potassium bromide

Potassium bromide
Identifiers
CAS number	7758-02-3 Y
PubChem	253877
ChEMBL	CHEMBL1644030 N
RTECS number	TS7650000
Jmol-3D images	Image 1
SMILES [K+].[Br-]
InChI InChI=1S/BrH.K/h1H;/q;+1/p-1
Properties
Molecular formula	KBr
Molar mass	119.002 g/mol
Appearance	white solid
Odor	odorless
Density	2.74 g/cm3
Melting point	734 °C, 1007 K, 1353 °F
Boiling point	1435 °C, 1708 K, 2615 °F
Solubility in water	53.5 g/100 mL (0 °C) 67.8 g/100 mL (25 °C) 102 g/100 mL (100 °C)
Solubility	very slightly soluble in diethyl ether
Solubility in glycerol	21.7 g/100 mL
Solubility in ethanol	4.76 g/100 mL (80 °C)
Refractive index (nD)	1.559
Structure
Crystal structure	Sodium chloride
Coordination geometry	octahedral
Dipole moment	10.41 D (gas)
Hazards
MSDS	MSDS at Oxford University
EU Index	Not listed
R-phrases	R20, R21, R22, R36, R37, R38
S-phrases	S22, S26, S36
Related compounds
Other anions	Potassium fluoride Potassium chloride Potassium iodide
Other cations	Lithium bromide Sodium bromide Rubidium bromide Caesium bromide
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium bromide (KBr) is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the US. Its action is due to the bromide ion (sodium bromide is equally effective). Potassium bromide is presently used as a veterinary drug, as an antiepileptic medication for dogs and cats.

Under standard conditions, potassium bromide is a white crystalline powder. It is freely soluble in water. In a dilute aqueous solution, potassium bromide tastes sweet, at higher concentration it tastes bitter, and when most concentrated it tastes salty to humans (these effects are due mainly to potassium ion; sodium bromide merely tastes salty at all concentrations). In high concentration potassium bromide strongly irritates the gastric mucous membrane, leading to nausea and sometimes vomiting (again this effect is typical of all soluble potassium salts).

Chemical properties

Potassium bromide is a typical ionic salt which is fully dissociated and near pH 7 in aqueous solution. It serves as a source of bromide ions- this reaction is important for the manufacture of silver bromide for photographic film:

KBr(aq) + AgNO₃(aq) → AgBr(s) + KNO₃(aq)

Aqueous bromide Br^- will also form complexes when reacted with some metal halides such as copper(II) bromide:

2 KBr(aq) + CuBr₂(aq) → K₂[CuBr₄](aq)

Preparation

A traditional method for the manufacture of KBr is the reaction of potassium carbonate with a bromide of iron, Fe₃Br₈, made by treating scrap iron under water with excess bromine:^{[citation needed]}

4 K₂CO₃ + Fe₃Br₈ → 8 KBr + Fe₃O₄ + 4 CO₂

Applications

Medical and veterinary

The anticonvulsant properties of potassium bromide were first noted by Sir Charles Locock at a meeting of the Royal Medical and Chirurgical Society in 1857. Bromide can be regarded as the first effective medication for epilepsy. At the time, it was commonly thought that epilepsy was caused by masturbation.^[1] Locock noted that bromide calmed sexual excitement and thought this was responsible for his success in treating seizures. In the latter half of the 19th century, potassium bromide was used for the calming of seizure and nervous disorders on an enormous scale, with the use by single hospitals being as much as several tons a year (the dose for a given person being a few grams per day).^[1]

There was not a better drug for epilepsy until phenobarbital in 1912. It was often said the British Army laced soldiers' tea with bromide to quell sexual arousal, but as doing so would also diminish alertness in battle it is likely to be an urban legend and similar stories were also told about a number of substances.^[2]

Bromide compounds, especially sodium bromide, continued to be used in over-the-counter sedatives and headache remedies (such as the original formulation of Bromo-Seltzer) in the US until 1975 when bromides were withdrawn as ingredients in all over-the-counter medicinal formulations, due to the chronic toxicity of bromide.^[3] Bromide's exceedingly long half life in the body made it difficult to dose without side effects (see below). Medical use of bromides in the US was discontinued at this time, as many better and shorter-acting sedatives were known by then.

Potassium bromide is presently in veterinary medicine to treat epilepsy in dogs, either as first-line treatment or in addition to phenobarbital, when seizures are not adequately controlled with phenobarbital alone. Use of bromide in cats is limited because it carries a substantial risk of causing lung inflammation (pneumonitis) in them. The use of bromide as a treatment drug for animals means that veterinary medical diagnostic laboratories are able as a matter of routine to measure serum levels of bromide on order of a veterinarian, whereas human medical diagnostic labs in the US do not measure bromide as a routine test.

Potassium bromide is not approved by the US Food and Drug Administration (FDA) for use in humans to control seizures. In Germany, it continues to be approved for use as an antiepileptic drug for humans, particularly children and adolescents. These indications include severe forms of generalized tonic-clonic seizures, early-childhood-related Grand-Mal-seizures, and also severe myoclonic seizures during childhood. Adults who have reacted positively to the drug during childhood/adolescence may continue treatment. Potassium bromide tablets are sold under the brand name Dibro-Be mono (Rx-only). The drug has almost complete bioavailability, but the bromide ion has a relatively long half life of 12 days in the blood,^[1] making bromide salts difficult to adjust and dose. Bromide is not known to interfere with the absorption or excretion of any other anticonvulsant, though it does have strong interactions with chloride in the body, the normal body uptake and excretion of which strongly influences bromide's excretion.^[1]

The therapeutic index (ratio of effectiveness to toxicity) is very small for bromide. As with other antiepileptics, sometimes even therapeutic doses (3 to 5 grams per day, taking 6 to 8 weeks to reach stable levels) may give rise to intoxication. Often indistinguishable from 'expected' side-effects, these include:

• Bromism These are central nervous system reactions. They may include:

depression,

lethargy, somnolence (from daytime sleepiness to coma)

loss of appetite and cachexia, nausea/emesis with exicosis (loss of body fluid)

loss of reflexes or pathologic reflexes

clonic seizures

tremor

ataxia

loss of neural sensitivity

paresis

cerebral edema with associated headache and papilledema of the eyes

delirium: confusion, abnormal speech, loss of concentration and memory, aggressiveness

psychoses

• Acne-form dermatitis and other forms of skin disease may also be seen, as well as mucous hypersecretion in the lungs. Asthma and rhinitis may worsen. Rarely, tongue disorder, aphten, bad breath, and obstipation occur.

Optics

Potassium bromide is transparent from the near ultraviolet to long wave infrared wavelengths (0.25-25 µm) and it has no significant optical absorption lines in its high transmission region. It is used widely as infrared optical windows and components for general spectroscopy because of its wide spectral range. In infrared spectroscopy, samples are analyzed by grinding with powdered potassium bromide and pressing into a disc. Alternatively, the samples may be analyzed as a liquid film (neat, as a solution, or in a mull with Nujol) between two polished potassium bromide discs.^[4]

Due to its high solubility and hygroscopic nature it must be kept in a dry environment. The refractive index is about 1.55 at 1.0 µm.

Photography

In addition to the manufacture of silver bromide previously mentioned, potassium bromide is used as a restrainer in black and white developer formulas. It improves the differentiation between exposed and unexposed crystals of silver halide and thus reduces fog.^[5]

References

1. ^ ^{a b c d} Goodman; Gilman (1970). "chapter 10: Hypnotics and Sedatives". The Biological Basis of Therapeutics (4th ed.). London: MacMillan. pp. 121–2. 
2. ^ Barbara Mikkelson (2 August 2007). "The Saltpeter Principle". Snopes.com. http://www.snopes.com/military/saltpeter.asp. 
3. ^ Adams, Samuel Hopkins (1905). The Great American fraud. http://books.google.com/?id=fd_S2Van52EC&dq=%22The+Great+American+Fraud%22&printsec=frontcover#v=twopage&q&f=true .
4. ^ W. Reusch. "Infrared Spectroscopy". VirtualText of Organic Chemistry. http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm. Retrieved 18 December 2007. 
5. ^ Anchell, Stephen; Troop, Bill (1998). The Film Developing Cookbook. Boston: Focal Press. p. 28. 

External links

• Veterinary use note at Auburn University
• Pesticide information