LITHIUM BROMIDE: STRUCTURE, PROPERTIES, USES, RISKS - CHEMISTRY - 2025

The lithium bromide is a neutral salt of an alkali metal whose chemical formula is LiBr. The formula expresses that its crystalline solid is composed of Li ⁺ and Br ^- ions in a 1: 1 ratio. Its crystals are white or light beige. It is very soluble in water and it is also a fairly hygroscopic salt.

This last property allows its use as a desiccant in air conditioning and refrigeration systems. Likewise, lithium bromide was used since the beginning of the 20th century in the treatment of certain mental health disorders, and its use was abandoned due to an inappropriate use of salt.

Cubic crystal structure of LiBr. Source: Benjah-bmm27 via Wikipedia.

LiBr is produced by treating lithium carbonate, Li ₂ CO ₃, with hydrobromic acid. After heating the medium, it precipitates out of the aqueous solution as a hydrate.

This salt irritates the skin and eyes on contact, while irritating the respiratory passages by inhalation. Ingestion of lithium salts can cause nausea, vomiting, diarrhea and dizziness.

Structure of lithium bromide

The LiBr formula makes it clear that the Li / Br ratio is equal to 1; for each Li ⁺ cation there must be a counterpart Br ^- anion. Therefore, this ratio must be kept constant in all corners of the LiBr crystal.

The Li ⁺ and Br ^- ions are mutually attracted, decreasing the repulsions between equal charges, to in this way originate a cubic gem-salt crystal; this is isomorphic to NaCl (top image). Note that by itself the whole set has a cubic geometry.

In this crystal, the Li ⁺ are smaller and have a light purple color; while the Br ^- are more voluminous and of an intense brown color. It is observed that each ion has six neighbors, which is the same as saying that they present an octahedral coordination: LiBr ₆ or Li ₆ Br; however, if the unit cell is considered, the Li / Br ratio remains 1.

This is the crystal structure preferably adopted by LiBr. However, it can also form other types of crystals: wurzite, if it crystallizes at low temperatures (-50 ° C) on a substrate; or body-centered cubic, or CsCl type, if the gem salt cubic crystal is subjected under high pressure.

Hydrates and glasses

The foregoing is applicable for anhydrous LiBr. This salt is hygroscopic, and can therefore absorb moisture from the environment, inserting water molecules within its own crystals. Thus, the hydrates LiBr · nH ₂ O (n = 1, 2, 3…, 10) arise. For each hydrate, the crystal structure is different.

For example, crystallographic studies determined that LiBr · H ₂ O adopts a perovskite-like structure.

When these hydrates are in aqueous solution, they can become supercooled and vitrify; that is, they adopt an apparently crystalline structure, but molecularly disordered. Under such conditions, hydrogen bonds in water become quite important.

Properties

Molecular weight

88.845 g / mol.

Appearance

White or light beige crystalline solid.

Odor

Toilet.

Melting point

552 ° C (1,026 ° F, 825 K).

Boiling point

1,256 ° C (2,309 ° F, 1,538 K).

Water solubility

166.7 g / 100 mL at 20 ° C. Note its high solubility.

Solubility in organic solvents

Soluble in methanol, ethanol, ether and acetone. Slightly soluble in pyridine, an aromatic compound and less polar than the previous ones.

Refractive index (ηD)

1,784.

Caloric capacity

51.88 J / mol · K.

Standard molar entropy (S

66.9 J / mol · K.

ignition point

1,265 ° C. It is considered non-flammable.

Stability

Stable. The anhydrous form, however, is extremely hygroscopic.

Decomposition

When decomposed by heating, it forms lithium oxide.

pH

Between pH 6 and 7 in aqueous solution (100 g / L, 20 ºC).

Reactivity

Lithium Bromide is not reactive under normal environmental conditions. However, you can experience strong reactions with strong acids with increasing temperature.

When dissolved in water, an increase in temperature occurs, caused by a negative enthalpy of hydration.

Production

LiBr is produced by reacting lithium hydroxide or lithium carbonate with hydrobromic acid in a neutralization reaction:

Li ₂ CO ₃ + HBr => LiBr + CO ₂ + H ₂ O

Lithium bromide is obtained as a hydrate. To obtain the anhydrous form, it is necessary to heat the hydrated salt under vacuum.

Nomenclature

The name 'lithium bromide' derives from the fact that it is a metal halide, so named according to the stock nomenclature. Other names, equally valid but less used, are lithium monobromide, according to the systematic nomenclature; and lithic bromide (the unique valence of +1 for lithium), according to traditional nomenclature.

Applications

Desiccant

LiBr is a hygroscopic salt, forming a concentrated brine capable of absorbing moisture from the air over a wide range of temperatures. This brine is used as a desiccant in air conditioning and refrigeration systems.

Fibers

It is used to increase the volume of wool, hair and other organic fibers.

Pharmaceutical Adducts

LiBr forms adducts with some pharmaceutical compounds, modulating their action. An adduct is the combination by coordination of two or more molecules, without producing a structural alteration of any of the joined molecules.

Sedative

Lithium bromide was first used as a sedative in certain mental disorders, discontinuing use in 1940. Currently, lithium carbonate is used; but in any case, lithium is the element that exerts the therapeutic action in both compounds.

Lithium is used in the treatment of bipolar disorder, having been useful in the control of manic episodes of the disorder. Lithium is thought to inhibit the activity of excitatory neurotransmitters, such as dopamine and glutamic acid.

On the other hand, it increases the activity of the inhibitory system mediated by the neurotransmitter gamma-amino-butyric acid (GABA). These actions could form part of the basis of the therapeutic action of lithium.

Risks

Contact toxicity

Skin irritation and sensitization, allergies. Serious eye damage or irritation to the eyes, respiratory tract, nasal passages and throat.

Ingestion

The main symptoms due to ingestion of lithium bromide are: gastrointestinal disorders, vomiting and nausea. Other adverse effects of ingesting salt include agitation, spasms, and loss of consciousness.

The condition known as "bromism" can occur with lithium bromide doses greater than 225 mg / day.

Some of the following signs and symptoms may occur in jokes: acne exacerbation and loss of appetite, tremors, speech problems, listlessness, weakness, and manic agitation.

References

1. Chemical Formulation. (2019). Lithium bromide. Recovered from: formulacionquimica.com
2. David C. Johnson. (July 10, 2008). New order for lithium bromide. Nature. doi.org/10.1038/454174a
3. Aayushi Jain & RC Dixit. (sf). Structural Phase Transition in Lithium Bromide: Effect of Pressure and Temperature.. Recovered from: ncpcm.in
4. Takamuku et al. (1997). X-ray Diffraction Studies on Supercooled Aqueous Lithium Bromide and Lithium Iodide Solutions. Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-80, Japan.
5. National Center for Biotechnology Information. (2019). Lithium bromide. PubChem Database, CID = 82050. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Wikipedia. (2019). Lithium bromide. Recovered from: en.wikipedia.org
7. Royal Society of Chemistry. (2019). Lithium bromide. Chemspider. Recovered from: chemspider.com