Iridium

Typical wastewaters

• municipal sewage (platinum group point sources) particulate iridium ending in sewage sludge and incinerator ash, up to 33 µg/kg in 91 UK samples, higher in Sheffield and London ash measured in the sludge, not the water; the converter pattern is Pt, Pd and Rh, so the iridium is attributed to point industrial sources

1 · Identity

Symbol, number

Ir, 77

Oxidation states in water

+3 and +4 in the chloro complexes IrCl₆³⁻ and IrCl₆²⁻ of refinery liquors and in the oxide IrO₂; the metal is inert. No natural water speciation was read.

Note

The element entry covers the metal, crucibles, spark plugs and the standard metre. This chapter says only where iridium touches water.

2 · Occurrence in water

Natural sources

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

Anthropogenic sources

Automobile catalysts are the main platinum group source to urban drainage and sewage, but the pattern they leave is platinum, palladium and rhodium; iridium in UK sewage sludge and incinerator ash reached 33 ppb and, with osmium and ruthenium, was higher in Sheffield and London ash and attributed to point industrial sources (Jackson 2010). Spent iridium catalysts and iridium oxide anode coatings are recycled, not discharged.

3 · Speciation

The metal is attacked by nothing in water. Iridium enters solution only as chloro complexes, IrCl₆³⁻ for Ir(III) and IrCl₆²⁻ for Ir(IV), made with chlorine in hydrochloric acid (element entry); on dilution and neutralisation these hydrolyse slowly to hydrous oxide. IrO₂ is insoluble and is the electrocatalytic coating of mixed metal oxide anodes. No stability constants or natural water speciation were read.

Solubility

Metal and IrO₂ insoluble; hexachloroiridates soluble.

Hydrolysis

Slow hydrolysis of the chloro complexes on neutralisation; not quantified in the sources read.

Complexation

Chloride in acid; nothing sourced for natural water.

Precipitates

Hydrous iridium oxide; iridium sulfide in refinery precipitation.

4 · Role in treatment

5 · Removal and control

Not relevant or not given for this element.

6 · Analytics

Sampling pitfalls

The metal resists digestion; fire assay or a chlorine and hydrochloric acid attack is needed to bring it into solution.

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

Low toxicity and no known biological role (element entry); no drinking water assessment exists.

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 anode coating note rests on the element entry and textbook electrochemistry; no coating wear or iridium release figure was read.

Gaps

• No source read gives iridium 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 or oral toxicity value for iridium was read.
• No removal study exists in the sources read; the removal list is empty on purpose.
• No GCC discharge standard was read.

Sources

Identity

Name and symbol

Iridium, Ir

Atomic number

77 protons

Position

group 9 · period 6 · d-block · transition metal

CAS number

7439-88-5

Atomic structure

Atomic mass

192.217 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁷
[Xe] 6s²⁴f¹⁴⁵d⁷

Electrons per shell

2, 8, 18, 32, 15, 2

Valence electrons

9 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,719 K (2,445.85 °C)

Boiling point

4,701 K (4,427.85 °C)

Density

22.42 g/cm3

Appearance

Silvery white

Thermal conductivity

147 W/(m·K)

Electrical resistivity

47.1 nΩ·m at 20 °C

Electrical conductivity

21.23 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

25.1 J/(mol·K)

Chemical properties

Oxidation states

+4, +3

Electronegativity

2.2 (Pauling Scale)

Ionisation energy

9.1 eV
1st 880, 2nd 1,600 kJ/mol

Electron affinity

1.565 eV

Atomic radius

empirical 141, covalent 141, van der Waals 202 pm

Ionic radius

Ir³⁺ 68; Ir⁴⁺ 63; Ir⁵⁺ 57 pm

Reactivity

The most corrosion-resistant metal known: no acid, not even aqua regia, attacks the bulk metal, and it keeps that inertness to about 2000 C; only the halogens and oxygen at high temperature, fused salts, and cyanide in the presence of oxygen react with it, giving mainly +3 and +4 compounds.

with water

Does not react with water.

with oxygen, air

No reaction in air at ordinary temperatures; at high temperature oxygen forms the blue-black dioxide: $\ce{Ir + O2 -> IrO2}$, the only well characterised binary oxide.

with acids

Not attacked by any acid nor by aqua regia; it dissolves only in concentrated hydrochloric acid with sodium perchlorate present, and it is attacked by molten salts such as sodium chloride and sodium cyanide.

with halogens

The halogens react at elevated temperature, chlorine giving the trichloride: $\ce{2 Ir + 3 Cl2 -> 2 IrCl3}$, and fluorine the volatile yellow hexafluoride: $\ce{Ir + 3 F2 -> IrF6}$, a strong oxidant.

Typical compounds

• IrO₂ iridium(IV) oxide blue-black rutile-type oxide, chlorine-cell anode coating
• IrCl₃ iridium(III) chloride the common starting halide for iridium compounds
• IrF₆ iridium hexafluoride volatile yellow +6 fluoride, decomposes in water
• H₂IrCl₆ hexachloroiridic acid soluble form obtained from chlorine and hydrochloric acid
• Ir₄(CO)₁₂ tetrairidium dodecacarbonyl very stable tetrahedral carbonyl cluster

Occurrence, production and use

Crustal abundance

1×10-3 milligrams per kilogram

Oceanic abundance

Not Applicable

Occurrence and sources

Iridium occurs uncombined in nature with platinum and other metals of this family in alluvial deposits. It is recovered as a by-product from the nickel mining industry.

• native iridium and osmiridium in river sediments; with other PGMs in sulfide ores PGM deposits of South Africa, Russia, Zimbabwe, Canada, United States
• crustal abundance 0.000037 ppm (BGS group figure for all PGMs via RSC); 0.001 mg/kg crust (PubChem)

Extraction, production

By-product of nickel refining

no separation chemistry stated; no production statistics published

Uses

Pure iridium is very brittle and is nearly impossible to machine. It is primarily used as a hardening agent for platinum. Platinum-iridium alloys are used to make crucibles and other high temperature equipment. Iridium is also alloyed with osmium to make the tips of fountain pens and compass bearings.

Iridium is the most corrosive resistant metal known. For this reason, the standard meter bar was created from an alloy of 90% platinum and 10% iridium. This bar was replaced as the definition of the meter in 1960 when the meter was redefined in terms of the orange-red spectral line of krypton-86.

A thin, worldwide layer of iridium exists in a layer of sediment that was put down at the end of the Cretaceous period. Since meteors and asteroids contain a higher percentage of iridium than the earth's crust, this iridium enriched layer is seen as evidence that the earth was struck by a large meteor or asteroid at that time. Dust from the impact would have spread around the globe, depositing the iridium. The dust also would have blocked the sun for a time, resulting in the extinction of many plant and animal species, including the dinosaurs.

Although its principal use is as a hardening agent for platinum, iridium is also used to make crucibles and devices requiring high temperatures. It is also used for electrical contacts.

The element forms an alloy with osmium which is used for tipping pens and compass bearings.

• Electrical and precision alloys: spark-plug contacts; osmium-iridium pen tips and compass bearings; platinum-iridium standards and crucibles
• Mining: co-product of PGM and nickel mining

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H319 Causes serious eye irritation Serious eye damage/eye 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 Ἶρις, the Greek goddess of the rainbow, for the strong colors of its salts

Iridium, a metal of the platinum family, is white (similar to platinum) but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work.

It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see Krypton).

Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. The specific gravity of iridium is to osmium's specific gravity. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm^3, respectively. These values may be more reliable than actual physical measurements for determining which element is heavier.