An iodide ion is an iodine atom with a -1 charge. Compounds with iodine in formal oxidation state -1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as carbon tetraiodide. This is the same naming scheme as is seen with chlorides and bromides The chemical test for an iodide compound is to acidify the aqueous compound by adding some drops of acid, to dispel any carbonate ions present, then adding lead nitrate, yielding a bright yellow precipitate of lead iodide. Most ionic iodides are soluble, with the exception of yellow silver iodide and yellow lead iodide. Iron(III) iodide does not exist because iron(III) ions oxidize iodide ions in aqueous solution. Aqueous solutions of iodide dissolve iodine better than pure water due to the formation of complex ions:
Potassium iodide behaves as a simple ionic salt, K+I-. Since the iodide ion is a mild reducing agent, I- is easily oxidised to I2 by powerful oxidising agents such as chlorine:
2 KI(aq) + Cl2(aq) 2 KCl + I2(aq)
Even air will oxidize iodide as evidenced by the observation of a purple extract when KI is rinsed with dichloromethane. Under acidic conditions, KI is oxidised even more easily, due to the formation of hydroiodic acid (HI), which is a powerful reducing agent.[1][2][3][4] KI forms I3- when combined with elemental iodine.
KI(aq) + I2(s) KI3(aq)
Sodium iodide crystals doped with thallium, NaI(Tl), when subjected to ionising radiation, emit photons (scintillate) and are used in scintillation detectors, traditionally in nuclear medicine, geophysics, nuclear physics, environmental measurements, etc. NaI(Tl) is the most widely used scintillation material and has the highest light output. The crystals are usually coupled with a photomultiplier tube, in a hermetically sealed assembly, as sodium iodide is hygroscopic. Fine tuning of some parameters (radiation hardness, afterglow, transparency) can be achieved by varying the conditions of the crystal growth. Crystals with higher level of doping are used in X-ray detectors with high spectrometric quality. Sodium iodide can be used both as single crystals and as polycrystals for this purpose.
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