2-calculator.
TG we-Calculate Editorial Team, "Molecular SO₂ Calculator — Wine Sulfite Management," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/molecular-so2-calculator
@misc{wecalculate_molecular_so2_calculator, title = {Molecular SO₂ Calculator — Wine Sulfite Management}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/molecular-so2-calculator}}, year = {2026}, note = {TG we-Calculate} }
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