Description of experiment
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Needed compounds:
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sodium dithionite : Na2S2O4 . 2H2O
ammonia : NH3
sodium hydroxide : NaOH
mercury (II) nitrate : Hg(NO3)2
nitric acid : HNO3
Class:
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elem=Hg
coordination
redox
precipitation
Summary:
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Mercury (II) salts give an oxide/hydroxide, which does not dissolve in
large excess amounts of alkaline solutions. Ammonia does not dissolve
the precipitate of mercury (II) oxide/hydroxide.
Description:
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Add a large excess amount of a solution of NaOH to a solution of mercury (II)
nitrate in dilute nitric acid (appr. 3 mol/l): Formation of a yellow
precipitate. It is not bright yellow, but a little brownish yellow.
Let precipitate settle for a few days and decant the liquid from the
precipitate. To the remaining precipitate a large excess amount of NH3
(appr. 5% by weight) is added: The precipitate does not dissolve (at
least, not all of it).
Add some solid sodium dithionite: The solid dissolves, the yellow precipitate
is converted to a dark compact grey precipitate. The grey precipitate is
finely divided metallic mercury.