Wine Molecular SO2 & Sulfite Addition Calculator
Model pH-dependent sulfur dioxide equilibrium, target molecular SO2 antimicrobial protection, and exact potassium metabisulfite (KMBS) dosing.
Wine Chemistry & Target
Sulfite Source & Binding Factor
Dosage Recommendation
Microbial Protection Status
Recommended Tools & Equipment
Tested hardware and components for high reliability
Engineering Fundamentals: Molecular SO2 & Wine pH Equilibrium
Sulfur dioxide (SO2) in wine exists in an aqueous chemical equilibrium between three forms: molecular SO2 (SO2·H2O), bisulfite ion (HSO3-), and sulfite ion (SO32-). Only the uncharged molecular SO2 fraction can penetrate microbial cell membranes to inhibit spoilage yeasts (Brettanomyces) and acetic/lactic bacteria.
1. Henderson-Hasselbalch Mathematical Relation
The fraction of free SO2 that exists in the active molecular state is governed strictly by wine pH:
Molecular SO2 = Free SO2 / [ 1 + 10^(pH - 1.81) ]Target Free SO2 = Target Molecular SO2 × [ 1 + 10^(pH - 1.81) ]
Because the relation is logarithmic, a wine at pH 3.80 requires over 3.5 times more free SO2 to achieve the same microbial protection as a wine at pH 3.20!
2. Potassium Metabisulfite (KMBS) Dosing
KMBS (K2S2O5) has a molecular weight of 222.3 g/mol and yields 57.6% active SO2 by mass:
KMBS (grams) = [ Liters × (Δ Free SO2 ppm) ] / (1000 × 0.576 × (1 - BindingLoss/100))
Frequently Asked Questions
Why does wine pH dramatically alter sulfur dioxide effectiveness?
SO2 exists in a pH-dependent dissociation equilibrium. At low pH (3.0), approximately 6% exists as active antimicrobial molecular SO2. At high pH (3.9), less than 0.8% is molecular, requiring over seven times higher total sulfite additions to stop spoilage organisms.
What molecular SO2 targets are recommended for white vs. red wines?
White and rosé wines target 0.8 mg/L molecular SO2 to prevent oxidation, browning, and malolactic fermentation. Red wines target 0.5 to 0.6 mg/L because natural red wine grape polyphenols and anthocyanins provide substantial additional antioxidant buffering.
What is the percentage of active SO2 in Potassium Metabisulfite (KMBS)?
Pure Potassium Metabisulfite (K2S2O5) contains exactly 57.6% available sulfur dioxide by weight. The remaining 42.4% consists of bound potassium.
Why do young wines bind a portion of added sulfur dioxide?
Acetaldehyde, pyruvic acid, alpha-ketoglutaric acid, and residual sugars bind free SO2 into bisulfite adducts within 24 to 48 hours. When calculating cellar additions, winemakers typically budget an extra 20% to 35% binding loss factor.