arsenic (III) oxide

Chemical properties

Arsenic(III) oxide is an odourless white powder which is only slightly soluble in cold water and more soluble in hot water. The aqueous solution has a sweetish metallic taste and reacts acidic: it contains arsenic acid H3AsO3, which is only stable in the aqueous solution. The solubility is better in alkaline solutions because arsenites, the salts of arsenious acid, are formed. In acidic solutions the solubility is worse. In concentrated hydrochloric acid the solubility is good again. Chloroarsenites are formed. With concentrated nitric acid arsenic acid and nitrogen oxides are formed.

In nature, arsenic(III) oxide occurs in two mineral modifications: The claudetite crystallizes according to the monoclinic crystal system, it formed at the mine edges of arsenic-containing ores. The arsenic bloom (arsenolite) is found as a whitish yellowish coating on weathered arsenic ores. Arsenic(III) oxide is obtained in the laboratory by heating elemental arsenic or natural “body cobalt” in an oxygen atmosphere. When heated, the arsenic oxidizes to arsenic(III) oxide, which initially evaporates due to the heat and resublimates on cooling.


When elemental arsenic is heated in air or under oxygen supply, arsenic (III) oxide is formed. However, such an experiment must never be carried out in schools, as the resulting product is very toxic and highly carcinogenic.

Industrially, arsenic (III) oxide is obtained by roasting the mineral arsenopyrite:

2 FeAsS + 5 O2 → Fe2O3 + 2 SO2 + As2O3

The resulting toxic gas resublimates in long, bricked channels to form a white powder. Working in such “poison huts” used to be associated with considerable risks for the workers. The arsenopyrite often contains gold as a by-product.


In addition to its high toxicity, a major reason why arsenic was used as a murder poison was its easy accessibility. It was frequently used as a poison for insects, mice and rats (e.g. in the form of “mouse butter”, i.e. fat with arsenic pellets) and was available in various preparations from pharmacies. A well-known poisoner who poisoned 15 people in this way was Gesche Gottfried, who died on the scaffold in Bremen in 1831.

In funeral science, arsenic(III) oxide has been used since the end of the 18th century to preserve corpses. In “arterial preservation”, a mixture of alcohol and arsenic was injected into the corpse’s bloodstream, usually through the carotid artery. A corresponding procedure was described by the British physician William Hunter (1718-1783) and first used in practice in 1775 by his brother John (1728-1793). Since formaldehyde was discovered as a preservative in 1855, arsenic(III) oxide lost its use in this area until the end of the 19th century.

The stimulating effect of low doses of arsenic has been known for some time. Particularly in the 19th century there was a fashion for eating arsenic in certain areas (in Austria in Tyrol and Styria, as well as in the southern states of the USA), in which arsenic was used as an intoxicating drug.

Arsenic was fraudulently administered to horses by horse dealers to make older, weaker animals look healthier (“rustlers”). This gave the horses a shiny coat and “blooming” appearance.

In Roman antiquity, arsenic was also used as a means of depilating pubic hair.

In medieval ophthalmology, arsenic (from the Latin arsenicum, white arsenic, arsenic trioxide, As2O3, or red arsenic, arsenic sulfide, AsS3) was a common ingredient in ocular fluid recipes for red eyes or the winged skin.


Arsenic(III) oxide is also known as arsenic. The lethal dose for a person is difficult to estimate, as there are people who tolerate considerably larger amounts. The lowest lethal dose for a human (LDLo oral) is given as 1.429 mg/kg. Converted to 50 kilograms of body weight, this is about 71 milligrams. Arsenic is thus more toxic than cyanide.

In acute intoxication, the intestinal capillaries become so permeable that large quantities of fluid are excreted. The symptoms are watery diarrhoea, nausea and colic. In the final stage, paralysis and cramps occur due to the loss of water. Death often does not follow immediately, but only occurs in two to three days. Arsenic was a frequently used murder poison until the 19th century. James Marsh (1794-1846) introduced the Marsh test in 1836 to detect >Arsenic. After the introduction of this analytical method into criminology, it was possible to prove the poisonous murder on the basis of the corpse. As a result, the number of arsenic poisonings decreased significantly. For a long time arsenic was also used as a rat and mouse poison.

Paracelsus (1493-1541) recommended arsenic in low concentrations as a remedy. Homeopathy still uses it today in very small doses as Arsenicum album. Other medical uses, however, were highly controversial: The so-called “Fowler’s solution” contained arsenic and potassium arsenate, a salt of arsenic acid. It was used as a tonic until the 20th century. The most common side effect was cancer. When taking small amounts of arsenic, a habituation effect occurred. In the past, there were so-called “arsenic eaters” in Austrian Styria who, after a while, even tolerated the lethal dose.

The carcinogenic effect of arsenic(III) oxide in humans is now considered proven. According to the GHS classification, this substance is classified in the highest category 1A within the hazard class carcinogenicity. Inhalation of the dusts causes lung cancer in particular. However, absorption into the body can also cause cancer of the bladder, kidney or skin.


Arsenic has long been notorious as a murder poison. Since late antiquity, it was by far the most widely used poison. The ironic French term poudre de succession (“inheritance powder”) for arsenic is derived from its use as a poison, as is the German term “Altsitzerpulver”. Many historical poison mixtures, such as Aqua Tofana, contained arsenic as an essential ingredient. By regularly taking small amounts, the human organism does not become accustomed to the poison, but absorption through the mucous membrane is significantly reduced (so-called arsenic resistance) and the minimum lethal dose is increased, so that oral doses that would be lethal for others are tolerated. Some rulers have therefore regularly taken small amounts of substances such as arsenic to protect themselves against poisonous attacks (mithridation). In old Latin texts, an attempt to assassinate a prince in order to avoid bloodshed is called coniuratio pulveraria, i.e. “a conspiracy with poisonous powder”. Such an attack was carried out, for example, on Margrave Jakob III of Baden-Hachberg in 1590.

For centuries, arsenic could not be chemically detected. If the murderer used the correct dose, known since the 16th century, the murder could hardly be proven to him. As late as 1840, 90 to 95 percent of all poisonous deaths were attributable to the use of arsenic. After the introduction of the Marsh sample in 1836, the number of murders using arsenic gradually decreased.