Chemical Reactivity of Indium
Indium (In) is a soft, silvery-white metal belonging to Group 13 of the periodic table, along with elements such as Aluminium and Gallium. It is classified as a post-transition metal. Its chemical reactivity lies between that of the highly reactive alkali metals and the much less reactive noble metals. Indium primarily exhibits a +3 oxidation state in its compounds, though a +1 oxidation state can also occur.
Reactivity with Water
Indium reacts with water, but this reaction is generally slow and not vigorous. With cold water, the reaction is barely perceptible. However, when heated, indium reacts more readily with hot water or steam. The products of this reaction are indium(III) hydroxide and hydrogen gas. The overall reaction can be represented as:
2 In(s) + 6 H₂O(l) → 2 In(OH)₃(s) + 3 H₂(g)
Reactivity with Air
In dry atmospheric air, indium is quite stable and does not readily react or corrode. However, in the presence of moist air, the surface of indium will slowly tarnish. This tarnishing occurs due to the formation of a thin, protective layer of indium(III) oxide (In₂O₃) on its surface. This oxide layer helps to prevent further corrosion of the metal, similar to how aluminium forms a protective oxide layer.
When indium is heated strongly in air or pure oxygen, it readily burns with a characteristic violet flame, producing indium(III) oxide.
4 In(s) + 3 O₂(g) → 2 In₂O₃(s)
Toxicity, Radioactivity, and Flammability
Toxicity
Bulk metallic indium is generally considered to have very low toxicity. It does not pose significant health risks through simple skin contact or brief exposure. However, prolonged exposure to indium compounds, particularly soluble ones, can be toxic if ingested or inhaled, potentially affecting organs such as the kidneys. Occupational safety precautions are typically followed in industrial settings where indium compounds are handled.
Radioactivity
Naturally occurring indium consists predominantly of two isotopes: Indium-113 (In-113) and Indium-115 (In-115). While In-113 is stable, In-115 is technically radioactive, undergoing beta decay. However, its half-life is exceptionally long (approximately 4.41 × 10¹⁴ years), which is vastly longer than the age of the universe. Due to this extremely long half-life, the radioactivity of naturally occurring indium is negligible and virtually undetectable for practical purposes. Therefore, for most practical applications and general understanding, indium is considered a non-radioactive element.
Flammability
As a bulk solid metal, indium is not flammable under normal atmospheric conditions. It does not easily catch fire. However, like many other metals, if indium is present in a very fine, powdered form, it can become combustible and potentially flammable under specific conditions, such as high temperatures and the presence of oxygen. In such powdered forms, it can burn with a violet flame, as mentioned in the reaction with air.
Example of a Chemical Reaction
A common chemical reaction for indium involves its interaction with acids. Indium reacts with strong acids, such as hydrochloric acid (HCl), to produce indium(III) chloride and hydrogen gas. This reaction demonstrates indium’s metallic character and its tendency to lose electrons to form positive ions.
2 In(s) + 6 HCl(aq) → 2 InCl₃(aq) + 3 H₂(g)
Indium is often obtained as a byproduct during the refining of zinc ores found in regions such as Rajasthan, India. It is a vital component in modern technologies, such as the production of Indium Tin Oxide (ITO) which is crucial for touchscreens in smartphones and tablets commonly used across India, as well as in solar panels.