Overview

Hydrogen iodide is a binary inorganic compound of hydrogen and iodine whose simple formula is HI. In its pure form it is a colorless gas with a noticeable odor; when dissolved in water it yields hydroiodic acid, a strongly acidic aqueous solution. Chemically it is classed as a hydrogen halide and is notable for its strong acidity in solution and its reducing behavior in the presence of oxidizers.

Physical and chemical characteristics

As a diatomic molecule, hydrogen iodide is heavier than air and is readily condensed to a liquid at low temperature. It is chemically reactive: exposure to atmospheric oxygen or other oxidizing agents promotes conversion of HI to elemental iodine and water, releasing iodine vapors and leading to gradual decomposition. In solution the HI molecule donates a proton to give iodide ions (I−) and hydronium, which is the basis of its designation as a strong acid.

Preparation methods

Several laboratory and industrial routes produce HI. Common approaches include:

  • Reduction of iodine with a chemical reductant — for example the conversion of iodine to HI can be accomplished by hydrazine or other reducing agents; this illustrates the reducing chemistry often associated with HI.
  • Treatment of iodide salts with a non-oxidizing acid such as phosphoric acid, which liberates gaseous HI without oxidizing iodide back to iodine.
  • Reaction between iodine and hydrogen sulfide (H2S) or other sulfide reagents in certain preparative contexts.
  • Direct combination of hydrogen gas and iodine vapor produces HI, but the equilibrium and kinetics make this route slow and less convenient for routine preparation.

Uses and applications

Hydrogen iodide and its aqueous form, hydroiodic acid, are used to prepare iodide salts and as reducing agents in organic synthesis (for example, in deoxygenation or cleavage reactions and in hydroiodination of alkenes). HI is also an intermediate in the manufacture of various iodine-containing compounds and can appear in some specialized analytical and synthetic procedures. Because concentrated HI can be diverted to illicit chemical production, its distribution and use are regulated in many jurisdictions.

Safety, handling, and distinctions

HI gas and concentrated hydroiodic acid are corrosive and can cause severe burns on contact; inhalation of vapors irritates the respiratory tract. It should be stored away from oxidizers and kept under dry, inert conditions to minimize oxidation to iodine. Distinguishing between gaseous hydrogen iodide (the anhydrous molecular gas) and aqueous hydroiodic acid (its solution) is important for both handling and reactivity: the gas is unstable in air, while the aqueous acid is commonly used in laboratory and industrial reactions. For further chemical context see general references on chemical compounds and sources covering iodide chemistry such as explanations of iodides.

For introductory reading on halogen hydrides and practical handling guides consult standard chemical safety texts and material safety data sheets available through institutional resources or suppliers (hydrogen, iodine, and related reagents are often cross-referenced) and specialized literature on inorganic synthesis and acid chemistry.