Potassium Iodide ( KI )

Potassium Iodide

Other Names: Potassium iodide, Potassium monoiodide
Chemical Formula: KI
Molar Mass: 166.00 g/mol
CAS Number: 7681-11-0
Appearance: White or colorless crystalline solid
Odor: Odorless
SMILES: [K+].[I-]

Potassium iodide is an inorganic ionic compound composed of potassium cations, K⁺, and iodide anions, I⁻. In the solid state it forms a cubic ionic lattice of the sodium chloride type. Potassium iodide is highly soluble in water and readily dissociates into hydrated potassium and iodide ions, making it a convenient source of iodide for analytical chemistry, inorganic chemistry, electrochemistry and organic synthesis. The iodide ion is readily oxidized compared with many other halide ions and participates in numerous redox and complex-formation reactions. Potassium iodide is also particularly important in iodine chemistry because iodide interacts with molecular iodine to form soluble polyiodide species, especially triiodide, I₃⁻.

Chemical and physical properties

Potassium iodide crystallizes in the cubic rock-salt structure with space group Fm3m and four formula units per unit cell. In this structure, both K⁺ and I⁻ ions have octahedral coordination. The ionic lattice is responsible for the relatively high melting temperature and characteristic mechanical and optical properties of KI crystals.

The compound has a melting point of approximately 681 °C. Its density at room temperature is approximately 3.1 g/cm³. Potassium iodide is highly soluble in water; published physicochemical data report a solubility of approximately 148 g per 100 g of water at 25 °C. It is considerably less soluble in organic solvents such as ethanol and acetone.

Potassium iodide crystals exhibit a broad optical transmission range extending from the ultraviolet through the visible region and into the infrared. Optical studies describe KI as a soft, hygroscopic cubic material with useful infrared transmission characteristics. These properties have resulted in its investigation and use as an optical material where transmission at longer infrared wavelengths is required.

In aqueous solution, iodide reacts reversibly with molecular iodine to form triiodide:

I₂ + I⁻ ⇌ I₃⁻

This reaction substantially increases the apparent solubility of molecular iodine in aqueous iodide solutions. Spectroscopic studies of iodine-potassium iodide systems show characteristic absorption associated with triiodide ions, and the equilibrium is strongly influenced by solvent composition and ionic environment.

Iodide is also an effective reducing species and can be oxidized to iodine by numerous oxidizing agents. This predictable redox chemistry is the basis for the extensive use of potassium iodide in iodometric analytical procedures.

Applications

Potassium iodide is widely used as an analytical reagent in iodometry. Oxidizing substances can convert iodide ions into molecular iodine, and the liberated iodine can subsequently be quantified, commonly by titration with standardized sodium thiosulfate. Scientific analytical methods have employed potassium iodide for determination of chlorine and other oxidizing species because iodide oxidation provides a sensitive and well-defined chemical response.

Potassium iodide is used for the preparation and stabilization of iodine-containing analytical solutions. Excess iodide converts poorly water-soluble molecular iodine into the more soluble triiodide ion. Iodine-potassium iodide systems are consequently used in spectrophotometric measurements, redox studies and investigations of iodine interactions with polymers and other materials.

In organic synthesis, potassium iodide is used as an iodide source in halogen-exchange reactions. Finkelstein-type reactions exploit the nucleophilicity of I⁻ to replace chlorine or bromine atoms with iodine in suitable organic substrates. Potassium iodide has been employed in both classical and catalytic halide-exchange procedures for preparing alkyl and aryl iodides.

Potassium iodide is also used in coordination and inorganic chemistry as a source of iodide ligands. Iodide can form complexes with numerous transition-metal ions and is used in studies of metal-halide coordination, redox chemistry and synthesis of metal iodide compounds.

High-purity potassium iodide single crystals are used and investigated in optical research because of their broad spectral transparency extending into the infrared. Studies of KI have examined its electronic band structure, ultraviolet reflectance, refractive properties and infrared absorption, making it an important model alkali-halide crystal in solid-state and optical physics.

Potassium iodide is additionally used as an iodide source during preparation and modification of semiconductor materials. For example, research on PbSe thin films has demonstrated that KI-containing processing systems can influence iodization, grain-boundary chemistry, infrared absorption and photoconductive properties.

Scientific references

1. PubChem, National Center for Biotechnology Information. Potassium Iodide, CID 4875. Molecular formula, molecular mass, CAS number, chemical structure, melting point and solubility data.
2. Palik, E. D. Potassium Iodide (KI). In: Handbook of Optical Constants of Solids, Volume III. Academic Press, 1997, pp. 807-820. DOI: 10.1016/B978-012544415-6.50133-3.
3. Page, L. J.; Hygh, E. H. Electron energy bands and the optical properties of potassium iodide and rubidium iodide. Journal of Physics and Chemistry of Solids, 1970, 31(2), 265-274. DOI: 10.1016/0022-3697(70)90106-X.
4. IUPAC-NIST Solubility Data Series. Potassium iodide-water systems. National Institute of Standards and Technology, Standard Reference Database 106.
5. Jordanov, N.; Daiev, K. Application of direct thermometric analysis in iodometry. Talanta, 1970, 17(12), 1215-1218. DOI: 10.1016/0039-9140(70)80037-6.
6. Barakat, M. Z.; Shehab, S. K.; Ibrahim, A. A. The microdetermination of soluble iodides with N-bromosuccinimide. Microchemical Journal, 1968, 13(4), 517-525. DOI: 10.1016/0026-265X(68)90027-1.
7. Spectrophotometric determination of the formation constant of triiodide ions in aqueous-organic solvent or polymer mixed media both in absence and presence of a surfactant. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2013. Experimental investigation of iodine-potassium iodide equilibria and triiodide formation.
8. White, J. L.; Poda, A. R.; et al. Determination of iodine number of activated carbon by the method of ultraviolet-visible spectroscopy. Materials Letters, 2021. Study of iodine, iodide and triiodide species in potassium iodide-containing analytical solutions.
9. Zhang, B.; et al. Synthesis of C2-Iodoindoles via an Aromatic Finkelstein-Type Halide Exchange under Mild Conditions. Organic Letters, 2026, 28(1), 236-241. DOI: 10.1021/acs.orglett.5c04613.
10. Photoconductive PbSe thin films and the role of potassium iodide. Journal of Alloys and Compounds, 2022, 905, 164155. DOI: 10.1016/j.jallcom.2022.164155.

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