Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
nitric acid : HNO3
sulphuric acid : H2SO4
hydrochloric acid : HCl
sodium sulfite : Na2SO3
sodium hydroxide : NaOH
ammonia : NH3
hydroxyl amine sulfate : (NH3OH)2 SO4
chrome alum : KCr(SO4)2 . 12H2O
ammonium dichromate : (NH4)2Cr2O7
sodium dichromate dihydrate : Na2Cr2O7.2H2O
potassium dichromate : K2Cr2O7
Class:
------
elem=Cr
coordination
redox
Summary:
--------
Chromium (III) apparently builds a coordination complex with hydroxyl amine,
but this complex does not simply form from chromium (III) salts and
hydroxyl amine. If chromium (III) is formed by means of reduction of
chromium (VI) in the presence of hydroxyl amine, then the complex is
formed. If hydroxyl amine is added to chromium (III) without redox reaction,
then another complex is created.
Description:
------------
This experiment consists of a set of sequences, with different conditions and
reagents in order to get insight in the behavior of chromium with hydroxyl
amine.
Sequence 1:
-----------
Add an excess amount of NaOH and some sodium dichromate to some water: Both
solids dissolve. The sodium dichromate dissolves first, resulting in an
orange liquid, the sodium hydroxide dissolves quickly also. The final liquid
is lemon-yellow and is strongly alkaline.
Add some solid hydroxyl amine sulfate: The liquid starts foaming. A brown
slurry appears. The liquid becomes completely turbid.
Add more water and a large excess amount of hydroxyl amine sulfate: The
resulting liquid becomes dark brown and completely opaque. A thick slurry
is formed.
Add much more NaOH: The solid dissolves and the liquid becomes clear and
dark brown.
Add an excess amount of dilute H2SO4 (2 mol/l): The liquid remains clear and
dark brown, even when it is gently boiled.
Add an excess amount of hydroxyl amine sulfate to the dark brown acidic
liquid: The solid dissolves, no further visible changes.
Add some sodium sulfite: The liquid remains dark brown (it does not turn
green or blue/green as one would expect). A strong odour of SO2 can be
observed. Apparently the dark brown compound is not a chromium (VI) compound,
which is reduced by sulfite or SO2.
Sequence 2:
-----------
Add an excess amount of hydroxyl amine sulfate to a solution of ammonium
dichromate in dilute H2SO4 (1 mol/l): A gas is evolved at the solid. The
solid quickly dissolves. The liquid changes from orange to bluish green,
a well-known color for chromium (III). Apparently the chromium - hydroxyl
amine complex is not formed when dichromate is reduced by hydroxyl amine
in acidic environments.
Sequence 3:
-----------
Add an excess amount of hydroxyl amine sulfate to a solution of ammonium
dichromate in dilute NH3 (appr. 1% by weight), such that the solution is
yellow instead of orange (excess base): The solid dissolves, the liquid
turns brown and remains clear.
Add an excess amount of H2SO4 (1 mol/l): The liquid remains brown and clear.
Sequence 4:
-----------
Add a little amount of solid potassium dichromate to a solution of hydroxyl
amine sulfate (no base or acid added): A gas is evolved at the crystals of
K2Cr2O7. The reaction proceeds quietly, but clearly visible. A brown
precipitate is formed, which sticks on the crystals of K2Cr2O7. The reaction
proceeds more and more slowly and finally comes to a halt, with the crystals
covered by a layer of a brown solid and the liquid above the crystals having
a orange/brown color.
Add an excess amount of HCl (10% by weight): All crystals of K2Cr2O7 quickly
dissolve, while a gas is developed at the crystals. The liquid becomes green/
brown and clear.
Add much more hydroxyl amine sulfate and HCl (10%): No visible changes, most
notably, the liquid does not become fully green.
Sequence 5:
-----------
Dissolve some chrome alum in water and add an excess amount of NaOH: The
liquid is dark green and clear (chromium (III) in strongly alkaline environ-
ment).
Add an excess amount of hydroxyl amine sulfate: The liquid becomes purple,
after a few tens of seconds it becomes grey. A gas is evolved. Apparently,
a complex is formed, but this complex differs from the complex formed from
dichromate. This complex is purple/grey, while the other one is brown.
Add an excess amount of nitric acid (2 mol/l) (nitric acid is used, because
of the fact that nitrate ion does not form complexes with chromium (III)):
The liquid remains grey/green. Development of gas continues, but it slows
down.
Sequence 6:
-----------
Dissolve some chrome alum in water and add an excess amount of hydroxyl
amine sulfate: Both solids dissolve and the liquid looks like a plain solution
of chrome alum in water. Now no special reaction is observed.
Add an excess amount of NaOH: The liquid becomes turbid for a little moment,
but then it becomes clear again and deep purple/grey (the same color as the
one, observed in sequence 5 after adding hydroxyl amine sulfate). A gas is
evolved from the liquid, especially at the places, where the pearls of NaOH
did not yet fully dissolve in the water.
Heat the liquid: The development of gas becomes much more vigorous. When the
source of heat is removed, the liquid remains very effervescent. At some
points in time there is hardly any effervescence and a few seconds later it
suddenly produces a lot of gas again. It looks as if a little bubble of gas
provokes more development of gas. When the liquid is shaken effervescence
becomes more vigorous as well. A smell of ammonia can be observed, this
probably is one of the decomposition products of hydroxyl amine (or is the
smell of hydroxyl amine the same as the smell of ammonia?). When the liquid
is kept hot (near the boiling point) for a while, then it becomes green, just
like a solution of a chromium (III) salt in a strongly alkaline environment.
Remarks:
--------
The results of all these experiments show that when dichromate reacts with
hydroxyl amine sulfate in acidic environment, then plain chromium (III) is
produced. When it reacts in alkaline environment, then a brown complex
appears to be formed, which even in strongly acidic environments is stable.
However, the brown complex cannot be created from chromium (III) and hydroxyl
amine in alkaline nor in acidic environments. In acidic environments, no
reaction can be observed at all, in alkaline environments another type of
complex is formed, which is purple/grey.