Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium ferrocyanide : K4 [Fe(CN)6] . 3H2O
hydrochloric acid : HCl
ammonia : NH3
sulphuric acid : H2SO4
sodium hydroxide : NaOH
disodium phosphate : Na2HPO4
phosphoric acid : H3PO4
ferric chloride : FeCl3 . 6H2O
Class:
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elem=Fe,P
coordination
Summary:
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Iron (III) builds a coordination complex with phosphates.
Description:
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Sequence 1:
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Add a solution of ferric chloride to a solution of disodium phosphate: An off-
white precipitate is formed in the form as a jelly mass. When the liquid is
shaken, then it becomes turbid and off-white. However, the precipitate slowly
dissolves on waiting. The liquid slowly becomes less turbid and after a minute
or two, the liquid has become clear again and it is almost colorless (when
observed carefully, one can see that it has a very faint yellow/brown color).
Add some dilute sulphuric acid (appr. 1 mol/l): The liquid remains clear and
almost colorless. A very slight change of color can be observed, now the
liquid is very pale yellow. The yellow color can be observed slightly more
easily, but still it is very pale.
Add an excess amount of a solution of sodium hydroxide: A brown precipitate
is formed.
Sequence 2 (remark: may be invalid, due to unknown impurities):
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Add some phosphoric acid (appr. 8% by weight) to a solution of ferric chloride:
The liquid becomes more intensely colored, although the concentration of iron
is reduced by a factor of 3 by adding the phosphoric acid. A reference tube
was used, with the same amount of ferric chloride solution, but with water
added. This reference tube was much lighter than the solution with phosphoric
acid.
Add a solution of sodium hydroxide: A brown precipitate is formed, but at some
places an off-white precipitate is formed. When the liquid is shaken, it is
brown and turbid. After a while, it has become less turbid, but it does not
become clear. A finely dispersed brown precipitate remains in the liquid.
Sequence 3a:
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Add a few drops of a concentrated phosphoric acid solution (appr. 85% by
weight) to a solution of ferric chloride: The liquid becomes completely
colorless and becomes completely clear. (This is in contrast with sequence 2,
but confirms sequence 1, the phosphoric acid, used in sequence 2 may be
contaminated with something else)
Add some ammonia (appr. 5% by weight): An off-white precipitate is formed,
which however, quickly redissolves again, leaving behind a colorless clear
liquid again.
Add much more ammonia: The off-white precipitate is formed again, together
with some brown/yellow precipitate. When the liquid is shaken, then it
becomes turbid pale yellow/off white. Now the liquid does not become
clear again.
Add an excess amount oh hydrochloric acid (appr. 10% by weight): The liquid
becomes clear again, but now it is bright yellow. After a while, however,
the color fades away slowly. Finally the liquid is clear and light yellow.
It does not become completely colorless again.
Sequence 3b:
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Add a few drops of a concentrated phosphoric acid solution (appr. 85% by
weight) to a solution of ferric chloride: The liquid becomes completely
colorless and becomes completely clear. (This is in contrast with sequence 2,
but confirms sequence 1, the phosphoric acid, used in sequence 2 may be
contaminated with something else)
Add a solution of potassium ferrocyanide: The liquid becomes dark blue and
a dark precipitate is formed.