Chemical Nature of Iridium
Iridium (Ir), with atomic number 77, is an extremely dense, hard, and corrosion-resistant transition metal. It belongs to the platinum group metals (PGMs) and is notable for being one of the most corrosion-resistant metals known. Its name originates from the Greek goddess Iris, referring to the striking colors of its compounds.
Reactivity with Water
Iridium exhibits exceptionally low reactivity with water. It does not react with water or steam, even at high temperatures. This remarkable resistance to aqueous environments is a key property that contributes to its use in highly demanding applications where corrosion is a major concern.
Reactivity with Air (Oxygen)
Iridium’s reactivity with air, specifically oxygen, is also very low. At ambient temperatures, Iridium does not oxidize or tarnish in air. This inertness makes it highly stable. However, when heated to very high temperatures (typically above 1100°C), Iridium can slowly react with oxygen to form iridium oxides, such as iridium dioxide (IrO₂). This oxidation is generally reversible upon further heating in the absence of oxygen.
Toxicity
In its metallic form, Iridium is generally considered non-toxic. Its inertness means it does not readily react with biological systems. However, like many heavy metals, certain Iridium compounds, particularly those in higher oxidation states, can exhibit toxicity and should be handled with appropriate precautions.
Radioactivity
Naturally occurring Iridium is composed of two stable isotopes: Iridium-191 and Iridium-193. Therefore, naturally occurring Iridium is not radioactive. However, several artificial radioactive isotopes of Iridium have been produced in laboratories, such as Iridium-192, which is used in industrial radiography and brachytherapy (a form of radiation therapy).
Flammability
Iridium is a metal and is not flammable. It does not ignite or sustain combustion under normal conditions, or even under extreme heating, exhibiting no flammability.
Illustrative Chemical Reaction
Due to its extreme inertness, Iridium’s direct chemical reactions are often limited to very specific and harsh conditions. A notable reaction involves its interaction with halogens at elevated temperatures.
Reaction with Chlorine Gas: Iridium reacts with chlorine gas (Cl₂) when heated to temperatures around 450-500°C to form iridium(III) chloride (IrCl₃). This reaction demonstrates that even noble metals can react under sufficiently energetic conditions with highly reactive non-metals.
2 Ir (s) + 3 Cl₂ (g) → 2 IrCl₃ (s)
Despite its limited direct reactivity, Iridium is extensively used as a catalyst or as an alloying agent in catalysts due to its stability and electronic properties. For instance, Iridium-based catalysts are employed in various industrial processes, including those relevant to the petrochemical industry and environmental controls, such as in catalytic converters for vehicles, which are increasingly common in India to reduce pollutant emissions.