Hydrochloric acid

Identity and aqueous chemistry

OxyChem’s aqueous HCl table pairs 20 °Bé with 31.45% HCl and specific gravity 1.1600 at 60/60 °F. Use that scale and temperature basis. Anhydrous hydrogen chloride is a gas at ordinary ambient conditions. Hydrochloric or muriatic acid is its aqueous solution. Both may use CAS 7647-01-0. HCl has molar mass about 36.46 g/mol. OxyChem handbook, page 4 NIST Chemistry WebBook, hydrogen chloride

The November 2016 ILO/WHO card covers anhydrous hydrogen chloride, including compressed liquefied gas. ICSC 0163, identity and physical information

In water, hydrochloric acid is a strong mineral acid with oxonium ions, H₃O⁺, and chloride ions, Cl⁻. Its salts are chlorides. A qualitative representation is HCl + H₂O → H₃O⁺ + Cl⁻. This representation does not supply an activity model or quantify every species in concentrated solutions. ITW Reagents, product 211020, chemical description

Commercial products and analytical claims

Nominal strength and grade describe different attributes. The following entries belong to named products, not universal definitions of technical or ACS grade. The manufacturer sheets print the impurity numbers below without explicit inequality signs. They are transcribed as specification entries, not measured lot results or inferred acceptance limits. ITW product 211020, specifications, Thermo Fisher specification, 033257.P6

The Thermo Fisher sheet also lists bromide at 0.005%, free chlorine reported as Cl at 1 ppm, heavy metals by ICP-OES at 1 ppm, and residue on ignition at 5 ppm. The retrieved PDF prints October 29, 2026 as its print date and says specifications may change. It does not define each percentage or ppm denominator, give all methods, or report measurement uncertainty. Thermo Fisher specification, complete one-page sheet

Synthesis: a shared HCl assay range does not establish equal impurity profiles. An SDS ingredient range serves a different purpose from a product assay specification. Neither is a current lot certificate of analysis. No current lot certificate was examined here. Free chlorine reported as Cl is also a different analytical entry from the chloride ion that accompanies aqueous HCl.

Concentration and equivalent quantities

A mass percentage of HCl expresses HCl-equivalent mass per total solution mass. Thus 37% by mass denotes 37 g HCl equivalent in 100 g solution. A mass-per-volume percentage instead has a volume denominator. Amount-of-substance concentration has units mol/L of solution. Molality has units mol/kg of solvent. These quantities cannot be exchanged numerically without their respective mass, volume, and temperature bases. NIST Guide to the SI, chapter 8, composition quantities

Derived dimensional relation: for HCl mass fraction w and solution density ρ in g/mL, the analytical HCl concentration is 1000 × ρ × w / 36.46 mol/L at the density's temperature. This is a composition conversion, not a preparation formula. The denominator is the HCl molar mass. NIST identity data

Derived chemistry: one mole of HCl corresponds to one mole of chloride and one acid equivalent for complete acid-base neutralization. The HCl equivalent mass is therefore about 36.46 g per equivalent for that reaction. In a solution with additional chloride salts, total chloride no longer uniquely determines HCl acidity. Other acids likewise prevent total titratable acidity from uniquely identifying HCl. The chemical distinction follows from the HCl formula and the separate identity of chloride salts. These are conditional stoichiometric deductions, not analytical results for a commercial lot. NIST HCl identity, ITW description of chloride salts

Properties require a material and reporting basis

Sources: ICSC 0163, November 2016, ATSDR physical properties, OxyChem handbook, page 35.

OxyChem separately plots total vapor pressure of aqueous acid and the vapor pressure of hydrogen chloride above that acid. Both plots use weight percent HCl, temperature curves, and mmHg, but describe different pressure quantities. Its boiling-point curve has a maximum near 20% HCl. The graph does not state its pressure basis, so it does not establish a precise boiling point at arbitrary pressure. OxyChem handbook, pages 36–38

The NIOSH Pocket Guide prints solubility as 67% at 86 °F, while ATSDR prints 67% at 68 °F. Neither entry defines that percentage's denominator. The ICSC instead prints 67 g per 100 mL at 30 °C. These statements cannot be treated as the same saturated-solution mass fraction or as a universal commercial strength. The temperature discrepancy remains unresolved here. The ATSDR page is archived and no longer updated. NIOSH hydrogen chloride entry, ATSDR physical properties, ICSC physical information

SDS evidence and unresolved product data

The ITW GB SDS retrieved from product 211020's endpoint is version 13.18, revised July 3, 2025. Its title says 37%, while section 3 gives hydrogen chloride as greater than 25% through 40%. Section 9 reports water at 68.0%, density 1.19 g/cm³ at 20 °C, and vapor pressure 23 hPa at 20 °C. It does not identify that pressure as total or partial. The water entry does not reconcile with a simple binary 37% HCl composition. Those entries therefore remain source-specific statements, not a coherent property set for exactly 37% acid. ITW GB SDS, sections 1, 3, and 9

That SDS leaves boiling point undetermined. The product page instead prints 85 °C without pressure or exact assay conditions. The discrepancy is retained rather than resolved by selecting one number. ITW SDS, section 9, ITW product technical data

For that dated GB product sheet, section 2 assigns metal corrosion category 1, skin corrosion category 1B, serious eye damage category 1, and specific target organ toxicity after single exposure category 3 for respiratory irritation. These are regional, product-specific hazard classifications, not a statement of worldwide current legal compliance or process compatibility. ITW GB SDS, section 2

Coverage limits

This reference distinguishes the substance, commercial mixtures, and analytical reporting bases. It does not establish reaction rates, corrosion rates, material compatibility, formulation performance, or a customer's qualified process. No primary activity model, full phase-equilibrium dataset, or current lot analysis was evaluated. Manufacturer property graphs support qualitative relationships within their printed axes, not precision interpolation or extrapolation.

The OxyChem handbook identifies itself as August 2022 v3. The ITW product page has no visible revision date. Source retrieval and review occurred October 30, 2026 PDT. Later source revisions can change product statements without changing the chemical identity.

Related reference

Surface-finishing processes describes the broader process subject. This substance reference supplies no bath recipe or operating settings.