The Marsh test is a laboratory test developed in the 19th century to check for traces of arsenic in a sample. It is named for James Marsh, the chemist who developed it in frustration because existing arsenic testing wasn’t always as sensitive as it needed to be. He also wanted to develop a method that would impress juries with its reliability, making it easier to prove the involvement of arsenic in poisoning cases. This was a problem so recurrent in the 19th century that one of arsenic’s nicknames was “inheritance powder,” referencing its use in the murders of inconvenient relatives.
People who used this poison skillfully could induce cholera-like symptoms in their victims. Deaths were often attributed to natural causes, and in cases where poisoning was suspected, it wasn’t always possible to identify arsenic. James Marsh set out to change that, and in the process, he developed a test so effective that it directly contributed to a drop in the frequency of arsenic poisonings.
His test involved reacting a sample from a case with zinc and acid. This produced a mixture of gases which could be passed through a heated tube, leaving deposits behind. By examining the deposits, the laboratory technician could determine if arsenic had been present in the original sample, and in what concentration. The concentration determination was made by comparing the Marsh test results with photographs of test results from samples of known concentration.
The relatively simple Marsh test could be used in court to show juries that arsenic had been involved in a death, and to demonstrate that concentrations of the poison were fatal. This made it much more difficult to kill people with arsenic without fear of detection. His test could also be used to check sample purity in other situations, like concerns about food contaminated with arsenic. During the 19th century, many foods and medications were contaminated by poor handling practices, lack of knowledge, or ingredient substitution, making quality testing critical for safety.
Some improvements were made to the Marsh test over time, including the use of testing to confirm that the reactive agents and equipment were arsenic-free before testing. This assured accurate test results free of contamination. Other testing is available to check for arsenic in modern laboratory facilities, but the Marsh test is still used in student education and demonstrations of what is known as “vapor-generation analytical chemistry,” a family of related techniques for identifying the individual components of known samples.