Description of experiment
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Needed compounds:
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sodium dithionite : Na2S2O4 . 2H2O
stannous chloride : SnCl2
sodium hypophosphite : NaH2PO2
sodium sulfite : Na2SO3
hydrochloric acid : HCl
arsenic pentoxide : As4O10
Class:
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elem=S,As,Sn
redox
Summary:
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Arsenic in oxidation state +5 is not easily reduced to elemental arsenic,
except by tin(II) chloride.
Description:
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Dissolve some arsenic pentoxide in moderately concentrated hydrochloric acid
(e.g. 20% HCl by weight): A colorless solution is obtained. In order to get all
of the arsenic pentoxide dissolved some heating is needed.
This liquid is called liquid A.
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To some of liquid A add a solution of sodium sulfite in moderately concentrated
hydrochloric acid: No visible reaction occurs. On heating there still is no
reaction. The liquid remains colorless.
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To some of liquid A add a solution of sodium hypophosphite in moderately
concentrated hydrochloric acid: No reaction occurs when the cold liquids are
mixed. On heating a small amount of arsenic is formed, but this reaction is
very slow and also seems to be incomplete. Hypophosphite is not capable of fast
and complete reduction.
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Prepare a solution of tin(II) chloride in moderately concentrated hydrochloric
acid and add this to some of liquid A: The liquid first turns brown (takes 10
seconds or so) and then it turns very dark grey. A grey mirror is formed on the
glass. Adter a few hours, there is a clear liquid in the test tube and a layer
of precipitated dark grey solid at the bottom. This dark grey solid is
elemental arsenic.
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Prepare a solution of sodium dithionite and add this to liquid A: Immediately a
dirty brown and somewhat slimy precipitate is formed. On shaking, the color of
the precipitate becomes brown with a somewhat yellowish hue. The resulting
material almost certainly is very impure arsenic with strong sulphur
contamination.