Work out sulphite (so2) addition instantly with clear inputs, formula shown and shareable results.
Only molecular sulphur dioxide is antimicrobial, and its fraction of free SO2 collapses as pH rises: about 6% at pH 3.0 but under 2% at pH 3.6. Targeting 0.8 mg/L molecular SO2 therefore requires far more free SO2 in a high-pH wine. Potassium metabisulphite is 57% available SO2.
Molecular fraction
Fraction = 1 / (1 + 10^(pH - 1.81))
Addition
SO2 (g) = volume x deficit / 1000; KMS (g) = SO2 / 0.57
Sulphite levels are legally capped and must be declared above threshold levels. Sulphites cause serious reactions in sensitive individuals; measure rather than estimate free and total SO2.
Because the same free SO2 gives radically different protection at different pH. A wine at pH 3.7 needs roughly three times the free SO2 of one at pH 3.2 for equal antimicrobial effect.
Legal maxima, typically 150-200 mg/L total for dry wines and higher for sweet, plus sensory thresholds. Labelling of sulphites above 10 mg/L is mandatory in most markets.