A hidden danger ?
I was reading about Kipp's gas generator which led me to the Marsh test for arsenic
https://en.wikipedia.org/wiki/James_Marsh_(chemist)
A sample suspected to contain arsenic is put in sulphuric acid with ARSENIC-FREE zinc,
any arsenic present will produce the scarily toxic arsine gas,
Wikipedia: LCLo (Lowest published), 250 ppm (human, 30 min), 300 ppm (human, 5 min), 25 ppm (human, 30 min)
which decomposes under heating to give a metallic deposit on glass, proving the presence of arsenic.
A very common hobby/school hydrogen source is zinc + acid (Kipp apparatus, Erlenmeyer etc.)
with the hydrogen gas produced considered combustible/explosive but non-toxic.
Probably wise for us to use guaranteed arsenic-free zinc ?
(e.g. recovered battery cups but not 'galvanized' iron etc.)
(a liitle research gave http://www.slideshare.net/prchandna/zinc-smelter-concept-not... which gives a figure of 1000 to 2000 ppm arsenic in zinc ore concentrate,
which is of course highly refined if required, but some lower grades may I suspect contain arsenic.)
(I have too many experiments running at the moment, especially gold recovery, later I may try to detect arsenic in galvanised iron, could be easy and
interesting)
P.S. when at school I learned that the zinc plus acid produce 'nascent hydrogen', which I just read in Wikipedia is now a ridiculed theory.
Drat ! ... not only have I a lot to learn, it seems I may have stuff to un-learn 
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