Potassium iodide
 Potassium iodide | |
| IUPAC name potassium iodide | |
| General | |
|---|---|
| Molecular formula | KI |
| Molecular weight | 166.00 amu |
| Appearance | White crystalline solid |
| CAS number | [7681-11-0] |
| MSDS | Potassium iodide MSDS |
| Other names | |
| Bulk properties | |
| Density | 3.130 g/cm3 |
| Solubility | Water: 127.5 g/100 cm3 (6 °C) |
| Melting point | 681 °C (953 K) |
| Boiling point | 1330 °C (1600 K) |
| Hazards | Irritant |
| Structure | |
| Coordination geometry | ? |
| Crystal structure | ? |
| Hydrates | none |
| Related compounds | |
| potassium bromide | sodium iodide
rubidium iodide calcium iodide |
Overview
Potassium iodide is a white crystalline salt with chemical formula Template:PotassiumTemplate:Iodine, used in photography and radiation treatment. It finds widespread application as an iodide source because it is less hygroscopic than sodium iodide, making it easier to work with. KI can turn yellow upon heating in air or upon standing in moist air for long periods, because of oxidation of the iodide to iodine.
Chemical properties
Potassium iodide behaves as a simple ionic salt, K+I-. Since iodide ion is a mild reducing agent, KI is easily oxidised to iodine by oxidising agents such as chlorine:
2 KI(aq) + Cl2(aq) → 2 KCl + I2(aq)
If dichloromethane is present, the purple colour of elemental iodine may be seen in the organic phase. Under acidic conditions, KI is oxidised even more easily, due to the formation of hydroiodic acid (HI), which is a powerful reducing agent.
KI also forms a complex ion I3- when combined with elemental iodine, and in fact this reaction is frequently used to render I2 more soluble in water:
The reaction is slightly reversible, allowing the iodine to be used in aqueous solutions for redox titrations.
Potassium iodide also reacts in some organic reactions as a source of iodide ion (see "uses" below).
Preparation
Potassium iodide may be prepared by the reaction of a potassium base with hydroiodic acid, for example:
HI + KHCO3 → KI + H2O(l) + CO2(g)
Alternatively iron(II) iodide, prepared using scrap iron and iodine (made from iodide rich brines or from Chile saltpeter, can be treated with potassium carbonate:
Uses
Potassium iodide is used in photography, in the preparation of silver(I) iodide for high speed photographic film:
KI(aq) + AgNO3(aq) → AgI(s) + KNO3(aq)
It may also be used to protect the thyroid from radioactive iodine in the event of a dirty bomb or other minor nuclear attack (it cannot protect against other causes of radiation poisoning, however). In a saturated solution, it is also used as an expectorant to treat lung congestion.
KI is often used as a source of iodide ion in organic synthesis. A useful application is in the preparation of aryl iodides from arenediazonium salts[5], for example:
Precautions
Irritant. Wear gloves and goggles.
Suppliers/Manufacturers
References
- N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984.
- Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
- The Merck Index, 7th edition, Merck & Co., Rahway, New Jersey, 1960.
- H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968.
- (a) L. G. Wade, Organic Chemistry, 5th ed., pp. 871-2, Prentice Hall, Upper Saddle RIver, New Jersey, 2003. (b) J. March, Advanced Organic Chemistry, 4th ed., pp. 670-1, Wiley, New York, 1992.