Sulfuric Acid Supplier Scorecard: The 10-Point Evaluation Sheet

Sulfuric acid, CAS 7664-93-9, is procured as technical-grade 93–98% H₂SO₄, battery-grade electrolyte, or oleum containing nominal 20–65% free SO₃. The molecular weight of H₂SO₄ is 98.08 g/mol, and the acidimetric equivalent is 49.04 g/eq. A 98% acid shipment exhibits a density of 1.836–1.846 g/cm³ at 20°C, a viscosity near 25 mPa·s, and a crystallization temperature near 3°C, which requires tank heating or insulation for unheated outdoor storage. The normal boiling point of the 98.3% azeotrope is 337°C, but mist and SO₃ vapor can evolve well below that temperature in hot-transfer operations. Procurement evaluation frequently applies the Sulfuric Acid Supplier Scorecard: The 10-Point Evaluation Sheet to separate qualified bulk tanker or isotainer sources from opportunistic brokers.

What Does the 10-Point Evaluation Sheet Require for Virgin Acid Purity?

Each point in the scorecard carries equal weight unless a downstream catalyst or electrochemical process requires a tightened impurity limit. The table below lists the ten evaluation criteria, the reference test method, and a representative pass band for virgin 98% technical acid. A supplier lot that fails any point is quarantined pending re-sampling; two consecutive failures in the same point trigger a vendor audit under ISO 9001:2015, clause 8.4.1. Scores are normalized to a 10.0 maximum because the sheet is designed as a pass/fail compliance matrix rather than a continuous quality index.

On receipt of a bulk tanker, the sampling plan follows lot-sampling guidance from ASTM E223. A dedicated 316L stainless-steel sampling thief is inserted through a PTFE-lined manway or nozzle under nitrogen padding; the receiving operator draws three samples: top, middle, and bottom. The samples are composited into a 1 L PTFE bottle, and the bottle is transported to the laboratory in secondary containment with a screw-cap seal. Sampling occurs only after the tanker has been grounded and the delivery pressure has been verified below the hose working pressure of 10.3 bar. Any sample showing visible haze or color is analyzed for APHA per ASTM D1209 before offloading is authorized. The retention sample is stored in a locked cabinet for 90 days to support retrograde batch comparisons, especially when a downstream ketone or surfactant reactor exhibits an unexpected color shift.

Receiving, Sampling, and Offloading Equipment Compatibility

Carbon steel is acceptable for concentrated sulfuric acid at ambient temperatures and flow velocities below 0.9 m/s because the acid maintains a protective iron sulfate film. Dilute acid below 85% at temperatures above 40°C is significantly more aggressive; PTFE or PVDF linings are specified for filling nozzles, hoses, and pump seals. Offloading pumps are sealless centrifugal or magnetically coupled units with a maximum speed of 1,750 rpm. A dry-air purge with a dew point below -40°C is applied to the storage tank vent after each receipt to reduce moisture ingress. The storage tank high-level alarm is set at 85% of capacity and the overfill trip at 90%; spill containment is sized for 110% of the largest tank in accordance with 40 CFR 112 for U.S. sites or equivalent national secondary containment codes.

Water absorption in a vented 98% acid storage tank is not uniform. Acid at the surface absorbs atmospheric moisture and forms a lower-density layer that can stratify above denser acid; this stratification is detected by top-to-bottom density differences greater than 0.005 g/cm³. A nitrogen blanket or silica-gel breather with a dew point below -40°C is applied to tanks that feed color-sensitive or sulfonation processes. The supplier scorecard does not directly measure surface moisture; instead, point 2 density and point 1 total acidity are trended across deliveries to detect slow moisture drift. A downward drift of 0.1% H₂SO₄ across monthly deliveries without a corresponding change in supplier lot number may indicate a leaking vent stack or an inadequately dried transfer hose.

When 20% Oleum Is Supplied for Sulfonation: Free SO₃ Assay and Transfer Line Temperature Control

Oleum with nominal 20% free SO₃ has a crystallization temperature in the range of 2–10°C depending on exact SO₃ content; transfer lines are heat-traced and maintained at 25–35°C. Free SO₃ is determined by the total acidity/free SO₃ method of ISO 910; scorecard point 3 assigns a band of ±0.5% of declared. A falling free SO₃ value accompanied by a rising water equivalent in the total acidity calculation indicates moisture intrusion during transfer. Sulfonation reactors require a stable SO₃ activity; a 0.2% drift in free SO₃ can alter the para-to-ortho sulfonic acid ratio in linear alkylbenzene sulfonation. Transfer hoses for oleum are fabricated from PTFE with carbon steel swivel joints; pump seals are flushed with dry nitrogen at 0.2–0.5 bar to prevent atmospheric moisture from converting free SO₃ to sulfuric acid mist.

Alkylation units processing isobutane with C₃–C₅ olefins maintain circulating acid strength between 89% and 93%. Makeup acid is specified at 98–99.5% H₂SO₄ to compensate for water and acid-soluble oil accumulation. A water mass fraction above 1.0% in circulating acid shifts the reaction toward tert-butyl alcohol and increases acid consumption. Scorecard iron limits are tightened for this service because iron catalyzes acid-soluble oil formation; fresh acid with more than 50 mg/kg iron is generally diverted to less sensitive applications. Published data for the exact threshold in a particular unit is limited because acid-soluble oil formation depends on olefin space velocity and isobutane-to-olefin ratio. The spent acid is typically regenerated or sold to fertilizer production; the supplier scorecard is therefore linked to regeneration yield because chloride and nitrate impurities can accumulate in the recycle loop.

Carbon Steel Run-Down Lines Demand a Velocity Limit Below 0.9 m/s

At transfer temperatures of 20–30°C, carbon steel piping carrying 98% acid is sized for a velocity below 0.9 m/s; at 1.2 m/s, the protective iron sulfate film may be removed at weld seams and elbows. Corrosion is generally less than 0.5 mm/y for 98% acid at 20°C and 0.9 m/s; above 1.5 m/s, erosion-corrosion can initiate. Piping dead legs and low-point drains are inspected because dilute acid can form by water absorption and attack carbon steel at rates in excess of 1.0 mm/y. Valves on the acid line are specified with PTFE or FEP bellows; elastomer seats based on natural rubber or EPDM are excluded unless specifically validated for acidic condensate.

Operator certification is verified before first hookup.

Before first hookup to a bulk tanker, the receiving operator’s training record is verified against the site’s process safety management system. The offloading sequence is interlocked with a manual deadman valve and a high-level shutdown set at 85% of tank capacity. Emergency eyewash and deluge shower stations conform to ANSI/ISEA Z358.1. The bulk tanker’s bottom outlet is connected only after the vapor return line is secured and the unloading pump is confirmed to be isolated from the tanker by a breakaway coupling.

Why Chloride and Nitrate Limits Are Tightened for Electrolyte Grade Acid

For lead-acid battery electrolyte, the scorecard points for chloride, nitrate, and iron carry the highest weight because chloride increases grid corrosion and nitrate can depolarize the lead oxide electrode. Chloride is typically limited to ≤10 mg/kg, nitrate as NO₃ to ≤5 mg/kg, and iron to ≤2 mg/kg for virgin battery-grade acid. These concentrations are verified by ion chromatography per ISO 10304-1 after 1:10 dilution with ultrapure water, and by photometric iron per ASTM E223. Acid that meets technical-grade iron limits may still fail the scorecard for traction or stationary battery service because the electrochemical degradation threshold is lower than the corrosion limit for carbon steel.

Shipments are classified as UN1830 for sulfuric acid with acid concentration above 51% and UN1831 for oleum. The ADR/RID dangerous goods declaration must list the proper shipping name and packing group. Under REACH, sulfuric acid is registered as a substance; the supplier’s extended Safety Data Sheet must align with Annex II of REACH Regulation (EC) No 1907/2006. The supplier scorecard documentation point is not closed until the tanker seal number, the delivery temperature log, and the batch CoA have been reviewed against the purchase specification.