# Sulfuric acid

Chemical identity, aqueous grades, oleum reporting bases, and the limits of product and thermodynamic data.

Reviewed: 2026-10-30

Tags: chemicals, acids

## Identity and composition

EcoServices reports **20% oleum as 104.5% H₂SO₄ equivalent**. Oleum strength uses free SO₃ by weight, while the acid-equivalent number expresses a conversion relative to the original sample mass. It can exceed 100% on that equivalent basis. Sigma-Aldrich ACS 320501 instead lists 95.0–98.0% sulfuric acid assay. H₂SO₄ is CAS 7664-93-9, 98.08 g/mol. [Sigma-Aldrich 320501 product page](https://www.sigmaaldrich.com/US/en/product/sigald/320501)

ICSC 0362 covers concentrated acid above 51% and below 100%. Oleum has separate CAS 8014-95-7 and composition-dependent properties. [ICSC 0362](https://www.inchem.org/documents/icsc/icsc/eics0362.htm) [ICSC 1447](https://www.inchem.org/documents/icsc/icsc/eics1447.htm)

## Aqueous assay and oleum equivalents

EcoServices’ *Oleum: Fuming Sulfuric Acid* brochure, in the manufacturer’s 2023-linked edition, defines oleum strength as percent by weight of free SO₃. Its product-information paragraph states that 20% oleum can also be reported as 104.5% H₂SO₄ equivalent. These are two reporting bases for the same named grade, not two literal mass fractions of H₂SO₄. [EcoServices brochure, p. 4, Product Information](https://www.ecovyst.com/wp-content/uploads/2023/08/EcoServices-Oleum-Brochure-Final-2023.pdf).

**Chemical-accounting deduction:** an equivalent percentage above 100 expresses acid-equivalent content relative to the original material mass. It cannot mean more than the entire sample mass is physically H₂SO₄. Free-SO₃ reporting is also not a measurement of every molecular species present. This distinction follows from the brochure’s paired reporting bases; it supplies neither an oleum preparation calculation nor a speciation model. [EcoServices, p. 4](https://www.ecovyst.com/wp-content/uploads/2023/08/EcoServices-Oleum-Brochure-Final-2023.pdf).

For an aqueous product, an H₂SO₄ assay, water content and impurity profile describe different attributes. **Conditional mass-balance deduction:** only a material explicitly established to contain H₂SO₄ and water alone permits the remainder after acid mass fraction to be assigned entirely to water. A product assay range alone does not establish that binary composition. Sigma’s separate impurity entries illustrate why the acid assay cannot stand in for a complete composition analysis. [Sigma-Aldrich 320501](https://www.sigmaaldrich.com/US/en/product/sigald/320501).

## Grade, specification and lot evidence

The following examples describe named catalog products, not universal requirements for sulfuric acid.

| Evidence | Meaning and limitation |
| --- | --- |
| Sigma-Aldrich 320501, ACS reagent | Lists 95.0–98.0% assay, ignition residue ≤5 ppm, Fe ≤0.2 ppm, Hg ≤5 ppb and heavy metals expressed as Pb ≤1 ppm. These are listed product limits, not measured values for a particular bottle. “As Pb” is the reporting basis of the heavy-metals entry. [Product page](https://www.sigmaaldrich.com/US/en/product/sigald/320501). |
| Fisher S9200PB17, SLR Extra Pure, Low In Nitrogen | The one-page product specification names a minimum 95% acid and gives descriptive physical properties. “Low In Nitrogen” appears in the product name; this sheet does not provide a numerical nitrogen limit or a trace-metal panel. [Manufacturer specification](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?countryCode=kr&erp_type=KR_E1_LCD&rootSku=S%2F9200&sku=S9200PB17). |

**Evidence distinction:** a typical property describes representative behavior; a specification states a product limit; a certificate of analysis can report results for its identified lot. Sigma’s documentation interface requires a lot/batch identifier for a specific certificate. No named-lot COA was verified for this article, so none of these figures establishes a historical lot result or the composition of current inventory. [Sigma documentation search](https://www.sigmaaldrich.com/US/en/documents-search?tab=coa).

## Physical properties require a state and basis

ICSC 0362 lists relative density 1.8 at 20 °C and water miscibility at 20 °C for its concentrated-acid scope. These are broad reference entries, not a density-versus-assay calibration. Its 10 °C melting entry and 340 °C decomposition entry do not define a freezing or boiling curve for every composition in that scope. [ICSC 0362, November 2016](https://www.inchem.org/documents/icsc/icsc/eics0362.htm).

Fisher’s S9200PB17 sheet gives a 10 °C melting point and 290 °C boiling point without a pressure or a composition-specific phase diagram. The distinction between its boiling entry and ICSC’s decomposition entry remains unresolved here: they name different phenomena and lack enough common conditions for substitution. The Fisher sheet is undated as a technical revision; its dynamically generated print date is not treated as an independently established edition date. [Fisher specification](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?countryCode=kr&erp_type=KR_E1_LCD&rootSku=S%2F9200&sku=S9200PB17); [ICSC 0362](https://www.inchem.org/documents/icsc/icsc/eics0362.htm).

For oleum, ICSC 1447 explicitly declines to give one molecular mass because free SO₃ varies. Its notes associate boiling points of 138, 116 and 60 °C with 20%, 30% and 65% SO₃ respectively, and melting points of 2, 21 and 5 °C with those same compositions. Pressure is not supplied there. These entries show composition dependence, including a nonmonotonic melting sequence; they are not an interpolation rule. The card labels water miscibility as involving reaction. [ICSC 1447, November 2016](https://www.inchem.org/documents/icsc/icsc/eics1447.htm).

## Thermodynamic and dissociation boundaries

Frank J. Zeleznik’s 1991 paper, *Thermodynamic Properties of the Aqueous Sulfuric Acid System to 350 K*, correlates experimental thermodynamic measurements as functions of temperature and composition. Its abstract distinguishes anhydrous acid, five crystalline hydrates and ice from the aqueous liquid, and describes tabulated liquid properties at sulfuric-acid mole-fraction intervals. Thus pure solid, hydrate, pure liquid and aqueous-mixture properties belong to explicitly different states. Its reported overall agreement of about ±0.75% is a correlation statement, not uncertainty certified for a commercial grade. [Original paper, abstract and symbols, pp. 1157–1158](https://www.nist.gov/system/files/documents/srd/jpcrd426.pdf).

An analytical acid concentration does not by itself specify hydrogen-ion activity or the distribution among aqueous ionic species. The original 1994 Clegg–Rard–Pitzer paper’s indexed abstract describes a model of H⁺–HSO₄⁻–SO₄²⁻–H₂O bounded to 0–6 mol/kg and 273.15–328.15 K. Only its abstract was available in this research; the publisher landing-page fetch was denied. No dissociation constant, numerical species fraction, pH prediction or transferable activity model is adopted here. [Original research DOI](https://doi.org/10.1039/FT9949001875).

## Coverage limits

The references establish chemical and reporting distinctions, not process performance, material compatibility, or customer-approved grades. No current lot, water determination, complete impurity analysis or application performance was verified. Safety-card data and product specifications serve different purposes; neither establishes global regulatory compliance. The Merck product 1.17048 SDS was available through a cached reader but its direct download was denied, so no SDS-specific classification or physical-property claim is adopted from it. Solvay’s oleum summary likewise appeared in a cached reader but live PDF verification failed; its typical-property table is excluded.

[Surface-finishing processes](../../processes/finishing/index.md) is a related subject, not evidence for a sulfuric-acid formulation or operating condition.