Sulfuric Acid

Sulfuric acid (H₂SO₄, CAS 7664-93-9) and oleum (fuming sulfuric acid, CAS 8014-95-7) are produced and shipped as industrial acids with total acidity expressed as weight percent H₂SO₄ equivalent. Concentrated acid at 98% H₂SO₄ has a density of 1.84 g/cm³ at 20 °C; oleum at 20% free SO₃ has an equivalent H₂SO₄ concentration of 104.5% by mass and a density near 1.9 g/cm³. The trade-inquiry phrase “Is Oleum on the Suspension List? Fuming Sulfuric Acid Export Status” is resolved by separating chemical-weapons control schedules from dangerous-goods transport classification and national sanction screening. Sulfuric acid and oleum are not identified in Schedules 1 through 3 of the Chemical Weapons Convention Annex on Chemicals; the primary cross-border instrument is the United Nations Model Regulations, under which oleum is assigned UN1831, Class 8, PG I.

What separates sulfuric acid and oleum in the dangerous-goods list?

Sulfuric acid containing more than 51% by mass is classified as UN1830, Class 8, PG II. Sulfuric acid with not more than 51% acid is UN2796, Class 8, PG II. Oleum is UN1831, Class 8, PG I because it contains free SO₃, which reacts violently with water and evolves sulfur trioxide vapour. Spent sulfuric acid is UN1832, Class 8, PG II. The packing group assignment for oleum imposes stricter tank, intermediate bulk container, and label requirements than ordinary concentrated acid. In trade compliance, the term “suspension list” frequently refers to a national or customer-specific hold list rather than a single global export prohibition. No global suspension under the United Nations Model Regulations exists for oleum, but carriers may place specific consignments on hold when the free SO₃ concentration or packaging configuration exceeds the limits stated in the shipper’s safety data sheet.

Dilution of concentrated sulfuric acid is an exothermic operation; the integral enthalpy of mixing for 98% acid with water exceeds 880 kJ/kg of acid. In production-scale blending, acid is metered below the liquid surface in a mechanically agitated vessel rather than adding water to strong acid or oleum. The addition order is inverted deliberately: acid is added to water, and the acid inlet is positioned below the impeller to avoid local stratification and vapour generation. Mixing tanks for this service are typically glass-lined or PTFE-lined steel. The resulting adiabatic temperature rise can exceed 100 °C when diluting from 98% to battery-electrolyte strength without jacket cooling or external heat exchange. Battery electrolyte is blended to a density of 1.28 g/cm³ at 25 °C, corresponding to approximately 37% H₂SO₄ by mass.

Storage shell metallurgy, vapour space conditioning, and nozzle velocities

Carbon steel is used for concentrated sulfuric acid storage because a ferrous sulfate film passivates the metal surface. The design of sulfuric acid and oleum storage tanks follows NACE SP0294, which addresses plate selection, weld preparation, bottom-to-shell details, and inspection for concentrated acid service. Oleum tanks require dry air or nitrogen padding; moisture ingress reacts with free SO₃ to produce H₂SO₄ mist and can accelerate corrosion at the vapour-liquid interface. Vent dryers or desiccant breathers are standard on oleum storage installations. Pump selection for oleum transfer often uses sealless or magnetically coupled pumps with wetted parts of cast iron or carbon steel. Published data for a single universal velocity limit in oleum piping is limited; equipment suppliers commonly specify low fluid velocity to avoid erosion of the passive sulfate film. Carbon steel is not applied without qualification to intermediate and dilute acid concentrations where the passive film is less stable.

In petroleum refinery sulfuric acid alkylation, fresh acid is maintained at 98% to 99.5% H₂SO₄, and spent acid is purged when the titratable acidity falls to approximately 88% to 92%. The spent acid stream contains water, acid-soluble oils, and residual hydrocarbons; storage at strengths below 85% requires different metallurgy and generates higher corrosion rates in carbon steel. Regeneration units concentrate the spent acid by thermal decomposition to SO₂, followed by catalytic oxidation to SO₃ and absorption in acid. Acid consumption in sulfuric acid alkylation is influenced by feed olefin type and acid-to-hydrocarbon ratio; design values are taken from licensor data rather than a single published universal figure.

When sulfur trioxide vapour pressure exceeds airborne exposure limits in oleum operations

Oleum liberates SO₃ vapour even at ordinary storage temperatures. The vapour space around 20% fuming oleum contains sulfur trioxide, which combines with atmospheric moisture to form sulfuric acid mist. Airborne inorganic acid exposure is regulated under 29 CFR 1910.1000 Table Z-1 with an 8-hour TWA PEL of 1 mg/m³ for sulfuric acid; NIOSH lists an IDLH of 15 mg/m³. Analytical determination of airborne sulfuric acid uses NIOSH Method 7903, which specifies filter washing and ion chromatography. Oleum storage vents are connected to packed scrubbers using 5% to 10% sodium hydroxide solution; scrubber liquor is recirculated and neutralized before discharge. Mist eliminators in the vapour line reduce aerosol carryover. The absence of an export suspension does not alter the requirement to control these workplace and environmental emissions.

Is Oleum on the Suspension List? Fuming Sulfuric Acid Export Status

Within the Chemical Weapons Convention Annex on Chemicals, sulfuric acid and oleum are not included in Schedules 1, 2, or 3. The dangerous-goods classification UN1831, Class 8, PG I remains the primary cross-border documentation requirement for oleum. Under the United States export administration framework, sulfuric acid and oleum are not identified as controlled items on the Commerce Control List for ordinary industrial use; published exporter guidance generally assigns these products as EAR99 unless they are specially destined for a prohibited end-use, sanctioned destination, or denied party. Under EU REACH, sulfuric acid and oleum are registered substances, and oleum does not appear in Annex I of the Prior Informed Consent Regulation for export notification. Because national suspension lists and customer-specific hold lists can change without technical notice, shippers must verify the consignee, destination, and end-use against current sanctions lists at the time of shipment. Published data for some jurisdictions is limited; the absence of a specific entry in the schedules above should not be read as a complete absence of trade controls under catch-all provisions or national security screening.

In the sulfate process for titanium dioxide production, ilmenite is digested with concentrated sulfuric acid at strengths between 85% and 98% H₂SO₄. The exothermic digestion requires acid-proof brick-lined reactors and controlled agitation; the subsequent hydrolysis stage adjusts acidity and temperature to precipitate hydrated titanium dioxide. The resulting weak sulfuric acid stream is recycled or neutralized. Because the digestion liquor contains metal sulfates and residual free acid, the materials selection for pumps, valves, and filter presses follows acid-resistant alloys or polymer-lined equipment. This application is mature, and the processing conditions are governed by site-specific energy balances rather than a single published ISO test method.

Battery electrolyte filling stations use conductivity and density sensors to maintain the target concentration within ±0.01 g/cm³ at a density set point of 1.28 g/cm³ at 25 °C. Temperature compensation is required because density changes with electrolyte temperature. Limitations include the requirement to use deionized water with chloride and iron concentrations below battery-grade limits; hard water introduces precipitation and self-discharge. Fill volume is controlled by cell type, and exposed terminals must be protected from acid mist. The blending operation is closed-loop and automatically stops when the recirculation loop deviates from the density set point.