Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
ammonia : NH3
potassium bromide : KBr
hydrogen peroxide : H2O2
sulphuric acid : H2SO4
pyrogallol : C6H3 (OH)3
hydroquinone : HO C6H4 OH
metol : (CH3 NH C6H4 OH)2 . H2SO4
p-aminophenol HCl : NH2 C6H4 OH . HCl
Class:
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elem=C,Br
redox
Summary:
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Diverse phenol-derivatives, can be oxidized easily by bromine, yielding
intensely colored oxidation products.
Description:
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Preparation of bromine water:
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Dissolve an excess amount of potassium bromide in some solution of
hydrogen peroxide (3% solution) and add some dilute sulphuric acid: The
solution turns pale yellow.
Heat to 60 C for a few minutes: The solution turns orange. A brown gas can
be observed above the liquid. Now we have some acidic bromine water with
an excess amount of bromide ions.
Sequence 1 with p-aminophenol HCl:
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Dissolve some p-aminophenol HCl in water and add a little amount of the freshly
made bromine water (still luke-warm): An intensely colored compound is formed.
The solution of p-aminophenol HCl, which was almost colorless, now becomes
turbid and its color becomes deep purple. It is difficult to see, whether
the deep purple color is due to a precipitate or that the precipitate is
dark brown/black and that the liquid has a deep purple color.
Add an excess amount of NH3 (5% by weight) to the purple liquid: The liquid
turns dark brown. A dark brown precipitate, floating around as little
pieces appr. half a mm in size, is formed.
Strongly dilute the liquid: The liquid becomes brown and a little turbid. No
small pieces of precipitate can be observed anymore.
Decant the liquid from the test tube: The glass has a light brown/sepia color.
Add some solid NaOH and add a little amount of water: The glass is cleaned
effectively. The liquid turns pink/brown and becomes a little warm (due to
heat of hydration of NaOH).
Sequence 2 with metol:
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Dissolve some metol in water and add a little amount of the freshly made
bromine water (which is cold now): When the bromine water is added no
reaction appears to occur. The liquid becomes pale yellow (dilution of
the orange bromine water). After a few seconds however, the color slightly
changes. It appears to be more brown.
Heat the liquid to appr. 50 C: The liquid slowly turns darker. Finally a
clear liquid is formed with a fairly dark purple/red/brown color. Solid
particles of metol, sticking to the glass are surrounded by a dark purplish
brown liquid.
Add an excess amount of NH3 (5% by weight): The color of the liquid changes
from dark purplish/red to dark brown/sepia. The liquid becomes slightly turbid,
but no clear precipitate can be observed.
Sequence 3 with hydroquinone:
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Dissolve some hydroquinone in water and add a little amount of the freshly
made bromine water (which is cold now): When the bromine water is added the
the liquid becomes brown red and clear. Slowly, however, the liquid becomes
turbid and darker brown red. The turbidity changes into precipitation of
countless many little needle-like crystals, which are dark-grey or black.
Their color is hard to observe, due to the metallic lustre. The liquid
turns pale yellow and clear and the needles go to the bottom and form a
glittering layer at the bottom of the test tube.
Some solid hydroquinone, sticking to the wet glass, becomes covered by a
grey or dark layer, when bromine vapor flows along the crystals.
Add an excess amount of NH3 (5% by weight): The liquid becomes dark red/brown
and almost clear. The needle-like precipitate dissolves.
Sequence 4 with pyrogallol:
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Dissolve some pyrogallol in water and add a little amount of the freshly
made bromine water (which is cold now): When the bromine water is added the
liquid turns dark red/brown/orange and clear. The color resembles the color
of a highly concentrated solution of bromine in water, the liquid does not
become as dark as with metol and p-aminophenol. The reaction between the
pyrogallol and the bromine proceeds fairly fast at the start, but lateron
the intensity of the red/brown color appears to increase on standing.
Add an excess amount of NH3 (5% by weight): The liquid instantaneously
turns very dark brown, almost black. As far as can be observed with such
a dark liquid it appears to remain clear (or just a little turbid).