At pH 3.0, the equilibrium distribution of sulfur dioxide in aqueous wine matrices shifts to approximately 6.8% molecular SO₂, the fraction that penetrates yeast and bacterial cell membranes and exerts preservative action; the remainder exists predominantly as bisulfite ion with negligible antimicrobial activity. In commercial winemaking, liquid sulfur dioxide is metered as a pressurized feedstock through a mass-flow-controlled diffuser into a recirculating must or wine stream, typically at a pre-fermentation dose of 50–80 mg/L total SO₂ for low-pH white juice and a post-malolactic finishing adjustment calculated to maintain 0.6–0.8 mg/L molecular SO₂ after binding by acetaldehyde, sugars, and anthocyanins. The relevant regulatory boundary is not a single fixed addition rate but the residual ceiling in the finished product: FDA 21 CFR 182.3616 recognizes sulfur dioxide as GRAS with current good manufacturing practice limitations, while Commission Delegated Regulation (EU) 2019/934 Annex I and Regulation (EC) No 1333/2008 Annex II set total SO₂ maxima by wine style and residual sugar, with the dry red wine ceiling at 150 mg/L and higher ceilings for sweet, liqueur, and botrytised wines specified in the same annex. Addition ratios are therefore calculated by aeration-oxidation or Ripper titration after a bench-top equilibrium trial, because the dose required to reach the same molecular SO₂ target can vary by a factor of three between pH 3.0 and pH 3.6. A closed-loop dosing skid with a 316L stainless steel lance, PTFE-lined static mixer, and downstream sampling port is standard on production lines handling 100–500 hL tank volumes; the lance is inserted into the lower third of the tank, and the recirculation pump runs at 1.5–2.5 tank volumes per hour during the 30–60-minute sulfiting cycle. The terminal products include dry red and dry white still wines, rosé wine, semi-sweet and sweet wines, and sulfited fruit must or juice concentrates that are later fermented or blended. A process limitation is that molecular SO₂ is continuously lost through oxidation and irreversible binding to carbonyl species, so the measured free SO₂ at bottling may be 15–25% lower than the theoretical value computed from the dosing table after three months of storage.

The pH-dependent speciation is derived from the Henderson-Hasselbalch relation using pKₐ₁ 1.86 for sulfurous acid at 20°C; the table below reports unbound model solution values only, because wine matrices with 20–40 mg/L acetaldehyde and variably polymerized tannins require empirical dose escalation beyond the pure aqueous equilibrium.

Corn Wet-Milling Steep Water Sulfiting: Redox Control and Starch Yield Retention

In the countercurrent wet-milling battery, sulfur dioxide is introduced into the steep water rather than the corn directly, maintaining a controlled reducing environment that disrupts disulfide bonds in the protein matrix surrounding starch granules and suppresses lactic acid bacteria growth. The addition ratio for yellow dent corn is typically 0.15–0.25 wt% (1,500–2,500 mg/L) expressed as total SO₂ in the fresh steep water, with the battery maintained at 49–54°C for 28–48 h; lower SO₂ concentrations below 1,000 mg/L may leave the endosperm protein shell insufficiently reduced, while concentrations above 3,000 mg/L increase vapour-phase SO₂ losses and accelerate corrosion of mild-steel evaporator bodies. Compliance in the United States is governed by FDA 21 CFR 182.3616 as a GRAS processing aid, and finished products containing more than 10 mg/L residual sulfite must bear declaration under 21 CFR 101.100(a)(4); buyers in the EU additionally apply Regulation (EC) No 1333/2008 to the final food matrix, although starch, dextrose, and high-fructose corn syrup produced through steeping normally retain less than 5 mg/kg SO₂ after refining. The production process is a seven-to-ten tank countercurrent steep system in which light steep water of 1,800–2,200 mg/L SO₂ is circulated from the second steep tank back to the first, and fresh liquid SO₂ is injected through a gas-dispersion ring at the suction side of the recirculation pump to preserve the setpoint measured by iodometric titration. The terminal product types include native corn starch, modified and oxidized starch, high-fructose corn syrup, dextrose monohydrate, and corn gluten feed or meal; the starch yield benefit is observed when the steep water redox potential is held between +150 mV and +250 mV Ag/AgCl during the final 8 h of steeping. Process control is compromised if the steep water pH drifts above 4.2 because the equilibrium shifts toward bisulfite and antimicrobial activity declines, requiring higher total SO₂ additions for equivalent endpoint performance.

Why Does the Edeleanu Raffinate Yield Deteriorate Below −5°C?

The Edeleanu extraction of aromatics from straight-run kerosene or lubricating-oil fractions uses liquid SO₂ as a selective polar solvent; at temperatures between 5°C and 15°C and total pressures of 3.5–5.0 bar g, the solvent-to-feed volumetric ratio is typically 1.5:1 to 3.0:1, with the higher ratio reserved for feedstocks containing 20–35% aromatic carbon as measured by ASTM D3238. The process is carried out in a jacketed rotating disc contactor or similar countercurrent column, with chilled SO₂ entering the top section and pre-chilled feedstock entering the bottom; the raffinate phase is the paraffinic overflow, while the extract phase contains dissolved aromatics and some sulphur compounds. After phase separation, the extract is heated and depressurised in a SO₂ recovery train; the solvent is recompressed and condensed for re-injection. The key process conflict is the lower temperature boundary: below −5°C, the selectivity for aromatics improves, but the viscosity of the lubricating-oil feedstock increases, the solubility of paraffins in the extract phase rises, and the interfacial settling rate in the contactor can fall by 40–60%, reducing raffinate yield and requiring longer residence time. Above +20°C, solvent capacity increases but aromatic selectivity falls, producing a raffinate with higher aromatic content. Product compliance is verified by ASTM D1319 fluorescent indicator adsorption for aromatic content, ASTM D2270 viscosity index for lubricating oil raffinates, and ASTM D943 for oxidation stability of hydrotreated base stocks. Terminal product types include dearomatised paraffinic hydrocarbon solvents, low-aromatic printing ink distillates, high-viscosity-index base oils, and white mineral oils that later undergo hydrotreating to remove residual sulphur and olefins. For feedstocks with more than 5% olefins, published data for this specific configuration is limited, and laboratory phase-separation screening is required before commercial solvent ratio selection.

When Liquid SO₂ Replaces Sodium Metabisulfite in Electroplating Rinsewater Cr(VI) Destruction

In electroplating rinsewater treatment, hexavalent chromium is reduced to trivalent chromium by gaseous sulfur dioxide under acidic conditions; the stoichiometric demand is approximately 1.85 kg SO₂ per kilogram of Cr(VI) according to the reaction 3SO₂ + 2H₂CrO₄ → Cr₂(SO₄)₃ + 2H₂O. Field practice at plants discharging under 40 CFR Part 437 and spending 20–30 min reaction time adds 2.0–3.0 kg SO₂ per kg Cr(VI) to maintain a final oxidation-reduction potential of +250 to +280 mV versus Ag/AgCl at pH 2.0–3.0. The reaction is pH-critical: above pH 3.5, the reduction rate becomes kinetically limited and residual Cr(VI) persists in the effluent; below pH 1.8, excess SO₂ off-gassing increases and chemical consumption rises without a corresponding rate benefit. In a production-scale 50 m³/day continuous treatment skid, liquid SO₂ is metered through a corrosion-resistant Hastelloy C-276 or PTFE-lined diffuser into a recirculation loop upstream of a static mixer, while 30–50% sulphuric acid is injected to maintain the pH setpoint; the skid generally includes two identical reduction tanks operated in series, each with a 15–20-minute hydraulic retention time. The endpoint is confirmed by diphenylcarbazide colourimetric measurement or ion chromatography, not by ORP alone, because metallic interference from copper and nickel plating wastes can shift the ORP response. The reduced Cr(III) stream is then neutralised with lime or caustic to pH 8.0–9.5, producing a mixed-metal hydroxide sludge that is thickened and filter-pressed; the treated effluent after filtration typically requires no further reduction, but may require cyanide oxidation and oil separation in integrated metal-finishing facilities. Terminal output includes a filter cake classified for hazardous waste under 40 CFR 261.24 if chromium concentrations exceed regulatory thresholds, clarified effluent suitable for discharge after final pH adjustment, and recovered sodium sulphate-bearing filtrate. Operational boundaries include sulphide precipitation incompatibility: SO₂ treatment before cyanide oxidation can generate toxic hydrogen cyanide at low pH in mixed-metal rinsewater, and SO₂ should not be added simultaneously with hypochlorite or permanganate because competing oxidants consume the reductant.

The Reed Sulfochlorination of N-Paraffins Proceeds at a Solution–Gas Interface, Not in Mixed-Phase Batch

For C12–C18 normal paraffins, liquid SO₂ is both a solvent and a reactant in the light-catalysed Reed reaction that produces alkane sulfonyl chlorides; the addition ratio used in continuous thin-film photoreactors is 1.5–3.0 mol SO₂ per mol n-paraffin with chlorine fed at 1.0–1.2 mol per mol n-paraffin, and the reaction mixture is held at 20–35°C while irradiated at UV wavelengths around 365 nm. The process is operated with the liquid film thickness controlled by lamp geometry and reactor coolant load, because UV penetration and gas–liquid mass transfer control the reaction rate; in bubble-column variants, the SO₂ feed is pre-saturated into the hydrocarbon phase through a sintered diffuser rather than injected as a simple sparge pipe. The intermediate sulfonyl chloride is then hydrolysed continuously with 20–25% sodium hydroxide at 60–80°C to yield sodium secondary alkane sulfonates, with hydrochloric acid recovered from the hydrolysis off-gas. Compliance for the final surfactant blends is established under EU Detergent Regulation (EC) No 648/2004 and REACH registration dossiers, and the detergent precursors are assessed using OECD Test Guideline 301B ready biodegradability screening before commercial notification; residual paraffin content in the sodium alkane sulfonate is typically controlled by vacuum stripping rather than by additional solvent extraction. The terminal product types are secondary alkane sulfonate (SAS) detergent powders and liquids, industrial textile wetting agents, and anionic emulsifiers used in emulsion polymerisation. The process boundary includes strict exclusion of free water from the photoreactor, because water hydrolyses the sulfonyl chloride before it leaves the reactor and causes a drop in selectivity to sulfonate; additionally, iron contamination from carbon steel piping promotes chlorination side reactions and must be avoided by 316L stainless steel or PTFE-lined transfer lines.

In cane and beet remelt sulfitation, liquid SO₂ is metered into clarified syrup or melter liquor 1.5–2.0 m upstream of a pH-controlled surge tank, with the addition rate held at 0.3–0.6 kg SO₂ per tonne of thin juice in beet processing and 0.5–1.0 kg SO₂ per tonne of remelt syrup in cane refineries, depending on the incoming colour measured in ICUMSA units. The objective is to lower the pH of the liquor to 4.0–4.5 and to reduce colour-forming carbonyl and amino precursors before the evaporation and crystallisation stages; the reaction is fast but not instantaneous, and the surge tank provides 30–60 minutes of retention time so that sulfite addition completes before the liquor enters the vacuum pan. Compliance for final white sugar is linked to Codex Alimentarius CXS 212-1999, which limits sulphur dioxide residual in white sugar to 15 mg/kg, and factory quality systems additionally monitor ICUMSA colour, ash, and turbidity after carbonation or sulphitation; in refineries shipping to the EU, the final product must also meet the general food-additive conditions of Regulation (EC) No 1333/2008 for SO₂ used as a processing aid in sugar. Downstream processing uses aeration and high-temperature evaporation to strip unbound sulfur dioxide from the treated liquor before crystallisation, while bound sulfite in the molasses fraction is more persistent and must be tracked in by-product syrup sold for fermentation. Terminal product types include refined granulated white sugar, soft brown sugar, liquid invert sugar, and candy syrups; the process also produces sulfite-containing molasses that may require declaration if sulfite residue exceeds 10 mg/kg in the destination food. Operational limits in sugar refinery use include the incompatibility of SO₂ with carbonatation systems: excess sulfite in the thin juice can dissolve calcium carbonate precipitate in the clarifiers, reducing the clarification effect and forcing an increase in lime consumption.