Hydroiodic Acid - HI

What is Hydroiodic Acid?

Hydroiodic acid also called Hydriodic acid is a colourless gas, which reddens litmus strongly, and produces dense white fumes in moist air. The chemical formula for hydroiodic acid is HI. Hydroiodic acid is a strong acid made by dissolving hydrogen iodide in water. However both differ in the physical state one is gas and the other is an aqueous solution. It is one of the strongest of all common halide acids due to high stability of its corresponding conjugate base.

Other names – Hydrogen iodide, iodane


Hydroiodic acid


1,700 g/cm³

Molecular Weight/ Molar Mass

127.911 g/mol

Boiling Point

-35.36 °C

Melting Point

-51 °C

Chemical Formula


Hydroiodic Acid Structure – HI

Hydroiodic Acid Structure

Physical Properties of Hydroiodic Acid – HI


Pungent odour


Colorless liquid

Covalently-Bonded Unit


Heat of vaporization

17.36 kJ/mol @ 25 deg C

Hydrogen Bond Donor



Soluble in water

Chemical Properties of Hydroiodic Acid – HI

  • Hydroiodic acid reacts with fuming nitric acid forms iodine, nitrogen oxide and water. The chemical equation is given below.
  • 2HNO3 + 6HI → 2NO + 3I2 + 4H2O

  • Hydroiodic acid reacts with a base say sodium hydroxide forms sodium iodide and water. The chemical equation is given below.
  • HI + NaOH → NaI + H2O

Uses of Hydroiodic Acid – HI

  • Used in the manufacture of iodides and as a reducing agent and in disinfectants and pharmaceuticals.
  • Used as one of the primary sources of iodine in organic and inorganic synthesis.
  • Hydroiodic acid in addition of potassium iodide, a logical choice as a source of a nucleophile in ketal deprotection.

Health Hazard – HI

Hydroiodic acid is a corrosive liquid that in contact with the skin can produce burns. Acid contact with the eyes may cause severe irritation. A strong irritant to the eyes, skin and mucous membranes is the gas hydrogen iodide.

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