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Concept
Lead(II) iodide
Summary
Lead(II) iodide (or lead iodide) is a chemical compound with the formula PbI2. At room temperature, it is a bright yellow odorless crystalline solid, that becomes orange and red when heated. It was formerly called plumbous iodide.
The compound currently has a few specialized applications, such as the manufacture of solar cells, X-rays and gamma-ray detectors. Its preparation is an entertaining and popular demonstration in chemistry education, to teach topics such as precipitation reactions and stoichiometry. It is decomposed by light at temperatures above , and this effect has been used in a patented photographic process.
Lead iodide was formerly employed as a yellow pigment in some paints, with the name iodide yellow. However, that use has been largely discontinued due to its toxicity and poor stability.
PbI2 is commonly synthesized via a precipitation reaction between potassium iodide KI and lead(II) nitrate Pb(NO3)2 in water solution:
While the potassium nitrate KNO3 is soluble, the lead iodide PbI2 is nearly insoluble at room temperature, and thus precipitates out.
Other soluble compounds containing lead(II) and iodide can be used instead, for example lead(II) acetate and sodium iodide.
The compound can also be synthesized by reacting iodine vapor with molten lead between 500 and 700 °C.
A thin film of PbI2 can also be prepared by depositing a film of lead sulfide PbS and exposing it to iodine vapor, by the reaction
The sulfur is then washed with dimethyl sulfoxide.
Lead iodide prepared from cold solutions usually consists of many small hexagonal platelets, giving the yellow precipitate a silky appearance. Larger crystals can be obtained by exploiting the fact that solubility of lead iodide in water (like those of lead chloride and lead bromide) increases dramatically with temperature. The compound is colorless when dissolved in hot water, but crystallizes on cooling as thin but visibly larger bright yellow flakes, that settle slowly through the liquid — a visual effect often described as "golden rain".
Official source
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