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Potassium bromide (KBr): structure, properties, uses

Potassium bromide is a white or crystalline solid salt, it is odorless, very soluble in water, and slightly soluble in ethanol and ether. It was used during the final part of the 19th century and much of the 20th century in the treatment of epilepsy and as a sedative.

Until 1975 it continued to be freely sold, the year in which its use decreased due to the toxic effects it produced on patients. Currently it is still used in several species of animals in the treatment of seizures, although from 1912 it was replaced by sodium phenobarbital.

Link type

Potassium bromide is a binary salt, product of the combination of a bromine atom with a -1 oxidation state, with a potassium atom with a +1 oxidation state. Therefore, an electrical attraction is established between the Br  ion and the K + ion .

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This type of electrical or electrostatic interaction is known as an ionic bond , which dissociates when the potassium bromide salt dissolves in water, releasing the K + and Br  ions .

These ions are disintegrated by the property of water to keep electrical charges of opposite sign separate.

Structure of potassium bromide

Fcc crystal structure of KBr: brown spheres correspond to Br- anions, while purple spheres correspond to K + cations

Potassium bromide has a face-centered cubic (fcc) crystal structure , the same as that observed in sodium chloride.

In this structure, each Br  anion is linked or coordinated with six K + cations , and in turn, each K + has six Br  as neighbors in the crystal lattices.

Nomenclature

Potassium bromide is classified as an alkali metal halide or halide. The names of these salts are constructed by removing the final letter from the halogen name (chlorine, bromine, iodine, fluorine, and astate), except for fluorine, and substituting the final letter for the suffix -ide. The name of the metal is then written without modification.

Potassium bromide properties

Clock disk with KBr granules. Source: Walkerma via Wikipedia.

Physical appearance

Colorless crystal, crystalline or white powder or granules.

Odor

Lavatory

Flavor

At a high concentration it has a bitter and spicy taste.

Molar mass

119.02 g / mol

Density

2.74 g / cm 3 at 25 ºC

Melting point

734 ºC

Boiling point

1435 ºC

Water solubility

535 g / L at 0 ºC

678 g / L at 25 ºC

1020 g / L at 100 ºC

1 gram of the salt dissolves in 1 mL of boiling water, which indicates that it is very soluble in water.

Glycerol solubility

217 g / L

1 gram of potassium bromide is dissolved in 4.6 mL of glycerol.

Solubility in ethanol

47.6 g / L at 80 ° C.

1 gram of potassium bromide is dissolved in 250 mL of ethanol.

Vapor density

7.14 in relation to air taken as unity.

Stability

Stable if stored under recommended conditions. Incompatible with oxidizing agents, strong acids, bromine trifluoride and bromine trichloride.

Decomposition

When heated to decomposition it emits harmful products, such as hydrogen bromide and potassium oxide.

Refractive index (nD)

1,559

pH

In aqueous solution it produces a neutral pH, around 7.

ignition point

1435 ºC

Dissolution and reactivity

In aqueous solution KBr dissociates completely to produce a neutral pH solution.

Salt is a source of bromide that can be used for the production of silver bromide, AgBr, a compound used in the development of photographs:

KBr (aq) + AgNO 3 (aq) → AgBr (s) + KNO 3 (aq)

Also, KBr is a weak reducing agent, incompatible with mercury and silver salts. Produces a violent reaction with bromine trifluoride. It can react with many salts, alkaloids and starch.

It also reacts with acids, for example concentrated sulfuric acid, releasing hydrogen bromide.

Applications

Treatment of epilepsy and seizures

The anticonvulsant properties of potassium bromide were first discussed by Sir Charles Locock, at a meeting of the Royal Medicine and Chirurgical Society in 1857, prompting its use in the treatment of epilepsy.

Potassium bromide was used as the priority drug in the treatment of epilepsy and as a sedative until 1912, when sodium phenobarbital was released.

Even so, it continued to be used as an adjunct to phenobarbital sodium in the treatment of seizures typical of epilepsy.

In 1975, due to its toxic nature, the FDA US (Food & Drug Administration) decided no longer to recommend the use of potassium bromide, both in humans and in animals. However, some countries continued to administer it.

Potassium bromide was used as a hypnotic and sedative in cattle, horses, sheep, and pigs. It was also used in the treatment of colic and tetany in horses, and in the treatment of seizures in dogs.

Infrared optics

Due to the transparency of potassium bromide in a range of wavelengths between ultraviolet and near infrared light, it is used as a component of windows for various electromagnetic radiation, especially infrared radiation.

Various techniques are used in this regard. One of them consists of making two crystalline potassium bromide discs, placing a film of the material to be analyzed in the infrared between the discs. The studies are carried out in the near infrared range between 780 nm and 3000 nm.

Photography

KBr was part of the materials used to process the first photographs of the 20th century

Potassium bromide has had numerous uses and applications in photography, for example: it has been used to formulate photographic developers, in the manufacture of photographic film, as a film thickener, and as a color photographic whitener.

It is used in improving the differentiation between exposed and unexposed silver halide crystals, in increasing the print density of thin negatives, and as a secondary halide, in combination with iodides, for processing paper negatives.

It has also been used in combination with mercury dichloride, copper sulfate, or potassium ferricyanur in photographic whiteners. It has also served as a limiter in alkaline developers used for gelatin plates and developing papers.

Chemical analysis

KBr is used in the determination of copper and silver titration, in the polarographic analysis of indium, cadmium and arsenic.

Other uses

It is used in lithography engraving processes, in petroleum processing , in the thermal stabilization of nylon, in the manufacture of fibers, and finally in the manufacture of special soaps.

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