A Certificate of Analysis for sulfuric acid functions as a lot-level analytical contract, not a generic product datasheet. The document should identify the production lot, the release specification, the sampling date, the numerical result, the test method, and the acceptance limit for every reported property. Sulfuric acid is supplied in multiple grades: technical, electrolyte, reagent, food processing, and semiconductor. The impurities that must be checked on a COA therefore depend on the grade and the downstream process. A buyer in phosphate fertilizer production may accept iron at 50 ppm without process disruption, while a semiconductor wet-etch operation must reject a COA that fails to report a full trace-metal panel with detection limits in the low ng/kg range. The correct reading practice is to match each COA line item to the test standard required by the user’s specification, then compare the reported result to the numerical limit for that grade.

Production route also shapes the impurity profile. Sulfur-burning plants typically produce acid with low arsenic and low chloride, whereas metallurgical off-gas plants may carry selenium, arsenic, and chloride. Spent acid regeneration can retain organic carbon unless purification is adequate. A COA review should begin by identifying the production route and the grade family so that the highest-risk impurities receive priority.

The first technical check is not an impurity but the total acidity and basis of expression. Sulfuric acid is sold as concentrated acid, oleum, or diluted grades. A COA may report weight percent H₂SO₄, weight percent total acidity, or free SO₃. In oleum, ISO 910:1977 calculates free sulfur trioxide from total acidity. A COA that reports only density cannot substitute for titrated assay because density varies with concentration and temperature. At 20 °C, 96% sulfuric acid has a density of approximately 1.8355 g/cm³, but density alone does not distinguish between a high-purity 96.0% acid and a contaminated 96.0% acid.

Water is an impurity in concentrated acid even though it is the solvent. A COA for 98.0% acid reports water by difference from total acidity. In processes such as alkylation, water content changes acid strength and may accelerate side reactions; therefore water must be read as an impurity parameter, not only as the balance of the assay.

How to Read a Sulfuric Acid COA: The Impurities Buyers Must Check

The buyer should first verify that every impurity has a defined method and a numerical result, not a text statement such as “complies.” A COA line item that reads “iron: passes” is not sufficient for lot acceptance. The essential impurity categories for concentrated sulfuric acid are iron, chloride, residue after ignition, oxidizable substances, sulfur dioxide, and, for regulated or reagent uses, arsenic and heavy metals. The buyer must also check the units. Iron may be reported in ppm, µg/g, or mg/kg; chloride may be reported as ppm or mg/L. Without a stated density at reference temperature, converting mass-based to volume-based values introduces error.

The sampling basis is part of the COA record. Bulk acid assays are representative only if the sample was drawn according to a defined procedure from the specific tank or drum lot. ISO 3165:1976 provides general rules for sampling liquid chemical products; ASTM E223-16 includes sampling provisions specific to sulfuric acid. If the COA does not identify the sampling standard and sampling point, the buyer cannot treat the impurity values as representative of a full tank truck or rail car.

For oleum and technical acid, the COA should also list free SO₃, sulfur dioxide, and density method. These fields are not impurity measurements in the strict sense, but they affect storage and unloading safety. A COA for oleum that omits free SO₃ does not provide enough information to calculate dilution heat or ventilation needs.

Which analytical signatures separate a reagent-grade COA from a technical-grade COA?

Total acidity may be identical between a technical and a reagent grade, but the impurity envelope is not. ACS reagent sulfuric acid specifications include iron at ≤ 0.5 ppm, chloride at ≤ 0.2 ppm, nitrate at ≤ 0.5 ppm, ammonium at ≤ 2 ppm, residue after ignition at ≤ 5 ppm, arsenic at ≤ 0.01 ppm, and heavy metals at ≤ 1 ppm as lead. Technical sulfuric acid often has iron limits in the 20–50 ppm range and may not report chloride or arsenic at all. The COA reader must not infer that an omitted field means zero concentration; it means the parameter was not tested or not controlled.

Color is another underused COA field. ACS reagent acid specifies APHA color ≤ 10. Technical acid can exceed this without violating assay because the color arises from organic impurities or iron complexes. A dark acid can still titrate at 98.0%; therefore, color must be read as an impurity index, not as an assay substitute.

The method codes themselves require scrutiny. A technical grade COA that reports iron by an in-house photometric method may not be equivalent to ASTM E223-16 if the sample digestion step differs. In acid containing suspended scale, total iron recovery depends on sample preparation. The buyer should require method alignment between the COA and the incoming inspection protocol; otherwise results cannot be compared across suppliers.

Iron determination by 1,10-phenanthroline photometry measures soluble Fe(II) after reduction; it does not fully recover iron bound in suspended scale unless the sample is digested. A COA using ICP-OES with acid digestion gives total recoverable iron and is preferable for technical acid from smelter gas sources.

Chloride is the most consequential unreported variable in technical-grade sulfuric acid COAs. In storage and dosing systems constructed from 316L stainless steel, pitting and stress corrosion cracking can initiate when the acid stream contains chloride above 50 ppm and the metal temperature exceeds 40–60 °C. The threshold drops when oxidizers such as hydrogen peroxide or nitric acid elevate the open-circuit potential. Chloride at 10 ppm may be sufficient for localized attack in stagnant zones of a corroded or sensitized weld. ASTM G48-11 Method A provides a comparative pitting test. A COA that omits chloride does not demonstrate that the acid is chloride-free; it demonstrates that the supplier did not test for it.

Ion chromatography in concentrated sulfuric acid requires sample dilution; chloride detection limits in the diluted sample can reach 0.01 ppm, but the diluent must be chloride-free. Turbidimetric chloride methods are subject to sulfate interference. The COA should state whether chloride is measured as Cl or as HCl equivalents.

Iron is another impurity that changes between the supplier COA and the buyer’s receiving tank. Concentrated sulfuric acid above 93% at ambient temperature forms a ferrous sulfate protective film on carbon steel that limits further attack. If humid air ingress forms dilute acid films on the metal surface, the local concentration can fall below the protective range, releasing iron and hydrogen. A COA iron value of 25 ppm at the loading point may not represent the iron content at the use point. Received-lot verification should include iron by photometric ASTM E223-16 or ICP-OES.

Trace Metal Panels for Semiconductor and Battery Electrolyte Sulfuric Acid

Semiconductor-grade sulfuric acid is used in SPM cleaning baths, where sulfuric acid is mixed with hydrogen peroxide. Trace metal contamination at 1–10 ng/g can deposit on wafer surfaces. A COA for this use must be a full element panel by ICP-MS with per-element detection limits, not a total heavy metals value. SEMI purity tiers define the required controls. Critical elements are Fe, Cr, Ni, Cu, Zn, Al, Na, K, Ca, and As. Battery electrolyte sulfuric acid requires low chloride and iron because iron can participate in redox side reactions at electrode surfaces. A COA that reports only “heavy metals ≤ 1 ppm” is insufficient for either application.

Oxidizable substances are reported as a sum parameter using permanganate reduction. This parameter is not interchangeable with total organic carbon. Permanganate-reducing impurities include sulfite, thiosulfate, organic compounds, and some metal ions. In SPM bath preparation, organic compounds can reduce hydrogen peroxide and alter the cleaning bath’s oxidation potential. A COA that reports only SO₂ does not quantify non-sulfur organic reducing compounds.

Lead-acid battery electrolyte specifications for sulfuric acid enforce low chloride and iron because these ions alter grid corrosion and open-circuit stability. Organic contaminants and oxidizable substances are controlled to limit self-discharge. A COA for this grade should report specific gravity, chloride, iron, and oxidizable matter; a general technical COA is not sufficient.

Residue after ignition has a direct mass-balance consequence that is often overlooked. A 1,000 L tote of 96% sulfuric acid at 20 °C has a density of approximately 1.8355 g/cm³ and a total mass of 1,835.5 kg. At a COA value of 5 ppm, the nonvolatile residue is 9.18 g per tote. In an evaporation or concentration system processing 50 tonnes/day, the same residue level represents 250 g/day of deposit-forming solids. The buyer should therefore require residue after ignition by ASTM E223-16 and use the value to estimate fouling intervals rather than ignoring it.

When a COA Report Omits Residue-on-Ignition and Arsenic Data

For food processing, pharmaceutical, and laboratory applications, the absence of residue and arsenic fields is a grade mismatch. The Food Chemicals Codex and ACS reagent specifications require arsenic, heavy metals, and residue after ignition. A technical-grade COA that lacks these fields cannot be substituted for a food-grade or reagent-grade material. The buyer should require the supplier to report the specified FCC or ACS method. If the method is unavailable, an ISO/IEC 17025 laboratory should perform the release testing. Arsenic is commonly measured by Gutzeit method at the 0.01 ppm level in ACS reagent acid; lower-level verification uses ICP-MS.

Selenium in acid derived from copper smelters can be present and is not captured by a lead-equivalence heavy metals value. In food processing, selenium is separately regulated. A COA that only reports heavy metals by lead equivalence does not quantify Se; a specific ICP-MS or hydride-generation AAS method is required.

Sulfur dioxide is a reducing impurity that is typically reported only for technical and oleum grades. In ASTM E223-16, SO₂ is measured by iodometric titration. In oleum unloading, vapor-phase SO₂ can present a toxic exposure risk. Nitrogen oxides are also relevant in nitration process streams; sulfonated organic systems can develop color when NOx is present. The COA reader should not confuse sulfur dioxide with oxidizable substances. SO₂ is a specific reducing species; oxidizable substances represent a sum parameter for all permanganate-reducing compounds.

Numerical COA values are meaningful only when the sampling procedure is defined. A result of 0.2 ppm chloride on a drum top sample is not equivalent to the same value on a circulated tank sample. The buyer should require the COA to state the sampling standard, the sample container, and the date. Without these fields, the COA is only a supplier data sheet and cannot be used as a release document. The acceptance decision should be based on a received-lot retest against the same method codes and grade limits.