Arsenic is an element of the periodic table that could be isolated for the first time in the 8th century. Used in industry, livestock and medicine, the metal arsenic is instantly associated with toxicology, being one of the great drivers of the development of this branch of chemistry and medicine.
Although at present the most effective way to detect the presence of arsenic in tissue samples is the use of Atomic Absorption Spectroscopy (EAA), for decades the Marsh method for the qualitative detection of arsenic helped save many lives and therefore It is still known as the most famous analytical method for the detection of arsenic .
In 1832, chemist James Marsh was called in to help solve a possible arsenic murder. He then devised a first method , which consisted of mixing the sample to be analyzed with hydrogen sulfide and hydrochloric acid, so that, if arsenic existed in the sample, arsenic trisulfide would form, which has a yellow color. Unfortunately, by the day of the trial, the arsenic trisulfide had deteriorated and the poisoner was released.
Wounded in his pride and angered by the consequences of his mistake, Marsh developed a new test for the detection of arsenic , which was the one that became famous and is known as the Marsh method.
Marsh’s method combines a sample suspected of containing arsenic with sulfuric acid and zinc arsenic, resulting in arsine gas , which is nothing other than arsenic trihydride. When arsine gas is subjected to elevated temperatures (for example, heated with a flame), the gas decomposes and releases elemental arsenic, leaving a residue that is somewhere between silver and black in color.
The sensitivity of this qualitative chemical analysis method allows the detection of arsenic in samples in which its concentration is greater than one fiftieth of a milligram (mg).
This level of ability to detect low concentrations of elemental or combined arsenic in samples of almost any type (food, water, blood, hair, etc.) made the Marsh method the usual procedure to determine the presence or not of arsenic , not only in cases of poisoning or in judicial matters. This method was also applied in the detection of accidental contamination or to guarantee the safety of the water intended for consumption in relation to this metal.
The first publication that refers to Marsh’s method is from 1836.
James Marsh devised a highly sensitive chemical method for the qualitative detection of arsenic and it was applied well into the 20th century. It was not without controversy when it was applied in judicial matters because, due to its high capacity to detect the presence of arsenic, some false positives were produced, in which the arsenic was due to cross contamination or poor taking of the sample to be analyzed . Or, at least, that’s what the detractors of this method argue.
