Arsenic

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Arsenic occurs in three distinct solid forms.

Gray arsenic is the most common. It has a metallic sheen and conducts electricity.

Yellow arsenic is metastable, is a poor electrical conductor and does not have a metallic sheen. It is prepared by cooling gray arsenic vapor in liquid air. It reverts to gray arsenic at room temperature.

Black arsenic can be prepared by cooling arsenic vapor at 100 ^oC – 200 ^oC. It is glassy, brittle and a poor electrical conductor.

As a result of its toxicity, arsenic compounds are used in wood preservation and insecticides.

Gallium arsenide (GaAs) is a semiconductor used in laser diodes and LEDs.

Small amounts of arsenic (less than two percent) can be used in lead alloys for ammunition.

Despite its potential toxicity, arsenic is also an essential element, necessary to our physiology. A level of 0.00001% is needed for growth and for a healthy nervous system.

Arsenic is a well-known poison. Arsenic compounds are sometimes used as rat poisons and insecticides but their use is strictly controlled. Surprisingly, arsenic can also have medicinal applications. In Victorian times, Dr. Fowler’s Solution (potassium arsenate dissolved in water) was a popular cure-all tonic that was even used by Charles Dickens. Today, organo arsenic compounds are added to poultry feed to prevent disease and improve weight gain.

Arsenic is added in small quantities to alpha-brass to make it dezincification-resistant. This grade of brass is used in plumbing fittings and other wet environments.

Arsenic is also used for taxonomic sample preservation.

It is also helpful in treating a certain type of leukemia (acute promyelocytic leukemia).