Description of experiment
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Needed compounds:
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sulphuric acid : H2SO4
sodium thiosulfate : Na2S2O3 . 5H2O
sodium hydroxide : NaOH
potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
potassium ferricyanide : K3 [Fe(CN)6]
silver nitrate : AgNO3
Class:
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elem=Ag,Fe,S
precipitation
Summary:
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Silver (I) ions form a precipitate, both with ferrocyanide and with
ferricyanide. The precipitate with ferricyanide is decomposed by alkalies,
the precipitate with ferrocyanide is more stable. Both compounds are
attacked by thiosulfate, which complexes the silver and causes the
solid to dissolve again.
Description:
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Add a solution of AgNO3 to
1) A solution of potassium ferricyanide;
2) A solution of potassium ferrocyanide.
The result of this is:
1) Formation of an orange/brown precipitate.
2) Formation of an off-white precipitate.
Add an excess amount of a solution of sodium hydroxide:
1) The precipitate becomes black. (What is this black compound?).
2) The precipitate becomes brown, but on shaking it becomes white again.
Probably some Ag2O is formed, but when sufficiently [Fe(CN)6]4- is
available, then this forms a precipitate with silver.
Wait for 2 days:
1) The black precipitate has become light yellow or white (exact color is
hard to determine, because of the fact that it is in a yellow liquid,
due to the presence of ferricyanide in solution). After rinsing several
times with water, the precipitate was light yellow, with an orange hue.
The color may be due to impurities, sticking to the precipitate.
2) The precipitate has not changed. It still is white.
Rinse the precipitates a few times in water and then add an excess amount
of a solution of sodium thiosulfate:
1) The precipitate dissolves and the liquid becomes light yellow, with an
orange hue. The liquid becomes clear, but the glass wall remains a
little bit dirty. Some white solid sticks to the glass, making its
appearance milky. This milky stain on the glass can easily be removed
by immersing the glass in dilute sulphuric acid (appr. 1 mol/l).
2) The precipitate dissolves and the liquid becomes light grey/green. The
liquid becomes almost clear, but a slight turbidity remains.