Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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dimethylamine borane : (CH3)2NH.BH3
potassium iodide : KI
hydrochloric acid : HCl
sodium hydride : NaH
sodium hydroxide : NaOH
disodium hydrogen arsenate heptahydrate : Na2HAsO4.7H2O
Class:
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elem=As
redox
Summary:
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Arsenate ion and arsenic acid cannot easily be reduced to the element.
Reduction to trivalent arsenic, however, can easily be accomplished with
iodide.
Description:
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Dissolve some solid Na2HAsO4.7H2O in water and add a few drops of hydrochloric
acid. Shake the liquid and then add a few granules of zinc metal: A colorless
gas is produced, this is hydrogen. The liquid remains clear and colorless.
Remark: It might be that some AsH3 is formed as well, but the experiment was
carried out in such a way that any chance of breathing some of the gas was
eliminated.
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Dissolve some Na2HAsO4.7H2O in water, add a few granules of NaOH and then add a
solution of dimethylamine-borane complex: No visible reaction. No gas bubbles
are produced, the liquid remains colorless and clear. There is a fairly strong
smell of dimethylamine.
Remark: No gas bubbles are produced, so no AsH3 is formed.
Heat the liquid until it starts boiling: The liquid remains colorless. No
elemental arsenic is formed, nor any bubbles of gas are produced. The smell of
dimethylamine (smell of stale piss) becomes stronger.
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Dissolve some solid Na2HAsO4.7H2O in water and add a few drops of hydrochloric
acid. Shake the liquid and then add a solution of potassium iodide: The
solution turns dark brown within in a few tens of seconds. The reaction is not
immediate, but the iodide is oxidized to iodine. The arsenic in the +5
oxidation state is capable of oxidizing the iodide to iodine.
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A final experiment to get any elemental arsine also failed. In this experiment,
some solid Na2HAsO4.7H2O is dissolved in water, a small amount of NaOH is
dissolved as well, and then some solid NaH (80% suspended in oil) is dumped
into the liquid: This reaction is quite violent and a lot of gas is produced.
Even a little flash of fire could be observed, but after the reaction there is
no visible evidence for the presence of any elemental arsenic.
WARNING: The latter experiment MUST be done outside, or in a real fume hood.
The reaction causes spraying of the liquid with the arsenate dissolved in it,
and it is imaginable that the colorless gas is rich in AsH3 (besides the H2
formed in this reaction).