Osmium

Typical wastewaters

• laboratory waste (osmium tetroxide stain and oxidant) OsO₄ (aq), a volatile molecular oxide dissolved without ionising; reduced in the fume hood to insoluble OsO₂ before disposal as heavy metal waste, never poured to sewer the reductant is not specified by the sources read
• municipal sewage (platinum group point sources) particulate osmium ending in sewage sludge and incinerator ash, up to 12 µg/kg in 91 UK samples, higher in Sheffield and London ash measured in the sludge, not the water; attributed to point industrial sources

1 · Identity

Symbol, number

Os, 76

Oxidation states in water

+8 as osmium tetroxide OsO₄, a volatile molecular oxide that dissolves in water without ionising; +6 as osmate from OsO₄ and alkali; +4 and +3 in the chlorides and the insoluble OsO₂. The metal does not dissolve.

Note

The element entry covers the metal, its density and the tetroxide hazard. This chapter is about OsO₄ in laboratory drains and the platinum group signal in sludge.

2 · Occurrence in water

Natural sources

Native osmium and osmiridium in platinum group deposits and river placers (element entry); dissolved osmium in natural water is at femtomolar levels and no measured value was read.

Anthropogenic sources

Laboratory use of OsO₄ as a biological stain and as a diol forming oxidant in organic synthesis; catalyst waste; platinum group metal refining. UK sewage sludge and incinerator ash reached 12 ppb osmium against 602 ppb platinum and 710 ppb palladium, and the osmium, iridium and ruthenium in Sheffield and London ash were higher than elsewhere and attributed to point industrial sources (Jackson 2010).

3 · Speciation

Osmium tetroxide is a molecular oxide, soluble in water to about 6 percent at room temperature and volatile from solution (NIOSH); it does not hydrolyse to an anion at neutral pH but is reduced by any oxidisable organic matter to black OsO₂ and lower oxides. In alkali it forms the +6 osmate K₂OsO₂(OH)₄ (element entry). The metal, OsO₂ and the sulfides are insoluble. No natural water speciation study was read.

Solubility

OsO₄ 6 percent in water at 25 C (NIOSH); the metal, OsO₂ and the sulfides insoluble.

Hydrolysis

OsO₄ stays molecular in neutral water.

Complexation

Chloro complexes in hydrochloric acid; not relevant to natural water.

Precipitates

OsO₂ from reduction; osmium sulfide from sulfide precipitation of refinery liquors.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Any oxidising preservation or digestion volatilises osmium as OsO₄; keep samples reduced or distil and trap the tetroxide on purpose. Osmium-187 interferes with rhenium-187 in ICP-MS.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

The metal is not toxic; the tetroxide is very toxic to lungs, skin and eyes (element entry, NIOSH). No oral or drinking water assessment was read.

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The only concentration is a sludge and ash maximum from one UK study; no water value was read.
• The NIOSH and OSHA numbers are workplace air limits and are placed under problems, not limits.
• The reduction step for OsO₄ waste is standard laboratory practice; the reagent and stoichiometry were not read and are not written.

Gaps

• No source read gives osmium in seawater, rivers, groundwater, drinking water or wastewater as a dissolved concentration.
• EU DWD 2020/2184 Annex I was not read this session.
• No aquatic toxicity or oral toxicity value for osmium or OsO₄ was read.
• No GCC discharge standard was read.

Sources

Identity

Name and symbol

Osmium, Os

Atomic number

76 protons

Position

group 8 · period 6 · d-block · transition metal

CAS number

7440-04-2

Atomic structure

Atomic mass

190.23 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁶
[Xe] 6s²⁴f¹⁴⁵d⁶

Electrons per shell

2, 8, 18, 32, 14, 2

Valence electrons

8 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

3,306 K (3,032.85 °C)

Boiling point

5,285 K (5,011.85 °C)

Density

22.57 g/cm3

Appearance

silvery, blue cast

Thermal conductivity

87.6 W/(m·K)

Electrical resistivity

81.2 nΩ·m at 0 °C

Electrical conductivity

12.32 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

24.7 J/(mol·K)

Chemical properties

Oxidation states

+4, +3

Electronegativity

2.2 (Pauling Scale)

Ionisation energy

8.7 eV
1st 840, 2nd 1,600 kJ/mol

Electron affinity

1.1 eV

Atomic radius

empirical 144, covalent 144, van der Waals 216 pm

Ionic radius

Os⁴⁺ 63; Os⁵⁺ 58; Os⁶⁺ 55; Os⁷⁺ 53; Os⁸⁺ 39 (4-coordinate) pm

Reactivity

The densest platinum-group metal, inert in bulk at ordinary temperatures, yet the easiest of the group to oxidise all the way to +8: the powder slowly gives off volatile, toxic osmium tetroxide in air, and hot oxidising acids or fused alkali oxidants attack it.

with water

Does not react with water.

with oxygen, air

The bulk metal is stable, but powdered or spongy osmium slowly gives off the tetroxide in air and the metal burns on heating: $\ce{Os + 2 O2 -> OsO4}$, a volatile solid boiling at 130 C with a strong smell.

with acids

Resists most acids including aqua regia; hot concentrated nitric acid oxidises it to OsO4, and fused alkali with an oxidant such as sodium peroxide converts it to osmate salts.

with halogens

Attacked by fluorine and chlorine at high temperature, fluorine giving the hexafluoride: $\ce{Os + 3 F2 -> OsF6}$, and chlorine the tetrachloride: $\ce{Os + 2 Cl2 -> OsCl4}$, with the lower halides stabilised by the heavier halogens.

Typical compounds

• OsO₄ osmium tetroxide volatile +8 oxide, biological stain and diol-forming oxidant
• OsO₂ osmium(IV) oxide dark, non-volatile, far less reactive oxide
• K₂OsO₂(OH)₄ potassium osmate the +6 osmate from tetroxide and alkali
• OsCl₃ osmium(III) chloride common starting halide for osmium complexes
• OsF₆ osmium hexafluoride yellow volatile +6 fluoride
• Os₃(CO)₁₂ triosmium dodecacarbonyl zero-valent cluster carbonyl

Occurrence, production and use

Crustal abundance

1.5×10-3 milligrams per kilogram

Oceanic abundance

Not Applicable

Occurrence and sources

Osmium occurs in iridosule and in platinum-bearing river sands in the Urals, North America, and South America. It is also found in the nickel-bearing ores of Sudbury, Ontario region along with other platinum metals. While the quantity of platinum metals in these ores is very small, the large tonnages of processed nickel ores make commercial recovery possible.

• native osmium and osmiridium (Os-Ir alloy) with other platinum metals PGM deposits of South Africa, Russia, Zimbabwe, Canada, United States
• crustal abundance 0.000037 ppm (BGS group figure for all PGMs via RSC); 0.0015 mg/kg crust (PubChem)

Extraction, production

By-product of nickel refining

no separation chemistry stated; no production statistics published

Uses

Metallic osmium is hard, brittle and very difficult to make. Powdered osmium is easier to make but emits osmium tetroxide (OsO4) when it is exposed to the air. Unfortunately, osmium tetroxide smells bad and is very poisonous. Because of these problems, osmium is primarily used to make very hard alloys. Osmium alloys can be found in ball point pen tips, fountain pen tips, record player needles, electrical contacts and other devices where frictional wear must be minimized.

The tetroxide has been used to detect fingerprints and to stain fatty tissue for microscope slides. The metal is almost entirely used to produce very hard alloys with other metals of the platinum group for fountain pen tips, instrument pivots, phonograph needles, and electrical contacts.

• Hard alloys: osmium-iridium tips for fountain pens, instrument pivots, needles, electrical contacts
• Chemicals: osmium catalysts

Safety, toxicity

Concentrations in air as low as 107 g/m3 can cause lung congestion, skin damage, or eye damage. Exposure to osmium tetroxide should not exceed 0.0016 mg/m3 (8-hour time weighted average - 40-hour work week).

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H315 Causes skin irritation Skin corrosion/irritation
• H318 Causes serious eye damage Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H413 May cause long lasting harmful effects to aquatic life to the aquatic environment, long-term hazard

Discovery and name

Discovered by

Smithson Tennant

Discovered

1803

First isolated

not in sources

Named by

not in sources

Origin of the name

after Greek osme, "a smell", for the smell of the volatile osmium tetroxide

The metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and the lowest vapor pressure of the platinum group. The metal is very difficult to fabricate, but the powdered or spongy metal slowly gives off osmium tetroxide, which as a powerful oxidizing agent and has a strong smell. The tetroxide is highly toxic, and boils at 130°C.

Density measurements show osmium to be a little more dense than iridium, and osmium is often cited as the heavier element. However, calculations of the density from the space lattice, which may be more reliable than these measurements, give a density of 22.65 for iridium compared to 22.61 for osmium. According to IUPAC, because of this apparent contradiction, no decision has been made as to which is heavier.