Description of experiment
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Needed compounds:
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sodium sulfite hepta hydrate : Na2SO3 . 7H2O
nitric acid : HNO3
silver nitrate : AgNO3
potassium dichromate : K2Cr2O7
Class:
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elem=Cr,Ag
precipitation
Summary:
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Silver (I) gives a bordeaux-red precipitate with dichromate, which
slightly dissolves in water.
Description:
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Prepare a saturated solution of potassium dichromate above some solid
potassium dichromate and add a solution, containing 2.5% by weight silver
nitrate: A bordeaux-red precipitate is formed, which quickly sinks to the
bottom.
Heat the liquid, until it starts boiling: A large part of the precipitate
dissolves and all solid potassium dichromate dissolves.
Add water, such that all solids just dissolve when the liquid boils: The
liquid is orange/red and clear.
Let cool down: Many glittering dark red plates/crystals appear in the
liquid, which slowly sink to the bottom. The plates are very thin, but
their area is fairly large (appr. 1 mm2). After one day of standing
the plates are at the bottom of the test tube. A thick layer of bordeaux-
red crystals is at the bottom. The liquid above the crystals is orange.
Decant the orange liquid, add water, shake and remove water, add water again,
shake, remove again and add water again: The red crystals dissolve in water,
albeit just in small quantities. The water becomes clear and pale yellow.
Decant the water again and add an excess amount of 2M HNO3: The liquid
becomes clear and pale yellow again.
Add a fairly large amount of satured solution of Na2SO3.7H2O: The liquid
has a pungent smell of SO2, indicating that it contains excess sulfite
and excess acid. It turns green (formation of chromium (III)). Part of
the bordeaux-red precipitate dissolves, but a new precipitate is formed.
The new precipitate is a white crystalline precipitate. On shaking and
settling again, the tube contains a thin bordeaux-red/purple layer, on
top of this there is a somewhat thicker layer of white crystals and
above that there is a bluish green clear liquid.
Shake the liquid for a few minutes: All bordeaux-red crystals dissolve and
much more of the white precipitate appears. The liquid becomes totally
opaque and greyish green. After standing for a minute or two, the liquid
is clear again and bluish green and a thick layer (1 cm) of a crystalline
precipitate has settled voluminously on the bottom of the test tube.
Heat the liquid and keep it close to its boiling point for a few minutes:
Already before the liquid starts boiling, most of the white precipitate
has dissolved. The free sulfur dioxide is driven off (the liquid starts
bubbling and gives off a strong pungent odour, but only for the first
half minute or so). When the liquid is boiling, it is completely clear,
but there are some larger pale yellow particles, which do not dissolve. When
shaking stops, these larger particles sink to the bottom quickly.
Let liquid cool down and wait for a few days, in between shaking a few
times: The liquid remains clear, besides the small amount of pale yellow
particles. A large amount of crystalline precipitate was expected after
a certain cool down period and after some shaking, but this did not
appear!