Glossary · Chemistry
Molecular sulphur dioxide
Also written molecular SO2. Each is a name for the same thing, which is why none of them has an entry of its own.
Definition
The undissociated fraction of free sulphur dioxide, and the only form that is antimicrobial. Its share of the free sulphite is governed by pH and falls sharply as pH rises.
- Subject
- Chemistry
- Also written
- molecular SO2
In more detail
A working target of around 0.5 to 0.8 mg/L molecular SO₂ is used for microbial protection in wine and is applied similarly in cider. Reaching that target needs perhaps 20 mg/L of free SO₂ at pH 3.2 and several times as much at pH 3.8 — which is why the practical advice is to fix the pH before increasing the dose.
Related terms
Terms that belong to the same part of the subject, or that this one is routinely confused with.
Chemistry
Free sulphur dioxide
The portion of added sulphite still in equilibrium in solution as molecular SO₂, bisulphite and sulphite ions, and therefore still available to protect the cider.
Chemistry
Sulphur dioxide
The antimicrobial and antioxidant additive of cider and wine-making, added as a metabisulphite salt or as gas. In solution it exists as molecular SO₂, bisulphite and sulphite, in proportions set by pH.
Chemistry
pH
A logarithmic measure of hydrogen ion activity, and in cider the single most important number for microbial safety. Apple juice usually falls between pH 3.2 and 3.9.
Fermentation
Sulphiting
Adding sulphur dioxide, usually as potassium or sodium metabisulphite, to juice or cider to suppress spoilage organisms, inhibit oxidation and enzymatic browning, and give a selected yeast a clear field.
Where to go next
- Cider glossary — The whole vocabulary, by subject and A–Z.
- Comparisons — Terms that are only really clear when set against the thing they are not.
- Search — Finds terms by their aliases and by their spellings in other languages.
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.