Tritium (10028-17-8) Physical and Chemical Properties
Tritium
Tritium (hydrogen‑3) is a radioactive isotope of hydrogen supplied as molecular gas or as tritiated water and used in tracer work, radiolabeling and specialized luminescent and fusion-related applications.
| CAS Number | 10028-17-8 |
| Family | Radionuclides (hydrogen isotope) |
| Typical Form | Colorless gas (or tritiated water) |
| Common Grades | EP, USP |
Tritium is the radioactive isotope of hydrogen with mass number 3 (nuclear composition: one proton and two neutrons). Chemically it belongs to the hydrogen isotopic series and, when molecular, exists primarily as diatomic molecular tritium (T2, often written as 3H2 or ditritium) and as tritiated water (HTO or T2O) after oxidation. Electronically and chemically, tritium behaves like other hydrogen isotopes: it forms the same types of covalent bonds and participates in the same acid–base equilibria as protium and deuterium, but isotopic mass differences alter vibrational zero-point energies, reaction kinetics, and diffusion behavior. In molecular form the computed descriptors for ditritium reflect a minimal polar surface area and no hydrogen-bonding functionality: SMILES "[3H][3H]" and a computed molecular weight of 6.032098563.
Tritium is a low-energy beta emitter; its decay produces an electron and an anti-neutrino, transforming to helium-3. The radiological properties (half-life, specific activity, beta energy spectrum) control many practical behaviors: tritiated water is chemically indistinguishable from ordinary water and is readily exchanged and transported in environmental and biological systems, producing the primary internal exposure pathway. Elemental tritium (T2 or HT) is a flammable diatomic gas that is much less readily assimilated by biological systems than HTO but poses containment and materials-compatibility challenges because of high diffusivity and hydrogen-induced embrittlement of many metals and degradation/permeation of polymers.
Common commercial grades reported for this substance include: EP, USP.
Basic Physical Properties (Density, Melting Point, Boiling Point)
Atomic Weight
Tritium is the hydrogen isotope of mass number 3 (nominal atomic mass 3). For molecular ditritium (T2) the computed molecular weight is 6.032098563 (dimension: \(\mathrm{g}\,\mathrm{mol}^{-1}\) when used as molar mass).
Appearance and Physical State
Tritium in its uncombined form is a diatomic gas. Experimental physical description notes occurrence as tritium gas (HT/T2) and as tritiated water (HTO), the latter being the dominant biologically relevant form.
Density
Measured and computed condensed-phase density-related properties reported include: critical volume \(57.1\ \text{cu cm/mol}\) (calculated); heat of sublimation \(1640\ \mathrm{J}\,\mathrm{mol}^{-1}\); entropy of vaporization \(54.0\ \mathrm{J}\,\mathrm{mol}^{-1}\,\mathrm{K}^{-1}\); and molar density of the liquid at \(25\ \mathrm{K}\) equal to \(42.65\ \mathrm{mol}\,\mathrm{L}^{-1}\).
Melting Point
Experimental melting point reported: \(-254.54\ ^\circ\mathrm{C}\) (\(20.62\ \mathrm{K}\)) at \(162\ \mathrm{mm}\,\mathrm{Hg}\).
Boiling Point
Experimental boiling point reported: \(-248.12\ ^\circ\mathrm{C}\) (\(25.04\ \mathrm{K}\)).
Chemical Properties (Reactivity and Oxidation States)
Oxidation States
Tritium exhibits the same formal oxidation states as protium: commonly \(-1, 0,\) and \(+1\) depending on chemical environment. As an isotope of hydrogen it forms hydrides (oxidation state \(-1\)), diatomic gas (oxidation state \(0\)), and protonic species (oxidation state \(+1\)) in acids and covalent compounds.
Reactivity with Air and Water
Elemental tritium (T2 or HT) is a flammable diatomic gas with reactivity characteristic of hydrogen: it will burn or form oxides in air under ignition conditions and will exchange with ordinary hydrogen in many reactions. Oxidation in moist air or by catalytic surfaces rapidly produces tritiated water (HTO/T2O). HTO behaves chemically as ordinary water and will distribute through aqueous phases and biological fluids; because HTO is chemically indistinguishable from H2O, it is the principal chemical form relevant for environmental transport and biological uptake.
Reactivity with Acids and Bases
Chemically tritium follows hydrogen chemistry. Tritium can be incorporated into acids as the cationic form (T+) and into metal hydrides as (T−). Tritium-labeled reagents and tritiated compounds participate in acid–base and redox equilibria analogous to protium- and deuterium-containing analogues, with kinetic isotope effects attributable to the higher mass of tritium.
Isotopic Composition
Stable Isotopes
Hydrogen has two stable isotopes: protium (\(^1\)H) and deuterium (\(^2\)H or D). Tritium (\(^3\)H) is the next heavier isotope but is radioactive rather than stable.
Radioisotopes
Tritium (\(^3\)H) is the radioactive hydrogen isotope. Multiple experimental half-life values and decay parameters are reported: half-life values appear as \(\text{12.33 years}\), \(\text{12.26 yr}\), and \(\text{12.323}\pm\text{0.004 years}\) in different measurements; decay proceeds by beta-minus emission (maximum beta energy \(\sim 18.6\ \mathrm{keV}\), mean beta energy \(\sim 5.7\ \mathrm{keV}\)). Reported radiochemical activity metrics include a maximum specific activity value of \(1078.9\ \mathrm{GBq}\,\mathrm{mmol}^{-1}\) and a molar activity of \(2157\ \mathrm{TBq}\,\mathrm{mol}^{-1}\).
Thermodynamic Parameters
Heat Capacity and Related Data
Direct heat-capacity values are not provided in the current data context. Reported phase-change energetic parameters include a heat of vaporization of \(1390\ \mathrm{J}\,\mathrm{mol}^{-1}\) and a heat of sublimation of \(1640\ \mathrm{J}\,\mathrm{mol}^{-1}\). Entropy of vaporization is reported as \(54.0\ \mathrm{J}\,\mathrm{mol}^{-1}\,\mathrm{K}^{-1}\).
Enthalpy and Gibbs Energy
No experimentally established standard enthalpy of formation or Gibbs free energy values for tritium-containing reference species are available in the current data context.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS number: 10028-17-8
- European Community (EC) number: 233-070-8
- UNII: YGG3Y3DAG1
- ChEBI: CHEBI:29298
- DSSTox Substance ID: DTXSID80881374
- InChI: InChI=1S/H2/h1H/i1+2T
- InChIKey: UFHFLCQGNIYNRP-JMRXTUGHSA-N
- SMILES: [3H][3H]
Synonyms and Common Names
Common synonyms and names found for the substance include: Tritium; Hydrogen-3; ditritium; (3H2)dihydrogen; Molecular tritium; Tritium, Radioactive; Tritium molecule; HYDROGEN, ISOTOPE OF MASS 3.
Industrial and Commercial Applications
Major Use Sectors
Tritium is used where its radiolabeling or low-energy beta emission is advantageous: nuclear weapons technology (yield boosting and fusion applications), radioluminescent devices and self-luminous signage, radiotracer applications in hydrology, environmental and biological research, and certain analytical/detection applications. It is produced commercially by neutron irradiation of lithium-6-bearing materials and is also generated in reactor systems through neutron capture by deuterium.
Typical Application Examples
- Radioluminescent sources: tritium mixed with phosphors provides long-lived low-level light sources for exit signs, instrument dials, and sighting devices.
- Radiotracer and labeling: tritium-labeled compounds are widely used in biochemical and environmental tracer studies because tritium emits beta radiation without accompanying gamma rays.
- Fusion and nuclear technology: tritium serves as a fuel component or fusion reactant and as a boosting agent in certain weapons systems.
- Research and process uses: calibration of detection equipment, sources for electron-capture detectors, and specialized radiochemical research.
If a concise application summary is required for procurement or specification purposes, selection is typically driven by desired activity (specific or molar activity), chemical form (gas, metal hydride, or aqueous HTO), and containment/packaging needs.
Safety and Handling Overview
Storage and Handling Considerations
Tritium presents both radiological and materials-compatibility challenges. Key considerations: - Forms and containment: elemental tritium (T2/HT) is a flammable, diffusive gas; tritiated water (HTO) behaves as ordinary water chemically and is the main pathway for biological uptake. Gaseous forms require gas-tight, low-permeability containment and pressure-rated vessels designed to mitigate embrittlement and helium buildup from decay. - Materials compatibility: tritium diffuses readily through many polymers and certain metals; many steels and hydride-forming metals are unsuitable for long-term service due to hydrogen-induced embrittlement or hydride formation. Polymers can absorb tritium and suffer radiation-induced degradation; common engineering polymers (e.g., LDPE, PTFE) exhibit permeability or radiolytic degradation and must be chosen only after compatibility assessment. - Storage formats: tritium may be stored as gas in ampoules/pressurized cylinders, bound in metal tritides, or as tritiated water in double containment. Metal-tritide storage reduces free-gas volume but can introduce pyrophoric metal fines or alloying complications on recovery. - Spill and release control: releases of HTO require airborne and surface contamination monitoring; control measures include isolation, absorbents for liquids, and containment/venting through tritium traps (e.g., water bubblers) for exhaust streams.
For detailed hazard, transport and regulatory information, users should refer to the product-specific Safety Data Sheet (SDS) and local legislation.
Occupational Exposure and Protective Measures
- Radiological characteristics: tritium is a weak beta emitter whose radiation does not penetrate intact skin; primary hazard is internal contamination by inhalation, ingestion, or dermal absorption (particularly for HTO).
- Biological behavior and monitoring: tritiated water is rapidly distributed in body water and has a short biological half-life in humans typically on the order of days (commonly reported values in the range of several to ~14 days for the body-water component, with longer retention components for organically bound tritium). Urine bioassay by liquid scintillation counting is the routine monitoring method for occupational exposure.
- Exposure limits and controls: conservative regulatory limits and derived quantities are used for work planning (examples include a maximum permissible body burden value reported as \(37\ \mathrm{MBq}\) (equal to \(1\ \mathrm{mCi}\)) and regulatory drinking-water guidance values at \(20{,}000\ \mathrm{pCi}\,\mathrm{L}^{-1}\)). Derived-air concentration concepts are applied to define acceptable airborne levels for continuous occupational exposure; engineering controls, glove boxes, fume hoods with tritium traps or bubbler systems, and stringent containment are recommended for work with curie-level quantities.
- Personal protective equipment (PPE): selection emphasizes prevention of internal uptake and control of surface contamination. For airborne tritium or significant activity, supplied-air respirators (SCBA or full-face supplied-air) are effective; laboratory procedures commonly use glove boxes, disposable labware, protective clothing, and frequent monitoring and change-out of contaminated gloves. Forced fluid intake and diuretics may be used clinically to reduce biological residence time after high inadvertent intake.
- Emergency and decontamination: immediate removal from exposure, removal of contaminated clothing, and thorough washing are primary steps. For significant internal intake, medical management focuses on supportive care and dose mitigation (e.g., forced fluids). Waste management for tritium-contaminated materials typically follows established radioactive-waste practices and may include solidification of aqueous wastes prior to disposal.
For detailed operational protocols and dose-assessment methodologies, facilities should use established radiation-protection programs, trained radiation-safety personnel, and the applicable legal and regulatory frameworks.
