Sulfuric acid, CAS 7664-93-9, EC 231-639-5, is manufactured through the contact process: dried sulfur is burned to sulfur dioxide, SO2 is catalytically oxidized to SO3 over a cesium-promoted vanadium pentoxide bed at 400–450 °C, and SO3 is absorbed in circulating 98.5 wt% sulfuric acid to avoid the mist formation that occurs when SO3 contacts water directly. The acid is a strong diprotic acid with pKa1 below −3 and pKa2 1.99 at 25 °C; density, viscosity, freezing point, and corrosion rate shift sharply across the water-acid system. Bulk storage vessels for 93–98 wt% acid are carbon steel with 3–6 mm corrosion allowance, while 20% oleum requires lined steel and controlled venting because free SO3 forms corrosive mist with humid air. Centrifugal pumps with silicon carbide mechanical seals are common in bulk transfer when suspended solids are absent.

Because concentrated sulfuric acid is strongly hygroscopic and releases approximately 970 kJ/kg on infinite dilution, industrial dilution is carried out by metering acid into water with jacket cooling to keep temperature below 60 °C. Reverse addition can cause localized boiling and splatter. Storage tanks are fitted with vent dryers, level radar, and either air pad or nitrogen pad depending on the acid’s organic contamination sensitivity.

Thermodynamic Boundaries and Commercial Strength Ranges

Commercial quotations for sulfuric acid are not specification-complete without a concentration, total acidity, or density value. The 98.3 wt% atmospheric azeotrope boils at 337 °C; pure 100% H2SO4 melts at 10 °C, while dilution moves the freezing point toward the water-H2SO4 eutectic near −64 °C at approximately 38 wt% H2SO4. Dynamic viscosity of 98 wt% acid at 20 °C is about 26.7 mPa·s. These values control storage heater sizing, pump suction line velocity, and vent system design. Representative commercial grades are listed in Table 1.

In lead-acid cells, the electrolyte is typically 35–38 wt% H2SO4 with specific gravity 1.265–1.285 at 25 °C. Chloride, iron, manganese, and arsenic are controlled because they reduce hydrogen overpotential and accelerate positive-grid corrosion. Stationary battery purchases often define maximum chloride of 1 mg/kg and maximum iron of 10 mg/kg in the incoming electrolyte acid. Deep-cycle and valve-regulated lead-acid cells require sulfate, antimony, and organic carbon limits that differ from general industrial acid and must be negotiated in the quotation phase.

What Limits Acid Consumption in Paraffin Alkylation?

During isobutane alkylation with C3–C5 olefins, sulfuric acid functions as the acid catalyst and the continuous phase of the reactor emulsion. Alkylation contactors operate at 4–10 °C with an isobutane-to-olefin ratio of 8:1 to 12:1 and a fresh acid strength of 98 wt%. Acid inventory in the contactor is typically 40–60% by volume. Once acid strength drops below 88 wt%, acid-soluble oil concentration rises, trimethylpentane selectivity declines, and emulsion viscosity increases, which increases mix-motor amps and refrigeration load. Spent acid is withdrawn at 85–90 wt% and either regenerated in a decomposition furnace to SO2/SO3 or sent to other acid-consuming operations. The make-up rate depends on olefin type and feed contaminants; propylene and butylene feeds consume acid differently because heavier acid-soluble oils are formed more rapidly from propylene-containing streams.

Acidulation of phosphate rock with 93 wt% sulfuric acid in a Prayon dihydrate wet-process reactor consumes 2.6–3.0 tonnes of acid per tonne of P2O5 recovered, with the exact ratio set by rock carbonate, organic matter, and silica content. Sulfate excess in the digestion slurry is maintained at 2–4 wt% SO4²− to produce filterable gypsum crystals. Local acid strength above 95 wt% forms hemihydrate or anhydrite scale on the reaction cooler and flash cooler surfaces. The filter feed is conditioned to 45–55 °C; excessive temperature accelerates gypsum crystal dissolution and blinds the vacuum filter cloth.

In the sulfate-route titanium dioxide process, ilmenite is digested with 85–95 wt% sulfuric acid in brick-lined batch digesters at 150–180 °C. Acid-to-ilmenite charge ratio, seed concentration, and hydrolysis rate set the rutile or anatase crystal fraction. Spent acid leaving hydrolysis contains 20–25 wt% H2SO4 and iron sulfate; it is evaporated for acid recovery, and ferrous sulfate heptahydrate is crystallized as a co-product. The acid recycle loop is sensitive to silica and alumina in the ore, which reduce evaporation capacity and increase scaling in titanium alloy heat exchangers.

When Acid Strength Drifts Below 93.2 wt% During Pickling

Temperature control in a continuous steel pickling bath with 10–20 wt% sulfuric acid at 60–90 °C determines oxide removal rate but also influences inhibitor stability. Free H2SO4 is maintained by automatic titration; when free acid falls below 5 wt% while FeSO4 exceeds 150 g/L, strip speed must be reduced or surface staining appears. Spent pickle liquor is cooled to 0–5 °C to crystallize ferrous sulfate heptahydrate, and the regenerated acid returns to the line. Thiourea-based or propargyl alcohol pickle inhibitors at 0.05–0.2 wt% reduce base-metal attack but lose effectiveness above 90 °C. Hydrogen uptake is controlled by limiting immersion time after scale removal and by maintaining inhibitor concentration in the bath.

When 98 wt% acid or 20% oleum is used for linear alkylbenzene sulfonation in falling-film reactors, the SO3-to-alkylbenzene molar ratio is held at 1.00–1.05 to limit dialkyl sulfone formation. Spent sulfuric acid from the digestion step is separated by settling at 50–60 °C; the acid layer may contain 1–3 wt% sulfone and branched alkyl sulfate by-products and is unacceptable for battery-grade blending without oxidative purification. For detergent sulfonation, the product sulfonic acid viscosity after water adjustment is normally 500–1,500 mPa·s at 25 °C, which sets the downstream neutralizer pump and heat exchanger size.

Storage terminals for concentrated acid keep the storage tank under nitrogen blanketing or calcium chloride drying vents to avoid atmospheric moisture pickup. Water absorption at the liquid surface creates dilute acid layers with higher corrosion rates. Unloading lines are sized for a flow velocity below 1.5 m/s in carbon steel to limit erosion-corrosion. For oleum transfer, the pipe must be heat traced above 20 °C, and the receiving tank must be fitted with a scrubber or vent condenser because free SO3 vapor pressure rises with temperature.

How to Get a Sulfuric Acid Quotation: What We Need From You

A binding commodity quotation for bulk, intermediate bulk container, or drum shipments requires the following from the requesting party: desired H2SO4 weight percent, oleum free SO3 content, or density/Baumé; total annual volume and spot parcel size; transport mode and UN classification; receiving site storage capacity and connection details; and any impurity ceilings for chloride, iron, arsenic, heavy metals, SO2, or residue on ignition. Total acidity should be referenced to ISO 910:1977; concentration may be checked by titration according to ASTM E223; specific gravity is measured under ASTM E100; sulfur dioxide content in sulfuric acid is determined by ISO 3423:1975. For battery-acid quotations, trace-metal ceilings are verified by ICP-OES after controlled dilution, and the supplier requires the customer’s maximum allowable antimony, copper, and organic carbon values.

Transport classification should be specific because concentrated acid, oleum, and spent acid are distinct entries: UN 1830 for sulfuric acid, UN 1831 for oleum, and UN 1832 for spent sulfuric acid. The receiving site should identify whether the shipment is governed by ADR/RID, IMDG, or DOT/49 CFR, because tank qualification, venting, and segregation rules differ. Full tank trailers commonly carry 20–24 t, ISO tank containers about 25,000 L, IBCs 1,200 kg, and drums 55 US gal. Requests that omit concentration cannot be priced because density, packaging, corrosion allowance, and freight weight all depend on acid strength; requests that omit impurity ceilings are defaulted to bulk technical-grade specification.