Potassium bromide

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Potassium bromide

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Potassium bromide (K Br) is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries. Its action is due to the bromide ion (sodium bromide is equally effective). Potassium bromide is presently used as veterinary drug, as an antiepileptic medication for dogs and cats. It is a white crystalline powder, 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:

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. Locock noted that bromide calmed sexual excitement and thought this was responsible for his success in treating seizures. There would not be a better drug for epilepsy until phenobarbital in 1912. It was often said the British Army laced the soldiers' tea with bromide to quell sexual arousal, however this is likely to be an urban legend and similar stories were also told about a number of substances.

Potassium bromide is used to treat epilepsy in dogs, either as first-line treatment or in addition to phenobarbital when the 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 this species.

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. KBr is sold under the brand name Dibro-Be mono (RX-only). When used for proper indications it shows promising results. The drug has almost complete bioavailability and an extremely long half-life of 6 weeks. One tablet contains 850 mg of potassium bromide. Potassium bromide is not known to interfere with the absorption or excretion of any other anticonvulsant.

The therapeutic index is very small for bromide. As with other antiepileptics, sometimes even therapeutic doses give rise to intoxication. Often indistinguishable from 'expected' side-effects, these include:

Loss of appetite, nausea/emesis, lethargy, propensity to sleep during the daytime, depression, loss of concentration and memory, confusion, headache, and

Bromism (central reactions reaching from somnolence to coma, cachexia, exicosis, loss of reflexes or pathologic reflexes, clonic seizures, tremor, ataxia, loss of neural sensitivity, paresis, papillar edema of the eyes, abnormal speech, cerebral edema, delirium, 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

KBr is transparent from the near ultraviolet to long wave infrared wavelengths (0.25-25 µm). KBr has no significant optical absorption lines in its high transmission region. It is used for optical windows and prisms. It must be kept in a dry environment due to high solubility and hygroscopic nature. The refractive index is about 1.55 at 1.0 µm.

In infrared spectroscopy, samples are analyzed by grinding with KBr powder, 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 KBr discs.