Selenium Trioxide (13768-86-0) Physical and Chemical Properties
Selenium Trioxide
Inorganic oxidizing reagent supplied as a hygroscopic crystalline powder for specialty materials manufacturing, surface processing and photovoltaic component applications.
| CAS Number | 13768-86-0 |
| Family | Inorganic oxides |
| Typical Form | Powder or crystalline solid |
| Common Grades | EP |
Selenium trioxide is an inorganic molecular oxide of selenium in the +6 oxidation state; it is best classified as an acidic oxide (selenium(VI) oxide) and as a strong inorganic oxidizing agent. The solid exists as a hygroscopic crystalline material with a dense, polymeric/oligomeric network in the solid state; the formal molecular composition in molecular-descriptor records is presented as \(\ce{O3Se}\) (commonly written as \(\ce{SeO3}\)), a neutral covalent oxide of selenium. Electronically, selenium in this oxide is in a high oxidation state (+6) and the molecule presents three oxygen atoms capable of accepting electron density — reflected in a calculated topological polar surface area of 51.2 Å2 and three hydrogen-bond-acceptor sites in descriptor tables.
Chemically, selenium trioxide functions as an acid anhydride: it reacts with water to give strongly acidic selenic acid \(\ce{H2SeO4}\). In practice this oxide behaves as a potent oxidant and as a water‑reactive material; aqueous contact generates selenic acid and selenate species, producing strongly acidic solutions and evolving heat under rapid hydrolysis. Toxicity and environmental profiles follow those of high‑valent inorganic selenium: the compound can be corrosive, is readily absorbed by inhalation or ingestion in particulate form, and aqueous releases are harmful to aquatic life.
Common commercial grades reported for this substance include: EP.
Basic Physical Properties
Density
Reported experimental density values indicate a high specific gravity for the crystalline solid. Density is listed as \(3.6\) at \(68\,^\circ\mathrm{F}\) (denser than water; will sink) and as \(3.6\,\mathrm{g}\,\mathrm{cm}^{-3}\). The high density is consistent with a compact, inorganic oxide lattice and with the presence of a heavy chalcogen atom (selenium) in the formula \(\ce{O3Se}\).
Melting or Decomposition Point
Melting‑point data are reported as \(244\,^\circ\mathrm{F}\) (equivalent on the same source to \(118\,^\circ\mathrm{C}\)). Thermal decomposition is reported at \(180\,^\circ\mathrm{C}\). Users should note that recorded thermal events may reflect onset of melt for a crystalline phase and subsequent decomposition at higher temperature; solid‑state samples may also undergo partial decomposition or sublimation depending on sample history and atmosphere.
Solubility in Water
Solubility in water is reported qualitatively as "very good." Practically, \(\ce{SeO3}\) reacts vigorously with \(\ce{H2O}\) to generate selenic acid, so the observed high solubility is coupled with chemical conversion rather than simple physical dissolution.
Solution pH (Qualitative Behavior)
Aqueous contact produces strongly acidic solutions because \(\ce{SeO3}\) hydrolyses to form selenic acid \(\ce{H2SeO4}\). Therefore aqueous solutions are strongly acidic (low \(\mathrm{pH}\)) and will behave like solutions of a strong acid/oxidant from an acid–base and corrosivity standpoint.
Chemical Properties
Acid–Base Behavior
Selenium trioxide is the acid anhydride of selenic acid and exhibits classical acidic oxide behavior: it hydrolyses to \(\ce{H2SeO4}\) on contact with water and will react with bases to give selenate salts containing \(\ce{SeO4^2-}\). As an oxide of a high oxidation‑state nonmetal it is not a base; in nongaseous environments it serves as an electron acceptor and proton‑forming species upon hydrolysis.
Reactivity and Stability
The compound is classified as a strong oxidizing agent and is water‑reactive. It reacts vigorously with water and can oxidize organic and inorganic reducing agents; reactions with reducing materials (including many organics, metals, cyanides, esters and thiocyanates) can be exothermic and may generate gaseous products or lead to fire/explosion if initiation occurs. The solid is hygroscopic and may give off corrosive and toxic fumes upon heating or decomposition. Control of dust, avoidance of contact with combustible materials, and exclusion of reducing contaminants are critical for stability management.
Molecular and Ionic Parameters
Formula and Molecular Weight
Primary computed molecular formula entries list the composition as \(\ce{O3Se}\) (equivalently written as \(\ce{SeO3}\)). Reported molecular weights and exact masses from computed descriptors are: - Molecular weight: \(126.97\,\mathrm{g}\,\mathrm{mol}^{-1}\) - Exact/monoisotopic mass: \(127.90127\)
Other computed descriptors: topological polar surface area \(51.2\), hydrogen bond donor count \(0\), hydrogen bond acceptor count \(3\), rotatable bond count \(0\), formal charge \(0\).
SMILES and InChI identifiers (inline code):
- SMILES: O=[Se](=O)=O
- InChI: InChI=1S/O3Se/c1-4(2)3
- InChIKey: VFLXBUJKRRJAKY-UHFFFAOYSA-N
Constituent Ions
In the pure solid state selenium trioxide is a neutral covalent molecular oxide (formal charge 0). In aqueous media it is converted to selenic acid \(\ce{H2SeO4}\) and, depending on pH, to the selenate anion \(\ce{SeO4^2-}\). There are no stable simple ionic constituents in the anhydrous solid; ionic speciation is a solution phenomenon following hydrolysis and acid dissociation.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS RN: 13768-86-0
- EC (European Community) number: 237-385-1
- UN number (transport reference present in records): 3283
- DSSTox Substance ID: DTXSID50895004
- ICSC Number: 0949
- Nikkaji Number: J43.597J
- Wikidata identifier: Q6070
Also recorded in descriptor fields: InChI and InChIKey (see Molecular section).
Synonyms and Common Names
Recorded synonyms and alternative names include (selected from supplied identifiers): - Selenium trioxide - Selenium oxide (SeO3) - selenonylideneoxidane - selenic anhydride - Trioxide, Selenium - Selenium(VI) trioxide - O3Se
Use of these synonyms in procurement and specification documents should be cross‑checked against registry numbers to avoid ambiguity.
Industrial and Commercial Applications
Functional Roles and Use Sectors
Selenium trioxide is used industrially as an oxide precursor and as a source of selenic acid for applications that require high‑valent selenium species. It has documented use in the manufacture of photoelectric cells and solar‑energy devices where selenium chemistry or thin‑film selenium derivatives are employed. More generally, materials in this chemical class can serve as oxidizers or precursors to selenate salts used in specialized manufacturing processes.
Typical Application Examples
Explicitly reported application: manufacture of photoelectric cells and solar energy devices. In practice this substance is selected where a high‑valent selenium oxidant or a source of selenic acid is required; selection is guided by its oxidizing power and ease of conversion to \(\ce{H2SeO4}\) or selenate species. No additional concise application summary is available in the current data context; in practice this substance is selected based on its general properties described above.
Safety and Handling Overview
Health and Environmental Hazards
Selenium trioxide is acutely and chronically toxic if inhaled, ingested, or in contact with skin. Exposure can produce respiratory irritation, bronchospasm, pneumonitis, gastrointestinal symptoms, and a characteristic garlic‑like breath odor. Repeated or prolonged exposure can cause systemic effects (including selenosis with hair and nail changes, neurological symptoms, and potential effects on reproductive parameters). The material is corrosive to skin, eyes and respiratory tract and can cause severe burns on contact. A chronic oral minimal risk level recorded in toxicological summaries is \(0.005\,\mathrm{mg}\,\mathrm{kg}^{-1}\,\mathrm{day}^{-1}\). Environmentally, aqueous releases are harmful to aquatic organisms; the oxidizing nature and mobility of selenate species make containment and wastewater treatment considerations important.
For emergency measures, standard recommendations include removal from exposure, decontamination by copious water irrigation (eyes/skin), respiratory support for inhalation, and avoidance of inducing vomiting after ingestion; seek prompt medical attention.
Storage and Handling Considerations
Handle as a strong oxidant and water‑reactive, corrosive solid. Key precautions: - Store dry, in a well‑ventilated area, separated from combustible, reducing, and organic materials. - Prevent dust generation and control airborne particulates; use local exhaust ventilation. - Use appropriate personal protective equipment (PPE): respiratory protection for dust/aerosols, chemical‑resistant gloves, eye/face protection, and protective clothing. - Avoid contact with moisture; do not allow to contact combustible absorbents (e.g., sawdust). - Spills: isolate area, prevent environmental release, avoid use of combustible absorbents; collect into appropriate containers for disposal under local regulation. For detailed hazard, transport and regulatory information, users should refer to the product‑specific Safety Data Sheet (SDS) and local legislation.
