Sulfurous Acid
Properties
| State | Exists only as an aqueous solution of SO2 in equilibrium with bisulfite |
| Color | Colorless to pale yellow |
| Solubility | Miscible with water (SO2 itself dissolves to 94 g/L at 25°C, equivalent to about 1.5 M H2SO3) |
| Melting Point | Not applicable — solution only |
| Boiling Point | Not applicable — decomposes back to SO2 and H2O on heating |
About Sulfurous Acid
Sulfurous acid, written H2SO3 (82.079 g/mol), is the textbook formula for what dissolved SO2 in water actually does — but the molecule itself has never been isolated. Spectroscopic evidence (Raman and IR studies on aqueous SO2 down to liquid-helium-cooled inert-gas matrices) shows that the dominant species in solution is hydrated SO2 (SO2·nH2O) plus the bisulfite anion HSO3⁻ in equilibrium, not free H2SO3 molecules. So when textbooks write SO2 + H2O ⇌ H2SO3 ⇌ H+ + HSO3⁻, the middle term is a useful bookkeeping fiction. That said, the chemistry of the system behaves as if H2SO3 were real: it is a weak diprotic acid with apparent pKa1 = 1.85 and pKa2 = 7.2, so at wine pH (3.0–3.6) the dominant species is HSO3⁻ with a small but biologically important fraction of molecular SO2; at neutral pH the system is mostly bisulfite; and above pH 7 it is mostly sulfite SO3²⁻. The bisulfite and sulfite salts (sodium metabisulfite Na2S2O5, sodium bisulfite NaHSO3, sodium sulfite Na2SO3, potassium metabisulfite K2S2O5) are the practical reagents that food, photography, paper, and textile industries actually buy and ship, because the free aqueous acid loses SO2 to the headspace and cannot be sold as a stable concentrate. The system serves as a mild reducing agent (S(IV) → S(VI)) and as a nucleophile that adds to aldehydes to form the bisulfite adducts used in classical aldehyde purification.
Where you'll encounter it
If you've ever opened a bottle of wine that smelled briefly like a struck match, that's the headspace SO2 in equilibrium with the dissolved bisulfite the winemaker added — about 25–50 mg/L total free SO2 typically. In a darkroom (the few that still exist), the stop bath after the developer is dilute acetic acid, but the fixer's preservative against air oxidation is sodium sulfite, which keeps thiosulfate from disproportionating. In a paper mill running the sulfite pulping process (now mostly displaced by kraft, but still active for specialty dissolving pulps and rayon-grade cellulose), wood chips are cooked at 130–160°C in a calcium or magnesium bisulfite cooking liquor that hydrolyzes lignin without breaking the cellulose chains as severely as the kraft sulfide route. And in any organic chemistry undergraduate lab, the classic separation of an aldehyde from a ketone by saturated NaHSO3 wash is the bisulfite addition reaction in action — aldehydes form crystalline bisulfite adducts that fall out of solution while ketones (mostly) do not react.
Common Uses
- Wine preservation as molecular SO2 against acetic acid bacteria and Brettanomyces
- Antioxidant and antimicrobial in dried fruit, fruit juice concentrate, and brined vegetables (E220–E228)
- Cooking liquor in calcium- and magnesium-bisulfite sulfite pulping for dissolving-grade cellulose pulp
- Reducing agent for dechlorinating municipal water treatment plant effluent before discharge
- Bleaching agent for wool, silk, straw, and mechanical pulp where chlorine would yellow the fibers
- Bisulfite addition reagent for purifying aldehydes by selective crystalline adduct formation
- Photographic fixer preservative as sodium sulfite to inhibit thiosulfate disproportionation
Safety Information
GHS: H314 causes severe skin burns and eye damage (concentrated solutions), H331 toxic if inhaled (SO2 evolved from solution). OSHA PEL for SO2 (the actual exposure form because the equilibrium liberates the gas) is 5 ppm 8-hr TWA; ACGIH TLV is 0.25 ppm STEL because asthmatics show bronchoconstriction at 0.5 ppm. The acid solution attacks copper, brass, and most non-stainless steels — use 316L, glass-lined, or PTFE-lined equipment for storage and transfer. Sulfite-induced asthma affects 5–10% of asthmatics, which is why FDA and EU regulations require 'contains sulfites' labeling on any food or beverage above 10 mg/kg. Spills are best handled by neutralization with dilute soda ash plus oxidation with peroxide to convert any residual sulfite to harmless sulfate before flushing.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.