hydrogen chloride
Enable an AI agent to recognise hydrogen chloride, assess its material and containment state, and determine which handling or use decisions require further evidence.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to recognise hydrogen chloride, assess its material and containment state, and determine which handling or use decisions require further evidence.
Hydrogen chloride is the heteronuclear diatomic hydrogen halide HCl: a colourless, nonflammable, pungent gas at ordinary temperature and pressure that is fully dissociated in water as hydrochloric acid and is handled industrially as anhydrous compressed gas, refrigerated liquid, or aqueous solution under the same CAS number.
It can be Classify an identified inventory as hydrogen chloride, an HCl-containing mixture or a related aqueous preparation.; Compare composition and impurity evidence against the requirements of a proposed use.; Evaluate whether documented containment and contacting materials cover the actual moisture and operating conditions.; Assess a proposed storage, transfer or dosing operation against linked approved procedures and verified prerequisites.; Track HCl through dissolution, reaction or treatment while linking resulting materials to their own models..
Distinguishing features
Require evidence for HCl identity; a chloride result alone cannot distinguish hydrogen chloride from a chloride salt.
Distinguish hydrogen chloride gas or liquefied material from an aqueous hydrochloric acid preparation by recording water content, solvent and phase.
Distinguish HCl from elemental chlorine, Cl2, using identification evidence rather than the shared presence of chlorine in their names.
Treat acidity as supporting context rather than unique identification, because other substances also produce acidic aqueous solutions.
Distinguish a material containing HCl from a substantially pure HCl inventory by recording its carrier medium and composition basis.
Scope
+ Identification of hydrogen chloride, HCl, and evidence supporting that identification
+ Physical state and conditions of a hydrogen chloride inventory or stream
+ Hydrogen chloride content, water content and decision-relevant impurities
+ Effects of moisture and dissolution on material identity, behaviour and handling
+ Compatibility, containment condition and evidence required for proposed operations
- Complete formulation and application models for hydrochloric acid solutions
- Elemental chlorine, chloride salts and other hydrogen halides as independent substances
- Engineering design and certification of cylinders, piping and process equipment
- Facility ventilation, emergency response and occupational health programmes
- Detailed models of production processes and downstream reaction products
Characteristics
- Chemical identity
- HCl; confirmed, provisional or unresolved identification Prevents confusion with chlorine, chloride salts and other acidic materials.
- Material presentation
- Gas, liquefied inventory, gas mixture, aqueous solution or other specified mixture Determines which phase, composition and neighbouring material models apply.
- Temperature
- K or °C, with measurement location and time Supports interpretation of physical state and operating conditions.
- Pressure
- Pa or bar, explicitly absolute or gauge Supports assessment of containment and phase conditions.
- Hydrogen chloride content
- Mass fraction, mole fraction or concentration with an explicit basis and reference conditions Separates purity from dilution and supports inventory and use decisions.
- Water content
- Mass or mole fraction, with method and detection limit Moisture can change material behaviour and the suitability of contacting equipment.
- Impurity profile
- Named constituents and concentrations, including reporting limits Determines whether a particular supply meets its intended use requirements.
- Contained quantity
- kg or mol; distinguish total material quantity from HCl quantity Supports inventory reconciliation and operation-specific consequence assessment.
- Contacting materials
- Linked vessel, valve, seal and piping materials with condition-specific compatibility evidence Compatibility must account for composition, moisture, temperature and pressure.
- Containment condition
- Verified intact, suspected loss, confirmed loss or unknown, with evidence timestamp Determines whether routine operations remain eligible for consideration.
Also called
Where this came from
wikidata · CC0 1.0
Also registered as vr.tr.hydrogen-chloride
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 6 bundles · 11 layers · 18 findings · 28 questions.
HCl identity and boundaries Establishes what material is being called hydrogen chloride and which related identities must remain distinct.
HCl identity, chloride detection and aqueous acidity are different claims and cannot substitute for one another.
Chemical identification
Records evidence that identifies HCl rather than a neighbouring chlorine-containing or acidic substance.
Evidence for HCl
Capture the identification claim, its evidence and the limits of the method used.
- What evidence identifies this material or constituent as HCl? provenance
- Does the evidence distinguish HCl from chloride salts, elemental chlorine and other acids present in the sample? boundary
Substance and preparation
Separates HCl substance identity from the identity of the material carrying it.
Carrier and solvent boundary
Record whether the subject is an HCl inventory or an HCl constituent in a separately characterised preparation.
- Is the subject hydrogen chloride itself, a gas mixture containing HCl or a solution containing its dissolved forms? definition
- Which properties and decisions require a linked mixture or hydrochloric acid solution model? boundary
Phase and inventory Describes the physical presentation and amount of hydrogen chloride under recorded conditions.
An HCl gas stream and an inventory containing liquid and vapour require different interpretations of pressure, sampling and quantity.
Physical state
Connects observed or inferred phase to temperature, pressure and composition.
Phase at recorded conditions
Record phase evidence and distinguish measured state from a prediction.
- What phases are present at the recorded temperature, pressure and composition? measurement
- What observation or validated property source supports the assigned phase? provenance
Amount and distribution
Distinguishes total material inventory from the amount and location of HCl within it.
HCl quantity basis
Capture the quantity basis and any assumptions about distribution between phases or locations.
- How much HCl is present, and how was that amount distinguished from the total mixture quantity? measurement
- Does the estimate account for HCl in each relevant phase and connected containment volume? boundary
Composition and moisture Captures HCl concentration, impurities and water exposure relevant to material suitability.
An anhydrous label or nominal purity does not establish the actual water content or all use-relevant impurities.
Purity and dilution
Interprets composition claims using explicit analytical and reporting bases.
Composition evidence
Record measured HCl content and impurities without treating an unspecified remainder as known.
- What is the HCl content, on what basis, and with what measurement uncertainty or specification tolerance? measurement
- Which impurities were measured, and which were unmeasured or below reporting limits? provenance
Water and dissolution
Tracks water content and contact with aqueous media as conditions affecting HCl behaviour.
Moisture status
Distinguish a documented moisture specification from measured water content and subsequent exposure.
- What measured water content or documented criterion supports describing this material as anhydrous or dry? measurement
- Has water ingress or aqueous contact changed the material enough to require reassessment or a linked solution record? boundary
Containment and compatible contact Relates the actual HCl material state to its containment and all contacting materials.
Suitability cannot be inferred from a generic acid-resistance claim without checking HCl composition and operating conditions.
Contact material suitability
Records compatibility evidence for surfaces, seals and components exposed to the material.
Condition-specific compatibility
Require compatibility evidence that covers the proposed HCl service conditions.
- Which materials contact the HCl, including seals, valves and any sampling components? definition
- What evidence supports their suitability at the actual water content, concentration, temperature and pressure? provenance
Containment and release evidence
Connects inventory condition to inspection and HCl-specific detection evidence.
Integrity and detection confidence
Record suspected or verified containment loss together with the scope and limitations of detection.
- What current inspection or monitoring evidence supports the containment condition? provenance
- What HCl concentrations were detected, where and when, and what detection limits or interferences apply? measurement
Use and transformation decisions Connects the assessed HCl state to proposed operations and resulting material identities.
An agent must distinguish eligibility for an operation from evidence that HCl has actually been transferred, dissolved or consumed.
Operation prerequisites
Evaluates proposed HCl operations against documented requirements and current evidence.
Operation eligibility
Record the proposed operation, governing procedure and any unmet conditions.
- For the proposed storage, transfer, dosing or dissolution operation, which approved procedure covers this HCl presentation? action
- Are the required composition, moisture, containment and equipment conditions verified, or what evidence prevents proceeding? action
Transformation and residual HCl
Tracks HCl across dissolution, reaction and treatment without assuming disappearance from a process label.
Fate and completion evidence
Record where HCl goes, what resulting materials are identified and how residual HCl is assessed.
- Was HCl transferred, dissolved or chemically consumed, and which resulting material records describe its fate? boundary
- What measurements and acceptance criteria establish residual HCl and completion of the intended transformation? measurement
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Anhydrous compressed hydrogen chloride gas (UN 1050)
- Refrigerated liquefied hydrogen chloride (UN 2186)
- Aqueous hydrochloric acid, HCl(aq) (UN 1789)
- Commercial concentrated hydrochloric acid (about 36-38 wt% HCl)
- Constant-boiling aqueous azeotrope (about 20.2 wt% HCl)
- Hydrogen chloride aerosol or mist (a form singled out in U.S. TRI reporting)
- Deuterium chloride (DCl; NIST isotopologue hydrochloric acid-d)
- Which of these kinds and varieties hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- CAS Registry Number - 7647-01-0 - Used for both anhydrous HCl and aqueous hydrochloric acid in most chemical inventories.
- PubChem CID - 313 - PubChem files hydrogen chloride and hydrochloric acid as one compound.
- EC number (EINECS) - 231-595-7 - ECHA/REACH substance identifier.
- ECHA InfoCard - 100.028.723 - European Chemicals Agency card for HCl.
- UN number - 1050 - Anhydrous hydrogen chloride, compressed; Class 2.3, subsidiary 8.
- UN number - 2186 - Hydrogen chloride, refrigerated liquid; Class 2.3, subsidiary 8.
- UN number - 1789 - Hydrochloric acid (aqueous); Class 8, packing group II or III.
- IUPAC InChI - InChI=1S/ClH/h1H - NIST standard InChI; Wikipedia also lists InChI=1S/HCl/h1H.
- InChIKey - VEXZGXHMUGYJMC-UHFFFAOYSA-N - Standard hashed InChI.
- ChEBI - CHEBI:17883 - ChEBI entity for hydrogen chloride.
- FDA UNII - QTT17582CB - Global Substance Registration System.
- EPA CompTox DTXSID - DTXSID2020711 - U.S. EPA DSSTox identifier.
- ChemSpider - 307 - Royal Society of Chemistry ChemSpider ID.
- RTECS - MW4025000 - NIOSH RTECS number.
- EU Index number - 017-002-00-2 - CLP/GHS annex index for hydrogen chloride.
- SMILES - Cl - Canonical SMILES used by PubChem/NIST for HCl.
- Which of these identifiers and schemes hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- OSHA PEL ceiling 5 ppm (7 mg/m³), 29 CFR 1910.1000 Table Z-1 (Occupational Safety and Health Administration)
- OSHA Process Safety Management highly hazardous chemical, threshold quantity 5,000 lb for hydrogen chloride / anhydrous hydrochloric acid, 29 CFR 1910.119 (OSHA)
- NIOSH REL ceiling 5 ppm (7 mg/m³); Pocket Guide IDLH 50 ppm; 2025 IDLH Value Profile 45 ppm (CDC/NIOSH)
- ACGIH TLV ceiling 2 ppm (American Conference of Governmental Industrial Hygienists, as cited in industrial stewardship summaries)
- ILO/WHO International Chemical Safety Card ICSC 0163 (ILO and WHO)
- UN GHS: compressed gas, toxic if inhaled, causes severe skin burns and eye damage (H314, H331; UNECE GHS / ICSC)
- UN Recommendations on the Transport of Dangerous Goods: UN 1050 and UN 2186 Class 2.3 + 8; UN 1789 Class 8 (UNECE)
- EU CLP/REACH: EC 231-595-7, Index 017-002-00-2 (European Chemicals Agency)
- U.S. Clean Air Act §112 hazardous air pollutant; CAA §112(r) RMP threshold 5,000 lb for anhydrous HCl (U.S. EPA)
- EPCRA §302 extremely hazardous substance, TPQ 500 lb for hydrogen chloride anhydrous/gas only (U.S. EPA)
- CERCLA/EPCRA reportable-quantity and TRI listing, including aerosol-only hydrochloric acid (U.S. EPA)
- CISA Chemical Facility Anti-Terrorism Standards: hydrogen chloride (anhydrous) and hydrochloric acid ≥37% as chemicals of interest (CISA)
- Canada Hazardous Products Regulations / WHMIS classification distinguishing HCl(g) from HCl(aq) (Health Canada)
- Which of these standards and regulation hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Shipped as a liquefied compressed gas in cylinders (anhydrous) or as aqueous acid in tanks and carboys.
- Industrial co-product of vinyl chloride, isocyanates, fluorocarbons, and chlorinated methanes/ethanes, and of inorganic routes to magnesium, fumed silica and titanium dioxide.
- Feedstock for organic chlorides (methyl chloride, ethyl chloride, chlorobenzenes) and for PVC-chain chemistry.
- Metal pickling, cleaning, electroplating, ore refining, and petroleum-well acidizing, usually as aqueous hydrochloric acid.
- Leather tanning and refining of fats, soaps and edible oils.
- Formed in volcanic gases and as a combustion product when PVC and other chlorinated plastics burn.
- Encountered in the body as gastric hydrochloric acid; commercial concentrated acid is typically 36-38% HCl in water.
- Which of these real-world use hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Molar mass - 36.46 (NIST 36.461) - g/mol
- Melting point (anhydrous) - −114.22 - °C
- Boiling point (anhydrous, 101.3 kPa) - −85.05 - °C
- Gas density - about 1.49 - g/L
- Relative vapour density (air = 1) - 1.27-1.27 - dimensionless
- Water solubility - 561 (60 °C) to 823 (0 °C); about 720 at 20 °C - g/L
- Vapour pressure (near 20 °C) - about 4.26-4.35 - MPa
- Aqueous pKa - about −3.0 to −5.9 - dimensionless
- Standard enthalpy of formation, gas (CODATA) - −92.31 ± 0.10 - kJ/mol
- Standard entropy, gas, 1 bar (CODATA) - 186.902 ± 0.005 - J/(mol·K)
- Workplace ceiling (OSHA PEL / NIOSH REL) - 5 - ppm
- IDLH - 45 (NIOSH 2025 profile) to 50 (Pocket Guide) - ppm
- Air concentration conversion - 1 ppm = 1.49 - mg/m³
- Odor threshold (ATSDR) - about 0.77 (only ~50% of distracted persons detect it at 5 ppm) - ppm
- Which of these typical measurements hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Inhalation of the gas or acid mist burns the nose, throat and lungs; laryngeal spasm and pulmonary edema are documented animal/human outcomes. Concentrations of 1,000-2,000 ppm are dangerous even briefly; IDLH is 45-50 ppm.
- Contact with moisture yields hydrochloric acid that causes severe skin and eye burns; liquefied HCl also causes frostbite.
- The vapour is heavier than air and can pool in pits, cellars and poorly ventilated low spots.
- Attacks many metals when wet, evolving flammable hydrogen; reacts violently with bases and with oxidants (the latter can release chlorine).
- Compressed or refrigerated containers are GHS compressed-gas hazards and can rupture if heated; a liquid leak should be oriented so gas, not liquid, escapes.
- White fumes in moist air create a corrosive acid mist; mists are still harmful but are often described as less dehydrating than the anhydrous gas.
- Which of these failure modes and hazards hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- English, Japanese and German (among others) treat the anhydrous gas and the aqueous acid as different named substances: hydrogen chloride / hydrochloric acid; 塩化水素 / 塩酸; Chlorwasserstoff / Salzsäure.
- Trade names muriatic acid and spirits of salt still mean impure aqueous HCl in construction, masonry and pool chemicals, especially in North America and Britain.
- Inventories often share CAS 7647-01-0 across gas and solution, while transport law splits them as UN 1050, UN 2186 and UN 1789.
- Occupational ceilings differ: OSHA and NIOSH 5 ppm versus ACGIH TLV 2 ppm.
- Canada's WHMIS assessment states separate classification cut-offs for hydrogen chloride versus hydrochloric acid concentration bands.
- Which of these regional variation hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Hydrochloric acid (aqueous HCl) - The acid is HCl dissolved in water, not the anhydrous molecule; transport UN 1789 (Class 8) versus UN 1050/2186 (Class 2.3+8), and the commercial liquid boils near 50-110 °C depending on titre rather than at −85 °C.
- Hydrogen fluoride (HF) - Different formula and CAS; HF boils near 19.5 °C because of hydrogen bonding, etches glass/silica, and is a weak acid in water (pKa ≈ 3.2) whereas HCl is a strong acid (pKa about −3 to −6).
- Hydrogen bromide (HBr) - Different hydrogen halide (Br not Cl); identify by formula/CAS and by boiling point (HBr is substantially higher than HCl's −85 °C).
- Chlorine (Cl2) - Elemental chlorine is a greenish-yellow oxidising gas, not a hydrogen halide; it does not form white acid fumes with moist air in the same way and is classified as an oxidiser rather than an anhydrous acid gas.
- Amine hydrochlorides and other solid hydrochlorides - Those are ionic salts of HCl, typically solids; they do not ship as UN 1050 gas and do not fume as anhydrous HCl until they release HCl.
- Hydrogen sulfide (H2S) - Both are pungent toxic gases that can accumulate low, but H2S is flammable with a rotten-egg odour, whereas HCl is nonflammable with a sharp acidic odour and forms hydrochloric acid with moisture.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of hydrogen chloride this model covers, and on what evidence? provenance
Sources
- Hydrogen chloride - Wikipedia, Wikimedia Foundation - IUPAC and systematic names, identifiers, melting and boiling points, density, water solubility, pKa, GHS statements, and the gas-versus-aqueous distinction.
- Hydrogen chloride - NIST Chemistry WebBook, National Institute of Standards and Technology - CAS 7647-01-0, InChI/InChIKey, molecular weight 36.461, CODATA ΔfH°(g) and S°(g), synonym list including UN numbers, and the DCl isotopologue.
- Hydrochloric Acid / Hydrogen Chloride, CID 313 - PubChem, NCBI/NLM - PubChem CID, EC number, UNII, dual UN numbers 1050/1789, industrial co-product routes, and the registry practice of filing gas and aqueous acid under one CID.
- Hydrogen chloride, anhydrous - CAMEO Chemicals, NOAA - UN/NA 1050, DOT poison-gas/corrosive labels, physical properties, acute concentration bands, and U.S. EPA/OSHA/CISA list thresholds.
- ICSC 0163 Hydrogen chloride - ILO/WHO International Chemical Safety Cards (INCHEM) - GHS criteria for the anhydrous gas, UN class 2.3 with subsidiary 8, UN 2186 and 1789 cross-references, and metal/base/oxidant incompatibilities.
- NIOSH Pocket Guide to Chemical Hazards: Hydrogen chloride - CDC/NIOSH - REL and OSHA PEL ceiling 5 ppm, IDLH 50 ppm, 1 ppm = 1.49 mg/m³ conversion, relative gas density, and target organs.
- Hazardous substance assessment: Hydrochloric acid and hydrogen chloride - Health Canada - WHMIS split of HCl(g) versus HCl(aq), UN 1050/2186/1789, and concentration cut-offs for skin, eye and metal-corrosion categories.
- Hydrochloric acid - Wikipedia, Wikimedia Foundation - Definition of hydrochloric acid as aqueous HCl, muriatic acid/spirits of salt names, commercial 37% density, and azeotrope boiling behaviour.
What the second pass must settle
- Does an existing Vercy world model already own hydrogen chloride or hydrochloric acid, and how should this registry entry link to that source of truth?
- Where should registry ownership fall between hydrogen chloride substance identity and aqueous hydrochloric acid preparations?
- Which authoritative property sources and validated methods should govern phase assessment and HCl identification in relevant mixtures?
- Which use-specific water and impurity criteria, supported by compatibility evidence, are needed for the intended HCl applications?
- Which operating jurisdictions and facility procedures supply the applicable exposure criteria, containment requirements and action thresholds?