The Marsh Test for Arsenic

poisons is arsenic, though the arsenic in question was generally not the pure chemical element itself but rather its oxide, As2O3 or diarsenic trioxide, otherwise known with equal inaccuracy as white arsenic. Indeed, since the most common form of this compound is actually composed of discrete As4O6 molecules, it should more properly be called tetraarsenic hexaoxide. For centuries proof of arsenic poisoning was problematic at best. True chemical tests for its presence did not appear until the late 18th century and most of these relied on precipitation of the arsenic as a characteristic colored compound, such as yellow As2S3(s) or diarsenic trisulfide (Hahnemann’s Test) (1). Unfortunately, when analyzing a sample of stomach fluid, for example, various other stomach contents were often coprecipitated with the arsenic leading to masking of the result and to a corresponding ambiguity in the evidence.