Sulfuric acid (CAS 7664-93-9) is a dense, highly reactive mineral acid with molecular weight 98.08 g/mol and EC listing 231-639-5. Concentrated merchant material is supplied predominantly at 93% to 98% H₂SO₄, while oleum grades contain free SO₃ above 100% acid equivalent. At 20 °C, 98% acid has a density of approximately 1.84 g/cm³ and a dynamic viscosity near 26 cP; 93% acid has a density near 1.83 g/cm³. The compound is miscible with water in all proportions, but dilution liberates sufficient heat to raise local temperature above the boiling point of water, so only acid-to-water addition is permissible. Under CLP Regulation (EC) 1272/2008, concentrated sulfuric acid is classified H314 for severe skin burns and eye damage; transport is governed by UN 1830 with corrosivity packaging group I.

Why does neutralization capacity vary with dilution and temperature?

The first dissociation of sulfuric acid in water is effectively complete in dilute solution, but the second dissociation constant at 25 °C is approximately 1.99, so HSO₄⁻ behaves as a weak acid after the first proton is consumed. In neutralization to sulfate endpoint, pH rises above 4 only after the second proton is neutralized. A wastewater neutralization station with a pH electrode calibrated according to ASTM D1293-18 or ISO 10523:2008 will show a buffered plateau between pH 2 and 4, which requires additional caustic before the discharge band of pH 6 to 9 is reached. Temperature directly affects reaction time: mixed neutralization tanks operating below 10 °C may require 20 min to 30 min residence time, while the same endpoint is typically achieved in 10 min to 15 min at 25 °C to 35 °C. Complete conversion to sodium sulfate consumes approximately 2 mol NaOH per 1 mol H₂SO₄; dissolved metals and bicarbonate alkalinity alter actual titrant demand.

Concentration Grade Purity Limits Define Handling and Storage Boundaries

Reagent-grade sulfuric acid meeting ACS Reagent Chemicals specifications is normally supplied at assay 95.0% to 98.0% H₂SO₄, with impurity limits for chloride at ≤0.2 ppm, nitrate at ≤0.5 ppm, and iron at ≤0.2 ppm. Electrolyte grades for lead-acid batteries are blended with demineralized water meeting ASTM D1193 Type II or ISO 3696 Grade 3, and finished electrolyte density is measured after temperature correction to 25 °C. A 98% acid with total iron above 50 mg/kg is generally avoided in electronics-grade or catalyst-manufacturing service because metal carryover affects downstream quality. Oleum grades of 20%, 30%, and 65% free SO₃ require vapour suppression systems because free SO₃ evolves at ambient storage and must be vented through scrubbing. For the 98% acid, the freezing point lies in the range of 3 °C to 10 °C, so heat tracing and insulation are needed in cold-climate tank farms.

Most merchant sulfuric acid is produced by the contact process from sulfur burning, spent acid regeneration, or metallurgical off-gas. A double-contact double-absorption unit that burns sulfur to SO₂, oxidizes SO₂ to SO₃ over vanadium pentoxide catalyst, and absorbs SO₃ in 98% acid can achieve SO₂ conversion above 99.7% and stack SO₂ concentrations below 200 ppmv during stable operation. The absorber is operated with 98% acid because at this concentration the equilibrium pressure of SO₃ over the liquid is minimized. Dilution water is metered precisely to hold concentration at 98%; deviation toward 93% sharply reduces SO₃ absorption efficiency, while deviation toward oleum increases vapour pressure and handling hazard. This production intensity explains why 98% technical grade remains the principal merchant specification for transport and large-scale consumption.

In concentrated form at ambient temperature, sulfuric acid can be stored in carbon steel tanks fabricated to API 650, provided a corrosion allowance of 3 mm to 6 mm is incorporated and acid velocity is maintained below 0.6 m/s to limit erosion-corrosion of the protective iron sulfate film. Dilution below 93% or moisture ingress accelerates attack, and heated acid above 40 °C requires corrosion-resistant alloys or fluoropolymer linings. Piping systems constructed under ASME B31.3 for concentrated acid services frequently use Schedule 80 carbon steel or Alloy 20; gaskets and seals in PTFE or fluoroelastomer are selected based on acid concentration and temperature. Hydrogen gas evolved from corrosion cells can accumulate in tank headspace and requires venting in accordance with API 2000. For dilute acid service, high-density polyethylene, PVDF, or fiberglass-reinforced plastic tanks are specified. Titanium is generally avoided above 5% acid by mass at room temperature because acid attack can initiate.

Vapour Pressure, Viscosity, and Respiratory Protection Thresholds

The vapour pressure of sulfuric acid increases with concentration and temperature, but the main airborne hazard is acid mist rather than volatile acid molecules. In open-surface pickling or anodizing baths above 55 °C, mist suppressants and local exhaust ventilation are specified because airborne droplet mass can meet or exceed the ACGIH TLV-TWA of 0.2 mg/m³ thoracic particulate. The OSHA PEL is 1 mg/m³ TWA and the NIOSH IDLH is 15 mg/m³. Viscosity of 98% acid near 26 cP at 20 °C declines rapidly with increasing temperature and dilution; this changes pump selection and flowmeter calibration because a positive-displacement gear meter sized at 25 °C will over-read if the acid is heated to 50 °C unless temperature compensation is active. Coriolis mass flowmeters with Hastelloy C-276 wetted parts are used in acid duty because they measure mass directly and avoid volumetric viscosity drift.

When Import Duty Structures Shift Landed-Cost Baselines

Sulfuric acid logistics are freight-intensive. A tariff differential of ten percentage points changes the marginal cost curve for regional buyers even before ocean freight, terminal storage, and local delivery are added. Sulfuric Acid Import Duty Worldwide: Thailand 30%, Malaysia 20%, Indonesia 15% under Harmonized System subheading 2807.00.00 produces a spread that can exceed the ocean freight difference between Thai and Indonesian discharge ports. On a notional bulk import of 1,000 tonnes at CFR USD 120 per tonne, the import duty charge is USD 36,000 in Thailand, USD 24,000 in Malaysia, and USD 18,000 in Indonesia, subject to MFN applied rates and excluding any preferential tariff under a regional trade agreement. Preferential origin status under the ASEAN Trade in Goods Agreement may reduce or eliminate these rates if the goods satisfy the applicable origin rule; published product-specific guidance for sulfuric acid under ATIGA is limited, so shippers must verify the current legal text and customs ruling before structuring contracts.

Supply-chain modeling should use duty-paid value rather than CFR value; the tariff is applied to the customs value, which normally includes ocean freight and insurance. A terminal receiving 5,000 DWT acid by barge may incur port charges, demurrage, and storage rent that vary independently of duty rate but interact with duty-paid inventory carrying cost. For this reason, the Thai rate of 30% acts as a direct barrier to small and mid-volume importers lacking bonded tank-farm capacity, while the Indonesian rate of 15% creates a landed-cost advantage for downstream fertilizer and metal-processing facilities near port storage.

Across wet-process phosphoric acid acidulation trains, the reaction between phosphate rock and sulfuric acid consumes 2.4 to 2.8 tonnes of 100% H₂SO₄ basis per tonne of P₂O₅ recovered, depending on rock reactivity, acid concentration, and gypsum filtration rate. Acidulation reactors operate at 70 °C to 80 °C and maintain slurry solids loading between 30% and 40% by mass; gypsum crystals are grown to 100 µm to 200 µm to maximize filter wash efficiency. Wet-process phosphoric acid plants use agitated multi-compartment reactors with agitator power density of 0.6 kW/m³ to 1.0 kW/m³; excessive shear shortens crystal growth, while insufficient shear causes settling and hot spots. Single superphosphate manufacture, by contrast, mixes 68% to 72% H₂SO₄ with ground phosphate rock in a pug mill or den, and the resulting exothermic reaction sets the mass into a porous den before granulation and curing.

Continuous carbon-steel pickling lines use heated 10% to 20% H₂SO₄ at 60 °C to 90 °C with acid inhibitors such as propargyl alcohol or thiourea to limit base metal loss. Acid consumption in strip pickling ranges from 10 kg to 30 kg H₂SO₄ per tonne of steel, and line speed is reduced when iron concentration in the pickle liquor exceeds 100 g/L FeSO₄. Spent liquor containing free acid and iron sulfate is either displaced to an acid recovery unit or neutralized with lime in a precipitation system. In lead-acid batteries, sulfuric acid electrolyte is filled at specific gravity 1.230 g/cm³ to 1.280 g/cm³ at 25 °C after formation charging; the electrolyte density is a direct indicator of state of charge, with a drop from 1.280 g/cm³ to 1.150 g/cm³ corresponding to near-full discharge. Sulfuric acid alkylation of isobutane and light olefins uses 88% to 98% acid as catalyst; acid consumption in a well-operated alkylation unit is 0.2 kg to 0.6 kg per barrel of alkylate. Spent alkylation acid containing acid-soluble oils is regenerated in a decomposition furnace at 1,000 °C to 1,200 °C, where acid is thermally cracked to SO₂ and then reconverted to fresh acid in a contact-process absorption train.

Sulfate-process titanium dioxide manufacture dissolves ilmenite or titanium slag in concentrated sulfuric acid at 150 °C to 180 °C in batch digesters; the subsequent hydrolysis step precipitates hydrated TiO₂, and the weak acid stream is either recycled to digestion or sent to neutralization. Acid consumption per tonne of finished TiO₂ is 3.5 to 4.5 tonnes H₂SO₄ on a 100% basis. Iron sulfate by-product is separated by vacuum cooling crystallization to produce ferrous sulfate heptahydrate, which can be sold or processed further for iron oxide and fresh acid recovery depending on local market conditions.

Occupational exposure control for sulfuric acid mist relies on capture velocity at open-surface tanks of 0.25 m/s to 0.5 m/s across the tank opening, backed by packed-bed or wet electrostatic mist eliminators. Acid-resistant PPE includes splash goggles and face shields tested to ANSI/ISEA Z87.1-2020, gloves of neoprene or butyl rubber, and body protection meeting acid-splash testing. Emergency showers and eyewash stations must conform to ANSI/ISEA Z358.1-2014 and be reachable within 10 seconds from the hazard zone. Spent sulfuric acid is neutralized with hydrated lime slurry to a target pH of 6 to 9 before discharge; sulfate in drinking water is subject to a secondary standard of 250 mg/L under U.S. EPA National Secondary Drinking Water Regulations. Dewatered calcium sulfate sludge is filter-pressed to 50% solids or higher and may be disposed to permitted landfill if it meets leachability criteria under local waste regulations.