Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
hydrochloric acid : HCl
sodium dithionite : Na2S2O4 . 2H2O
potassium iodide : KI
sodium tungstate : Na2WO4.2H2O
Class:
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elem=W
redox
Summary:
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Tungstate ions, when reduced, produce a deep blue color. Tungstate is
a weak oxidizer.
Description:
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Sequence 1
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Dissolve some sodium tungstate (Na2WO4.2H2O) in some water: The solid
dissolves and the liquid becomes clear and colorless.
Add some hydrochloric acid (10% by weight): The liquid becomes turbid and
pale yellow. A pale yellow precipitate is produced.
Add some solid KI: The solid dissolves, the liquid remains turbid and pale
yellow. Even after a few minutes no visible changes could be observed. It
appears that tungstate is not capable of oxidizing iodide in acidic
environments.
Add some solid Na2S2O4.2H2O: Where the solid touches the liquid, it
instantaneously causes an intense blue coloration. The little crystals,
sinking to the bottom leave thin dark blue traces in the pale yellow
liquid. When shaking, the liquid becomes dark blue and opaque.
Add an excess amount of sodium hydroxide: The solid dissolves and the
liquid becomes almost clear and pale yellow (milky). The milkyness
may be due to decomposition products of the dithionite. All dark blue
material disappears.
Sequence 2
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Dissolve some sodium tungstate and a little amount of sodium dithionite
in water: The liquid becomes clear and colorless.
Add some hydrochloric acid (10% by weight): The liquid becomes intense blue
and a precipitate is formed (it is expected that this is the pale yellow
precipitate, as formed in sequence 1, contaminated with the intense blue
compound).
Remove most of the liquid: Some of the blue precipitate sticks to the glass.
Shaking with water does not dissolve the precipitate and it remains blue.
Add some water and a few grains of sodium hydroxide: The sodium hydroxide
dissolves and the precipitate, which sticked to the glass also dissolves.
The liquid becomes clear and pale blue (almost colorless).