Sulfuric Acid Questions Buyers Ask Us Every Week (FAQ)

Sulfuric acid, CAS 7664-93-9, is purchased as technical-grade 93 wt%, 98 wt%, and oleum. The technical questions that recur on purchase orders involve assay conversion, moisture ingress, freezing behaviour, materials selection, contaminant limits, unloading safety, and transport classification. The references cited below are to ASTM E223-23, ISO 910:1977, and ISO 913:1977; this document is not a replacement for a lot-specific certificate of analysis or regulatory safety data sheet.

Buyers commonly ask whether an oleum shipment should be evaluated as free SO3 or as equivalent H2SO4. For a 20% free SO3 oleum, the equivalent H2SO4 content is 104.5 wt%, because each 1 kg of free SO3 converts to 1.225 kg of H2SO4 on hydration. Titrations following ASTM E223-23 report total acidity; ISO 910:1977 sets out the calculation route for total acidity and free SO3 in oleum. A workable lot-acceptance lower bound for technical-grade 98% acid is 97.8 wt%; buyers should also require a density check at 20 °C because a 1.0 °C offset shifts the density reading by approximately 0.001 g/cm³. Density stratification in a poorly mixed bulk tank is best resolved by bottom sample ports, not top-hatch grab samples.

Residue on ignition is another recurrent basis-of-purchase issue. In sulfonation and nitration process feeds, residue on ignition is often controlled below 500 mg/kg, but a standard technical-grade offer may not include this figure unless explicitly nominated. Buyers should define the analytical method and sample preparation because iron results can vary by 5 mg/kg between laboratories using different digestion and matrix-matching protocols.

Does Atmospheric Moisture Ingress Change the Assay of Stored 98% Acid Before Unloading?

Atmospheric moisture ingress is a leading cause of off-spec receiving after ambient storage. A bulk 98% sulfuric acid tank with an open vent or a failed desiccant bed absorbs water during breathing cycles. At 20 °C, the equilibrium water vapour pressure over 98% acid is below that of ambient air at RH >60%, so water transfers from the air into the acid. Vent dryers on storage tanks should deliver make-up air with a dew point at or below -40 °C; the bed is typically a desiccant or membrane dryer sized for the tank breathing rate. During discharge, a nitrogen pad with a dew point at or below -40 °C is preferred over ambient air. A documented sampling interval of 30 days for assay by ASTM E223-23 is common, with an action limit when the assay falls to 97.8 wt% for 98% acid.

External heating in 93% acid circuits is not normally required for freeze protection because the freezing point of 93 wt% H2SO4 is approximately -35 °C. In contrast, 98 wt% H2SO4 solidifies at about -2 °C, and unheated 98% lines in exposed pipe racks can plug during winter shutdown. Where heat tracing is used, the maximum skin temperature is kept below 50 °C to avoid local boiling at the metal surface. At 20 °C, the dynamic viscosity of 98% acid is approximately 25 mPa·s, rising to about 60 mPa·s at 10 °C. Pump sizing must account for this viscosity increase and for the specific gravity of 1.836 at 20 °C.

Alloy Selection in 93% and 98% Acid Handling Circuits

Carbon steel is acceptable for 93% and 98% sulfuric acid at ambient temperature when the acid remains above 88 wt% and the flow velocity is kept below 0.9 m/s. Below 88 wt%, the corrosion rate of carbon steel rises sharply, and dilute-acid piping is usually specified in PTFE-lined carbon steel, polyvinylidene fluoride, or high-silicon cast iron. 316L stainless steel is serviceable below 10 wt% or above 90 wt% within narrow temperature limits but is unsuitable in intermediate concentrations at elevated temperature because the active-passive transition produces local attack. Corrosion coupons tested in accordance with ASTM G31-21 should be used to confirm a material change; a carbon steel corrosion allowance of 3 mm is common for bulk acid tanks but does not cover hydrogen grooving under high turbulence. Gasket materials are PTFE envelope or flexible graphite; EPDM is not used in concentrated acid because sulfonation embrittles the elastomer. Pumps in leak-critical service are frequently ANSI B73.1 magnetic-drive or canned-motor units with PTFE or ETFE liners; double mechanical seals with pressure-balanced barrier fluid are used where sealless pumps are not practical.

Recurring contaminant questions concern iron, chloride, nitrate, and residue on ignition. Iron is determined by flame atomic absorption or by 2,2'-bipyridyl spectrophotometry; a common technical-grade upper limit is 25 mg/kg. Chloride is often limited to below 10 mg/kg in acid used for electronics or food-contact processes because chloride accelerates pitting in stainless steel. Nitrate and nitrosyl species are limited in acid destined for nitration or pharmaceutical sulfonation where uncontrolled redox side-reactions occur. The analytical matrix is not interchangeable; the buyer should define the exact sample preparation and wavelength.

Analytical methods and typical technical-grade acceptance windows are summarised below.

When a Buyer Requests Limits on Iron, Chloride, and Nitrate Below Standard Technical-Grade Specifications

When a buyer requests iron below 5 mg/kg, chloride below 5 mg/kg, and nitrate below 5 mg/kg, the specification moves from technical grade toward electronic or reagent grade. These levels cannot be verified reliably by simple density and visual colour checks; inductively coupled plasma mass spectrometry or ion chromatography with matrix matching is required. For acid used in semiconductor etching, sub-ppb metal limits may apply, and the packaging must be fluoropolymer-lined or high-purity quartz. The buyer should also require lot-specific filtration and particle count data because off-spec acid in electronics is more commonly caused by particulate contamination than by bulk acidity loss. In pharmaceutical sulfonation, nitrate and nitrosyl impurities can generate nitrosating species that compete with the intended electrophile; a pre-shipment nitrosyl analysis is therefore specified when the acid is used for nitrosamine-sensitive syntheses.

During tanker discharge, the highest-risk operations are splash loading, overflow, and vent-line blockage. Closed-loop unloading through dry-break couplings and laminar flow restrictors reduces mist formation. The receiving tank should be anchored to an overflow line with a capacity equal to the maximum tanker unloading rate, commonly 30 m³/h to 45 m³/h for a 20 t road tanker, and the vent should be sized for the displaced air volume. Emergency eyewash stations and safety showers conforming to ANSI Z358.1-2014 are positioned within 10 s travel distance. Where acid mist can accumulate above the PEL, area monitoring for sulfuric acid mist is set with an alarm at 50% of the 1 mg/m³ OSHA TWA.

Is the Correct Transport Description UN1830 or UN1831 for Oleum?

The correct UN number depends on free SO3 content and physical form. Sulfuric acid with more than 51% acid and not meeting oleum criteria is UN1830 under 49 CFR 172.101. Oleum, or fuming sulfuric acid, is assigned UN1831; spent sulfuric acid is assigned UN1832. A 20% free SO3 oleum is always UN1831, not UN1830, because the free SO3 hazard changes emergency response and packaging requirements. For 98% acid, the proper shipping description is UN1830, Class 8, Packing Group II. For oleum, the assignment is UN1831, Class 8, Packing Group I. Bill-of-lading discrepancies at the receiving dock are one of the most frequent delays; the buyer should confirm the UN number against the SDS before scheduling unloading.

The principal classification and exposure limits for bulk sulfuric acid are listed below.

Spent sulfuric acid disposal or regeneration inquiries typically involve acid strength after use, dissolved metals, and organic contamination. Spent acid with 70–80 wt% H2SO4 and high iron or hydrocarbon content may be regenerated thermally in a 1000 °C decomposition furnace; the SO2-bearing gas is converted back to sulfuric acid through a contact process. Lower-strength spent acid below 30 wt% is more frequently neutralised with lime or sodium hydroxide, producing gypsum or sodium sulfate; the choice depends on local waste-treatment permit limits for sulfate discharge. Dilution heat release is the reason acid must be added to water, never water to agitated acid; the heat of dilution can raise a small water volume above its boiling point and create a steam spray. For a 98% acid dilution to 10 wt%, the exotherm is approximately 880 kJ/kg of acid; cooler or jacketed mixers are required when batch volume exceeds 1 m³. This scenario also drives the minimum wall thickness evaluation in dilution tanks: the design should account for local temperature spikes above 90 °C at the addition point, even when the bulk tank is kept below 40 °C.