Sulfuric acid (H₂SO₄, CAS 7664-93-9, EC 231-639-5, molar mass 98.079 g mol⁻¹) is supplied as a dense mineral acid with a density of 1.84 g cm⁻³ at 20 °C for commercial 98% w/w material. Product documents and storage signage for this substance depend directly on the CLP classification of sulfuric acid: H314, H315, and the concentration thresholds that separate the two hazard categories. The harmonised entry under Annex VI of Regulation (EC) No 1272/2008, Index 016-020-00-8, classifies the neat substance as Skin Corr. 1A with H314. The same entry sets specific concentration limits that move diluted process streams into Skin Irrit. 2 with H315 and Eye Irrit. 2 with H319. These limits are mass fraction values expressed as weight/weight, not volumetric ratios, and they control Safety Data Sheet Section 2, GHS labelling, and downstream packaging decisions independently of pH.

Commercial production grades include chamber acid at approximately 93% w/w, standard oil-of-vitriol at 98% w/w, and oleum with free SO₃ at 20–25%. The viscosity of 98% w/w acid near 20 °C is approximately 26.7 mPa·s. The freezing point of 98% w/w acid lies near 3 °C; outdoor storage in unheated tanks therefore requires concentration management or heat tracing to avoid solidification and pumping failure. Concentrated acid is hygroscopic, and a vented storage tank in high-humidity air can absorb moisture and drift to 96–97% w/w over weeks. This drift does not alter the H314 classification, but it affects density-based inventory records and freeze-protection settings.

How Does the Harmonised CLP Entry Assign H314 and H315 Across Dilution Ranges?

Annex VI of Regulation (EC) No 1272/2008 establishes a two-band concentration split for the dermal classification of sulfuric acid. At or above 15% w/w, the preparation is classified as Skin Corr. 1A with hazard statement H314; this statement communicates severe skin burns and eye damage and carries the signal word Danger. From 5% w/w up to but not including 15% w/w, the preparation is classified as Skin Irrit. 2 with H315, supplemented by Eye Irrit. 2 with H319 in the harmonised entry. Below 5% w/w, the harmonised entry does not classify the preparation for dermal corrosion or irritation. The table summarises the harmonised concentration bands.

The 15% w/w value is a specific concentration limit under Annex VI, not a generic CLP mixture threshold. For a mixture containing sulfuric acid, this harmonised value overrides the generic 5% w/w cut-off that would otherwise apply to a corrosive component. A 10% w/w sulfuric acid solution is therefore classified as H315, not H314, under the harmonised entry, even though its pH is below zero and it remains corrosive to carbon steel. Where sulfuric acid is blended with other skin-corrosive substances, the CLP Annex I summation rules apply; the mixture enters H314 when the summed concentration fractions relative to the relevant specific or generic limits reach 1.

At the 15% w/w boundary, the label changes from GHS07 Warning to GHS05 Danger. A 14.9% w/w solution carries H315 and H319, while a 15.1% w/w solution carries H314. Because evaporation and drag-out can shift water content on a production floor, the concentration boundary is also an operational control boundary. Sampling for concentration verification should be cooled below 40 °C before density or titrimetric measurement to prevent evaporative concentration drift in the sample itself.

High-Acidity Transfer Systems and the 15% w/w Corrosivity Boundary

Carbon steel ASTM A36 remains the standard construction material for storage of commercial 93–98% w/w sulfuric acid under low-velocity conditions below 0.6 m s⁻¹, because the iron sulfate film passivates the metal. This passivation is not available below the 15% w/w boundary. Dilute acid at 10% w/w, although classified H315, attacks carbon steel by direct acid dissolution; transfer lines in this concentration band are therefore lined with PTFE or constructed from PVDF. Pumps specified for 5–15% w/w acid are magnetic-drive polypropylene or PTFE wet-ended units, and manual valves are PTFE-lined diaphragm type to avoid crevice attack on stainless steel bodies.

Inline concentration measurement on a dilution skid normally uses a Coriolis mass flow meter supplying density output. Temperature correction from a Pt100 probe allows the control loop to hold the product below the 15% w/w classification boundary. pH probes are not acceptable as the primary control instrument in this range because glass electrode acid error and non-linear response below pH 0 make pH data unreliable for calculating mass fraction. Titrimetric verification against a standard base remains the reference method for batch records.

Personal protective equipment on lines crossing the 15% w/w boundary is selected from concentration-specific permeation data. For 98% w/w acid, butyl rubber gloves tested according to EN 374-1 with breakthrough times above 480 min are specified; face shields to EN 166 and acid-resistant clothing to EN 14605 are also required. Secondary containment for H314 concentrated acid uses carbon steel bunds with an acid-resistant coating or HDPE lining because sulfuric acid attacks calcium hydroxide in concrete and forms gypsum. Safety showers and eyewash stations positioned within 10 s of transfer points follow ANSI/ISEA Z358.1.

In continuous steel strip pickling, a bath initially maintained at 10% w/w sulfuric acid and 75 °C can drift upward as evaporation removes water and acid is replenished. The process fluid crosses from H315 to H314 when free acid concentration exceeds 15% w/w, changing emergency eye-wash signage, secondary containment classification, and PPE requirements at the line. Acid recovery from spent pickle liquor typically combines evaporation and ferrous sulfate separation; the recovered acid concentration must be monitored before return to the bath to prevent uncontrolled excursion through the threshold.

When Oleum Is Diluted to Process Strength Below 15% w/w

Oleum containing free SO₃ at 20–25% w/w is adjusted to process acid through multi-stage dilution because the first water-contact step releases a large heat of mixing. The integral heat of solution for concentrated sulfuric acid to infinite dilution is approximately −96 kJ mol⁻¹. In a continuous dilution unit, oleum or 98% acid is metered through a PTFE feed lance into agitated water; jacket cooling and recirculated chilled water remove enough heat to prevent local boiling. Reversing the addition order can produce a steam burst and aerosol release of acid mist. The stream passes from neat H314 to H315 only after the mass fraction drops into the 5–15% w/w range, but the thermal hazard is highest while the mixture remains above that band.

Automotive lead-acid battery electrolyte is supplied at approximately 37% w/w sulfuric acid. The product is well above the 15% w/w boundary and is therefore classified H314. Battery filling equipment uses acid-resistant peristaltic pumps with PTFE or acid-resistant elastomer tubing; fill volume control is set by plate geometry and separator type. If electrolyte is diluted by wash water on a fill deck, it may enter the 5–15% w/w H315 range, but drainage neutralization with lime or sodium hydroxide remains mandatory because residual acid below 5% w/w can still corrode concrete and carbon steel.

pH Adjustment and Sulfonation Processes Operate Under Separate Threshold Controls

Cooling water and wastewater pH control stations commonly store sulfuric acid at 50% or 98% w/w and dilute it at the point of injection. The stored acid is H314; a dilution skid that delivers 10% w/w acid produces an H315 dosing stream only if the blending system holds the mass fraction within the 5–15% w/w band. Injection quills for this service are fabricated from PVDF or PTFE and discharge into a static mixer to prevent local high-acid zones. A pH setpoint of 6–9 in the treated effluent corresponds to negligible residual sulfuric acid, but this does not change the label of the concentrated storage tank. If a site uses 20% w/w acid for pH trim instead of 10% w/w acid, the stream remains H314 and must be labelled Danger.

Effluent neutralization systems using lime slurry or sodium hydroxide are interlocked with pH analysers and flow meters so that acid dosing stops on loss of sample flow. A 10% w/w sulfuric acid solution requires a higher volumetric feed rate than 98% acid to neutralize a given alkalinity, which increases pump duty. This is an operational consequence of selecting an H315-range dilution over an H314-range concentrate for pH trim.

Sulfonation processes that produce linear alkylbenzene sulfonic acid for surfactant manufacture use sulfuric acid as a diluent and recycle stream alongside SO₃ falling-film reactors. Reactor SO₃ concentrations in the inert gas feed are typically 4–8% v/v, and organic film temperatures are held below 50 °C to limit side-product formation. The absorber bottoms contain sulfuric acid at 90–98% w/w with dissolved organics; this stream remains far above the 15% w/w H314 threshold. Steam desorption at 170–180 °C strips organic components before re-concentration, but published data for the exact dermal classification threshold of the contaminated recycle stream is limited. The starting concentration, organic loading, and process temperature therefore require a task-specific SDS and mixture classification review beyond the harmonised pure-acid bands.

In semiconductor cleaning, mixtures of sulfuric acid and hydrogen peroxide generate peroxymonosulfuric acid in situ. The mixture cannot be classified solely from the sulfuric acid harmonised entry because the oxidising agent introduces ancillary hazards. Published data for this specific configuration is limited; manufacturers prepare a separate mixture assessment under CLP. This is outside the direct H314/H315 sulfuric acid concentration thresholds and requires additional labels for oxidising or acute toxicity endpoints where applicable.

Packaging and transport classification follow independent concentration thresholds. ADR transport rules assign UN1830 to sulfuric acid with more than 51% acid and UN2796 to sulfuric acid with no more than 51% acid; both are Class 8 PG II. The CLP health hazard boundary at 15% w/w does not alter the UN number, but it changes the GHS pictogram on packaging from GHS05 to GHS07. For consumer-facing products, CLP packaging provisions for child-resistant fastenings and tactile warnings apply to H314 Skin Corr. 1 packaging; H315 Skin Irrit. 2 does not automatically trigger those specific packaging requirements but still requires hazard labelling and safe-use instructions under CLP.