Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
sodium sulfide : Na2S . 3H2O
disodium hydrogen arsenate heptahydrate : Na2HAsO4.7H2O
Class:
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elem=S,As
coordination
Summary:
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Arsenate does not form thioarsenates when sulfide ion is added. On
acidification, the sulfide is oxidized though by the arsenate.
Description:
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Add a solution of sodium sulfide to a solution of Na2HAsO4.7H2O: No visible
change can be observed. The liquid remains colorless and clear.
Add a few drops of 15% hydrochloric acid: The liquid becomes turbid and a light
yellow precipitate of sulphur is formed. There also is a smell of H2S. Most of
the sulfide is oxidized by the arsenate to sulphur, the arsenate being reduced
to arsenite.