Osmium tetroxide: Structure, CAS Number, Properties and Uses

Osmium tetroxide, OsO4 (CAS 20816-12-0, PubChem CID 30318), is a platinum group metal compound that is 74.8% osmium by mass and carries a GHS Danger classification for acute toxicity. It sublimes at room temperature, catalyzes the industrial-scale conversion of alkenes to diols at 1 to 2 mol% loading, and stains biological membranes for electron microscopy, three roles that depend directly on the tetrahedral, volatile structure described below. Its identity, structure, PGM content, physical properties, manufacture, hazard profile, catalytic uses and commercial grades follow section by section.

What Is Osmium Tetroxide?#

Osmium tetroxide is the tetroxide of osmium (Os), element 76, with the formula OsO4 and CAS number 20816-12-0. PubChem catalogs the compound under CID 30318, gives its IUPAC name as tetraoxoosmium and its InChIKey as VUVGYHUDAICLFK-UHFFFAOYSA-N, with a molecular weight of 254.2 g/mol; the Hill-system formula O4Os appears in registry databases, but the conventional formula OsO4 is used throughout this page. Older electron-microscopy literature and some supplier catalogs also call the compound osmic acid. Osmium tetroxide is one of the platinum group metal compounds and catalysts cataloged with a dedicated CAS registry entry on this site, built on osmium (Os), element 76.

Osmium Tetroxide Structure and Bonding#

Osmium tetroxide adopts a tetrahedral, d0 molecular geometry, with four oxygen atoms bonded to a central osmium(VIII) center that carries no unpaired d electrons. That electron count makes OsO4 a discrete covalent molecule rather than an ionic oxide, and it is why the compound is volatile enough to sublime at room temperature and lipophilic enough to cross cell membranes, plastics and food packaging, none of which a purely ionic metal oxide would do. The same d0 tetrahedral logic explains ruthenium tetroxide's comparable behavior: RuO4 sits one row above osmium in the periodic table's PGM block, and it too behaves as a volatile molecular oxide rather than a stable ionic solid.

PGM Content of Osmium Tetroxide#

Osmium makes up 74.8% of osmium tetroxide's mass, calculated by dividing osmium's atomic weight of 190.23 g/mol by the compound's molecular weight of 254.2 g/mol (190.23 divided by 254.2 equals 0.748). Because OsO4 is a single, real discrete compound rather than a mixture, this percentage holds for the pure substance without the grade-dependent caveats that a supported catalyst or an alloy would need. The remaining 25.2% of the molecule's mass is oxygen, four atoms bonded to each osmium center, consistent with the tetrahedral structure described above. Refiners value distillate and refining intermediates against this contained-osmium fraction, the same assay logic used for other PGM compounds on this site.

Physical and Chemical Properties#

Osmium tetroxide is a colorless to pale yellow crystalline solid with an acrid, chlorine- and garlic-like odor; most commercial samples appear yellow-tinted because of trace osmium(IV) oxide impurity. The compound melts at 40.25°C and boils at 129.7°C, though it sublimes well below that boiling point at room temperature, driven by a vapor pressure of 7 mmHg at 20°C. Its odor threshold is as low as 0.0019 ppm, according to the New Jersey Department of Health's Right to Know Hazardous Substance Fact Sheet #1441, a concentration far too faint to serve as a reliable warning of hazardous exposure.

OsO4's density is 4.9 g/cm3, a fraction of metallic osmium's own osmium density of 22.587 g/cm3, and its 40.25°C melting point sits roughly 3,000°C below the metal's own conventional osmium melting point of 3,033°C. The CDC/NIOSH Pocket Guide to Chemical Hazards separately records a specific gravity of 5.10 for osmium tetroxide, a third data point that sits closer to the 4.9 g/cm3 figure than to a divergent 1.04 g/cm3 value that also circulates in secondary compilations, though the two still differ by roughly 4% and neither source resolves the gap to more than one significant figure. Published water-solubility figures for OsO4 conflict across sources by an order of magnitude and are not stated here as a single number; the compound is established to be soluble in organic solvents.

How Is Osmium Tetroxide Made?#

Osmium tetroxide forms whenever finely divided osmium metal or PGM refining residue reacts with oxygen, through a laboratory route and an industrial route that share the same oxidation chemistry. In the laboratory, osmium powder reacts with atmospheric oxygen slowly at room temperature, while bulk synthesis needs heating to 400°C to proceed at a useful rate. Industrially, refiners distill OsO4 directly out of PGM refining residues as a dedicated, closed, early step in the flowsheet, separating it from the similarly volatile ruthenium tetroxide by controlling oxidation potential and distillation conditions, since osmium oxidizes more readily than ruthenium.

The manufacturing sequence runs as four steps:

1. React osmium powder with atmospheric oxygen at room temperature, or heat to 400°C for faster bulk conversion.
2. Distil the volatile tetroxide from PGM refining residues, in a closed vessel early in the refining circuit because of OsO4's toxicity.
3. Precipitate the distillate as ammonium hexachloroosmate(IV), (NH4)2OsCl6, CAS 12125-08-5, molecular weight 439.0 g/mol.
4. Calcine the precipitate under hydrogen to reduce it to osmium sponge, the feedstock for further osmium processing.

Osmium metal used as the laboratory starting material is available as osmium powder in research quantities.

Safety and Hazard Classification#

Osmium tetroxide carries the GHS signal word Danger together with ten acute and chronic hazard statement codes: H300, H301, H310, H314, H318, H330, H334, H370, H372 and H373. GHS classification reported to PubChem: signal word Danger, ten hazard statement codes and 17 separate classification notifications on file for CID 30318, one of the most hazardous classification profiles of any compound cataloged on this site.

Occupational exposure limits for osmium tetroxide converge tightly around 0.002 mg/m3, measured as osmium (as Os) rather than as the whole OsO4 molecule, despite differing between agencies. The New Jersey Department of Health's Right to Know Hazardous Substance Fact Sheet #1441 lists an OSHA permissible exposure limit of 0.002 mg/m3 as an 8-hour time-weighted average, a figure cross-checked against ChemicalBook's technical data; OSHA's own general-industry Table Z-1 carries no compound-specific line item for osmium tetroxide, so this 0.002 mg/m3 figure is the working limit reported by secondary compilers rather than a dedicated Table Z-1 entry, with Cal/OSHA the agency that does list it. ACGIH's threshold limit value matches at 0.002 mg/m3 TWA with a 0.006 mg/m3 short-term exposure limit, and NIOSH's recommended exposure limit is 0.002 mg/m3 (0.0002 ppm) over a 10-hour workday with a 0.006 mg/m3 ceiling not to be exceeded in any 15-minute period. NIOSH sets the immediately-dangerous-to-life-or-health level at 1 mg/m3.

That IDLH figure matters alongside OsO4's vapor pressure: 7 mmHg at 20°C paired with a 1 mg/m3 IDLH means an open vial at room temperature can generate hazardous headspace concentrations without any heating or agitation. The CDC/NIOSH Pocket Guide to Chemical Hazards also names hydrochloric acid and easily oxidized organic materials as key incompatibilities for osmium tetroxide, alongside the IDLH threshold above. Acute inhalation osmium toxicity data corroborate the low exposure limits.

PubChem's aggregated mirror of the EU's Classification and Labelling Inventory, used here in place of direct ECHA access, records 58 individual notifications across 6 distinct classifications for osmium tetroxide: every notifier classifies it Acute Tox. 1 and 2 and Skin Corr. 1B, and 70.7% add Resp. Sens. 1, with GHS05 (corrosive), GHS06 (acute toxic) and, in most notifications, GHS08 (health hazard) pictograms under signal word Danger. The same compound is listed on the Australian Inventory of Industrial Chemicals as "Osmium oxide (OsO4), (T-4)-" and carries a New Zealand HSNO approval, HSR002793, for controlled use, per PubChem's regulatory-information mirror of both inventories. In the United States, osmium tetroxide carries a CERCLA reportable quantity of 1,000 lb (454 kg), above which any release to the environment must be reported to the National Response Center, and it is regulated as RCRA hazardous waste code P087 when discarded; the EPA's CompTox Chemicals Dashboard separately lists the compound under substance identifier DTXSID5042245 for CAS 20816-12-0.

OsO4 also irreversibly stains the human cornea on contact, which can cause blindness, and it penetrates plastics and food packaging, so it must be stored in glass under refrigeration rather than in standard laboratory containers. For transport, osmium tetroxide is regulated as UN 2471, transport hazard class 6.1 (toxic), packing group I, and is listed on the US Toxic Substances Control Act inventory, per the same New Jersey fact sheet cross-checked against ChemicalBook's regulatory data. Osmium(IV) oxide, OsO4's lower-valent sibling, is explicitly non-toxic and non-volatile in its stable crystalline form, per Wikipedia's osmium dioxide entry, a contrast covered further under osmium toxicity and in the related-compounds table below.

Handling osmium tetroxide safely follows four rules:

1. Store OsO4 in glass under refrigeration.
2. Handle it only in a fume hood.
3. Keep it away from plastic and food-grade packaging.
4. Treat any eye exposure as a medical emergency.

What Is Osmium Tetroxide Used For?#

Osmium tetroxide is used for electron-microscopy lipid staining, catalytic syn-dihydroxylation, latent fingerprint development and PGM refining, with dihydroxylation the only large-scale industrial catalysis among the four. In biological transmission electron microscopy, OsO4 binds lipids and unsaturated bonds, providing the primary contrast mechanism for imaging cell membranes, and the same binding chemistry reveals block-copolymer microstructure in materials-science samples. Latent fingerprint development draws on the same reactivity toward organic surface residue. In PGM refining, OsO4's volatility is the separation mechanism itself, distilled directly out of process residues as described above. Catalytic OsO4 also serves as a benchmark cluster-chemistry precursor: three equivalents of OsO4 react with 24 equivalents of carbon monoxide to give the cluster Os3(CO)12 plus 12 equivalents of carbon dioxide, in near-quantitative yield.

The four established uses are:

• Electron-microscopy lipid staining, the primary contrast mechanism for biological membranes.
• Catalytic syn-dihydroxylation, converting alkenes to 1,2-diols.
• Latent fingerprint development, revealing prints through organic-residue reactivity.
• PGM refining, exploiting OsO4's volatility for distillation-based separation.

Catalytic Cycle: Dihydroxylation#

The Upjohn dihydroxylation runs on catalytic osmium tetroxide, typically 1 to 2 mol%, with N-methylmorpholine N-oxide (NMO) as the stoichiometric re-oxidant that returns reduced osmium back to OsO4 and closes the catalytic cycle, converting an alkene to a syn-1,2-diol.

The related Sharpless asymmetric dihydroxylation adds cinchona-alkaloid ligands from the DHQ or DHQD families, commonly supplied as AD-mix-alpha or AD-mix-beta, to convert alkenes into enantiomerically enriched vicinal diols. K. Barry Sharpless received a share of the 2001 Nobel Prize in Chemistry for this and related asymmetric oxidation work, since applied to anti-HIV agents, beta-lactam antibiotics and other natural-product analogues. A safety-driven variant avoids handling neat OsO4 at all: potassium osmate, K2OsO2(OH)4, is oxidized by NMO to generate osmium tetroxide in situ, so the operator never measures out the volatile tetroxide directly.

That handling constraint is also why osmium catalysis never scales to bulk chemical production. OsO4's threshold limit value of 0.0002 ppm (0.002 mg/m3) is roughly five to six orders of magnitude tighter, on a parts-per-million basis, than common solvent exposure limits such as toluene's 20 ppm, acetone's 250 ppm or ethanol's 1,000 ppm, and about 4.5 orders of magnitude tighter on a milligram-per-cubic-meter basis.

Commercial Forms, Purity and Price#

Osmium tetroxide is sold as neat crystals in several purity grades and as pre-made dilute solutions for safer handling, the standard commercial split for a compound this hazardous to weigh out directly. ReagentPlus and ACS reagent grades run 98.0% to 99.8% purity as the neat solid, per Sigma-Aldrich's catalog data (product 201030 at 99.8% purity), while Thermo Fisher offers 99.9%-plus trace-metal-basis crystals for high-purity synthesis and Ladd Research supplies AR-grade crystals for electron-microscopy preparation. Far more common for routine handling are pre-made dilute aqueous solutions at a stated weight percentage, such as Sigma-Aldrich's 4 wt% in water grade (product 251755) and comparable 1 to 4 wt% solutions from EMS Diasum and Polysciences, all shipped and stored refrigerated in tightly sealed secondary containment.

Laboratories that need osmium's dihydroxylation chemistry without handling the volatile tetroxide at all typically substitute potassium osmate dihydrate, K2OsO4·2H2O (CAS 10022-66-9), a non-volatile surrogate covered further in the related-compounds table below. No independently verified reagent-grade price exists for osmium tetroxide itself; buyers instead track osmium price today for the underlying metal's live market value, and investors researching the metal directly can compare it against crystalline osmium as an investment.

Where Osmium Tetroxide Chemistry Stops: Jewelry, Converters and Medicine#

This page covers osmium tetroxide as a laboratory reagent and refining intermediate only, not as jewelry, converter scrap or a medical treatment. Catalytic converters recover platinum, palladium and rhodium, not osmium, so scrap converters carry no recoverable OsO4 value despite osmium's own PGM classification. Osmium's role in jewelry alloys and any osmium-related pharmacology are separate subjects, covered without dosing or clinical content on osmium compounds elsewhere on this site.

Related PGM Compounds#

Osmium tetroxide shares its chemistry with three other named osmium compounds and one sibling PGM tetroxide, tabulated below by formula, CAS number and relationship to OsO4. Potassium osmate dihydrate's [OsO2(OH)4]2- anion is octahedral, with Os=O bonds reported at 1.75 Å and Os-OH bonds at 1.99 Å, and the complex formally satisfies the 18-electron rule, an unusual feature among metal-oxo complexes, according to Wikipedia's entry on the compound.

History and Naming#

Osmium was discovered in 1803 by Smithson Tennant, an English chemist working in partnership with William Hyde Wollaston on a shared batch of South American crude platinum ore. Wollaston processed the ore's soluble fraction into palladium and rhodium, while Tennant fused the insoluble black residue with sodium hydroxide and then distilled the volatile oxide from the acidified solution, the same fusion-and-distill chemistry that industrial refiners still use to recover osmium today. Tennant announced both osmium and iridium to the Royal Society in a letter dated 21 June 1804. He named the new element osmium from the Greek osme, "a smell," describing the tetroxide's odor as chlorine-like and slightly garlic-like; the name therefore comes from OsO4's smell, not from any odor in the metal itself, which is odorless in bulk form. Read more on osmium discovery.

Frequently Asked Questions#

What is osmium tetroxide used for? Osmium tetroxide's four established uses are electron-microscopy lipid staining, catalytic syn-dihydroxylation of alkenes, latent fingerprint development and PGM refining. Its dihydroxylation role, run at 1 to 2 mol% loading with NMO as the stoichiometric re-oxidant, is the compound's only large-scale industrial catalysis.

How expensive is osmium tetroxide? No independently verified reagent-grade price exists for osmium tetroxide itself; suppliers price it by grade and pack size rather than by a public benchmark. Buyers instead track osmium price today for the underlying metal's live market value, since OsO4's cost follows osmium's contained-metal value plus the cost of refrigerated, secondary-contained shipping.

What does osmium tetroxide do to the body? Osmium tetroxide irreversibly stains the human cornea and can cause blindness on contact, and its GHS classification carries ten hazard statement codes reported to PubChem covering acute oral, dermal and inhalation toxicity plus organ-specific effects. Inhalation studies report an LC50 of 423 mg/m3 in rats and mice over 4 hours and 1,316 mg/m3 in rabbits over 30 minutes.

Is osmium safe to touch? Bulk osmium metal is safe to touch; it is chemically unreactive at room temperature and does not carry the tetroxide's hazard profile. Osmium tetroxide is a different matter: it sublimes readily at room temperature, so opening a container releases vapor that can contact skin and eyes, and it should always be handled with gloves in a fume hood.

Is osmium rarer than gold? Osmium is rarer than gold by annual mine output, at roughly 500 to 550 kilograms a year worldwide, per SFA (Oxford) and the International Platinum Group Metals Association's 2026 osmium fact sheet, against gold's far larger annual production, but the comparison depends on which measure is used. See how rare is osmium for the full breakdown by crustal abundance, mine output and above-ground stock.

What are osmium tetroxide's CAS number, formula and PGM identity? Osmium tetroxide's identity triple is CAS 20816-12-0, molecular formula OsO4 (PubChem Hill formula O4Os) and PGM osmium (Os), element 76. PubChem catalogs it under CID 30318 with molecular weight 254.2 g/mol and the IUPAC name tetraoxoosmium.

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