LYCOS RETRIEVER 
Arsenic: Forms
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Arsenic is a chemical substance, which is released from the Earth’s crust via natural processes and from certain human activities. It can exist in inorganic or organic form, inorganic arsenic being generally considered more toxic.
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Arsenic is usually estimated either in the form of magnesium pyroarsenate or as arsenic sulphide. For the pyroarsenate method it is necessary that the arsenic should be in the arsenic condition, if necessary this can be effected by heating with nitric acid; the acid solution is then mixed with "magnesia mixture" and made strongly alkaline by the addition of ammonia. It is then allowed to stand twenty-four hours, filtered, washed with dilute ammonia, dried, ignited to constant weight and weighed, the filter paper being incinerated separately after moistening with nitric acid. From the weight of magnesium pyroarsenate obtained the weight of arsenic can be calculated.
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Arsenic had been a popular way of poisoning people since the Middle Ages. Arsenic itself is not very poisonous but another form of arsenic - arsenic oxide - is extremely poisonous. The symptoms of arsenic poisoning could be confused with those of many other illnesses, and it was ... very difficult to detect arsenic after the death so it provided a practical way of murdering someone. Indeed white arsenic became known as 'inheritance powder'.
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Arsenic cannot be found in nature as a free element. It exhibits a broad range of chemical reactivity with an ability to form alloys with other elements and covalent bonds. Arsenic precipitates readily in oxidation-reduction, methylation-demethylation and acidbase reactions.
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Arsenic could ... be given as a series of smaller doses, producing a more subtle form of chronic poisoning characterized by a loss of strength, confusion and paralysis. Eventually, the arsenical of choice emerged as so-called white arsenic or arsenic trioxide (As2O3); the fatal dose was known to be an amount equivalent in size to a pea.
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