Can you do "real" samples? like hair, or soil?
Of course! I analyze only real samples (my specialliy are minerals). I have analyzed dozends of real samples...soil, minerals, wood, alloys (e.g. an aluminium can), samples of metal (e.g. metallic lead to determine the percentage of antimony), coal, ashes, washing powders and so on....It's really funny to analyze real samples... and I think it's quite boring to analyze a mixture of salts made by the teacher.
If I remember correctly, the presence of arsenic shows up as little dark spots in the solution, correct?
Not really...the nacent hydrogen produced by Zn and sulfuric acid reduces the arsenic compounds to arsine, and the arsine is heated => AsH3
--------HEAT--------> As + 1.5 H2
You identify the arsenic "mirror" (Very thin foil of elementar arsenic) because of it's black color. By the way....I once smelled the arsine gas, and the rotten garlic odor is pretty disgusing. I smelled ti only once because the mineral contained so much arsenic that a huge amount of arsine was created, so that the fume hood couldn't remove the whole gas, but fortunately nothing bad happened. (a more disgusting odor is the H2
Se Odor (Rotting radishes))
Do you do this as an undergraduate experiment? Is it part of a specific toxicology course or is it done in general chemistry? It sounds interesting, I hadn't actually heard of it until now.
Not really, I have 8 hours of analytical laboratory per week in my school, and If I have to analyze a sample I make several tests for many different ions....one of this tests is the Marsh test.