Skip to content
CiderHQ
Search

Tool

Is my sulphite actually doing anything?

The active fraction of free sulphur dioxide at a given pH — the relationship that explains why a low-acid bittersweet juice is genuinely hard to protect, and why blending in sharp fruit is a microbiological decision.

Molecular SO₂

This has to be a measurement. It cannot be inferred from how much was added — a large and unpredictable fraction binds immediately.

Cider juice is normally 3.0–4.0. Bittersweet fruit sits at the high end.

Molecular SO₂

0.40 mg/L

Active fraction: 1.6%

How this is worked out
  1. Formula: molecular SO₂ = free SO₂ ÷ (1 + 10^(pH − 1.81))
  2. Substituted: 25.0 ÷ (1 + 10^(3.60 − 1.81))
  3. Result: 0.40 mg/L — 1.6% of the free SO₂ is in the active form
What this assumes (4)
  • The 1.81 constant is the first dissociation constant of sulphurous acid at typical cider temperature. It shifts slightly with temperature and ionic strength.
  • This tool reports what a measured free SO₂ figure amounts to. It does not tell you how much to add: an addition is a food-safety decision that depends on legal limits, on the juice, and on what you are protecting against, and it should be made from the guidance of your supplier or regulator rather than from a web calculator.
  • Free SO₂ has to be measured. It cannot be inferred from how much was added, because a substantial and unpredictable fraction binds immediately to aldehydes and sugars in the juice.
  • At pH values above about 3.8 the molecular fraction is so small that sulphite is not a practical defence, whatever the free figure says. Acid adjustment or blending is the answer, not more sulphite.

Why this is a pH question

Sulphur dioxide only works as an antimicrobial in its undissociated molecular form, and the share of free SO₂ in that form collapses as pH rises. At pH 3.0 something like six per cent of it is active. At pH 3.8 it is around one per cent. The same addition that comfortably protects a sharp juice does almost nothing to a bittersweet one.

That is why blending sharp fruit into a low-acid blend is a microbiological decision as much as a flavour one, and it is why traditional West Country practice — which is built on high-pH bittersweet juice — depended on other defences entirely: a large healthy yeast population, a cold cellar, and full vessels.

Free SO₂ has to be measured rather than inferred. A substantial and unpredictable fraction of any addition binds immediately to aldehydes and sugars in the juice and is no longer available.

Related tools

Where this reading comes from

Guides that reach this calculator, and what the number decides once you have it.