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Lead iodide: structure, properties, obtaining, uses

The compound PbI 4 , that is, with lead in oxidation state +4, seems to be non-existent, probably due to the reducing capacity of the iodide ion (I  ). PbI 2 is a golden yellow solid that is poorly soluble in water.

Solid lead iodide (PbI 2 ). W. Oelen / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

It can be obtained using an ion exchange reaction between an iodide salt and a lead salt that are both soluble in water.

It has semiconductor properties, so most of its current applications are in photovoltaic devices, detectors of certain radiation and sensors.

Structure

In lead iodide, the bond between its atoms is only partially ionic. Atoms form layers with a hexagonal structure and these are linked together by weak Van der Waals forces.

These forces are neither ionic nor covalent, they are weak interactions between the electronic shells of atoms.

Three-dimensional two-layer structure of crystalline PbI 2 . Gray = lead; violet = iodine. Benjah-bmm27 / Public domain. Source: Wikimedia Commons.

Nomenclature

  • Lead iodide
  • Lead (II) iodide
  • Lead diiodide
  • Plumbous Iodide

Properties

Physical state

Bright yellow crystalline solid. Hexagonal crystals.

Hexagonal crystals of lead iodide. Alessandro e Damiano / CC BY (https://creativecommons.org/licenses/by/4.0). Source: Wikimedia Commons.

Molecular weight

461 g / mol

Melting point

410 ºC

Boiling point

954 ° C, boils with decomposition.

Density

6.16 g / cm 3

Solubility

Slightly soluble in water: 0.076 g / 100 mL at 25 ° C. Soluble in hot water. Insoluble in alcohol and cold hydrochloric acid (HCl).

Chemical properties

Its oxidizing and reducing properties are weak. However, it can exhibit redox reactions.

Although it is very poorly soluble in water, it dissolves in concentrated solutions of alkaline iodides such as potassium iodide (KI). It is soluble in concentrated sodium acetate solution (CH 3 COONa). It dissolves freely in sodium thiosulfate solution (Na 2 S 2 O 3 ).

Some authors indicate that the PbI + ion can be generated in water and if there is an excess of iodide ion (I  ), more complex species such as PbI  and PbI 2- , among others, can be formed.

Other physical properties

It behaves like a semiconductor, that is, it may or may not conduct electricity depending on the conditions to which it is subjected.

It is a direct gap semiconductor, that is, for one of its electrons to pass from the valence band to the conduction band, it only needs to have an amount of energy equal to the forbidden bandwidth.

Due to the high atomic number of its elements (Pb = 82, I = 53) it has a high photoelectric capacity. Its 2.5 eV gap band enables highly efficient photovoltaic performances at temperatures up to 250 ° C.

Obtaining

It can be prepared by reacting a water soluble lead compound with hydroiodic acid (HI) or with a soluble metal iodide. For example, an aqueous solution of lead acetate is mixed with potassium iodide:

Pb (CH 3 COO) 2 + 2 KI → PbI 2 ↓ + 2 K (CH 3 COO)

This type of reaction is known as “ion exchange” because cations and anions are exchanged between salts.

In the example mentioned, potassium acetate is very soluble in water and remains dissolved, while potassium iodide, being less soluble, precipitates and can be filtered. Purification is carried out by recrystallizing the compound from water.

The precipitation of PbI 2 can be observed in the following image showing a test tube where lead (II) nitrate (Pb (NO 3 ) 2 ) and potassium iodide (KI) were mixed in aqueous solution. This effect is called “golden shower.”

PbI gold shower 2 . Stefano sct / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Applications

As a semiconductor

It is used as a detector for high-energy photons such as X-rays and gamma rays. It can be used in photovoltaic devices, photocells, LED lights, optical detectors and in sensors for biological classification and diagnosis.

If it is introduced into nanostructures, it can be used in photocatalysis and solar cells. Furthermore, many PbI 2 nanoparticles have luminescent properties.

Lead iodide is used in detectors in medical X-ray devices. Author: lkcjjang40090. Source: Pixabay.

Solar cells

PbI 2 is an intermediate in the synthesis of perovskites destined for solar cells. This type of photovoltaic cells contain lead methylammonium iodide (CH 3 NH 3 PbI 3 ) on a TiO 2 base .

Such devices have high efficiency and low cost, which is why they have been the subject of much study and research.

Solar cells on the roof of a house in the countryside. Solar cells in which PbI 2 is used  are very efficient and inexpensive, so they could be used in homes in the future. Author: Manfred Antranias Zimmer. Source: Pixabay.

However, since CH 3 NH 3 PbI 3 can be decomposed with rainwater, it has been studied how polluting these cells can be both when they are in use and when they are discarded.

When CH 3 NH 3 PbI 3 comes into contact with water, it decomposes into methylamine (CH 3 NH 2 ), hydroiodic acid and PbI 2 . The latter, although poorly soluble in water, can release amounts of the toxic Pb 2+ ion over time .

The studies are not conclusive, since the place where the lead release occurs must be taken into account to determine if the amount may be harmful in the short term. On the other hand, a sustained release can bioaccumulate and be very dangerous.

Other apps

  • It is sown as an aerosol in the clouds to produce rain.
  • In filters for far infrared astronomy.
  • In photography, prints, films for recording optical images, photographic emulsions.
  • In brake lining. In lubricating greases.
  • Mercury vapor arc lamps. On electrosensitive paper.
  • Thermoelectric materials, thermal batteries with iodine.

Risks

For security

It should be stored away from oxidants such as perchlorates, peroxides, permanganates, chlorates and nitrates. Contact with chemically active metals such as potassium, sodium, magnesium and zinc should also be avoided. In all these cases a violent reaction can occur.

If it is subjected to strong heating, poisonous lead and iodine gases are generated.

To health

It is very harmful to human beings. It has been confirmed that it is carcinogenic to animals, therefore, it can reasonably be inferred that it is also carcinogenic to humans.

It can cause headaches, irritability, reduce memory, and disturb sleep. The lead contained in this compound can cause permanent damage to the kidneys , brain, nerves, blood cells and risk of high blood pressure.

It must be handled as a teratogen (a compound that can cause a birth defect ). It can also cause iodism, the symptoms of which are congestion of the nasal passages, headache, irritation of the mucous membranes and skin rash, among others.

For the natural environment

It is classified as a toxic pollutant. It should be kept away from water sources and drains. To prevent it from contaminating, dikes should be built whenever it is necessary to retain it.

It is very toxic to aquatic life with effects that last over time, as it bioaccumulates.

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