7g of ammonium carbonate is dissolved in 20 mL of water and 20 mL of concentrated
ammonia solution is added. While stirring, this is then added to a solution containing 5g
cobalt sulfate in 10 mL of water. 3 mL of 30% hydrogen peroxide is then slowly added. (Handle
H2O2 carefully. If spilled on the skin, wash the affected area immediately with water;
hydrogen peroxide can cause severe skin burns).Next, pour the solution into an
evaporating dish in a fumehood and concentrate it over a gas burner to 30-35 mL. Do not
allow the solution to boil. During the evaporation add, in small portions, 2g of ammonium
carbonate. Suction filter (with water aspirator) the hot solution and then cool the
filtrate in an ice bath. Under suction, filter off the red product crystals of
[Co(NH3)4CO3]2SO4.3H2O. Wash the product in the filtration apparatus using a saturated
solution prepared from a small portion of the precipitate.
Record the yield and show the product to your demonstrator.
2g of the previously prepared tetraamminecarbonatocobalt(III) complex is dissolved in 55mL of 0.3M sulfuric acid. Evolution of carbon dioxide should occur. The clear solution is then treated with 20-25 mL of ethanol, added in small portions. The precipitate is filtered off, washed with 50% ethanol/water until free of acid. (Check with litmus paper.) Finally, the product is dried in air. Record the yield and show the product to your demonstrator.
hexol.3g of cobalt(II) chloride hexahydrate is dissolved in a boiling solution of 2g of
ammonium chloride in 5mL of water. This is then added to 0.15g of decolourising charcoal
in a 100 mL flask and cooled by running water on the outside of the flask. The original
vessel is rinsed with 8 mL of concentrated ammonia and these washings are added to the
flask with the charcoal. The flask is then cooled to 10C or below.
8 mL of "20 volume" hydrogen peroxide is then slowly added in portions and the
falsk is briskly shaken. When all the oxidising agent has been added, the mixture is
heated gradually to 60C and maintained at this temperature until the pinkish tint in the
liquid disappears (about 30 minutes). During this time the flask should be shaken
frequently. At the end of the heating period, crystals should start to separate.
The flask is then cooled in an ice-bath for about 10 minutes and the crude solid filtered
off vai a Buchner funnel. Portions of the filtrate are used to rinse and solid remaining
in the flask on to the filter. The product is allowed to drain for 5 minutes and then is
recrystallised as follows:
Transfer the product to a beaker containing a boiling mixture of 25 mL of water and 1 mL
of concentrated hydrochloric acid. When all the solid (except the charcoal) has dissolved,
filter the hot liquid, via a Buchner funnel. 3 mL of concentrated hydrochloric acid is
then added to the filtrate and the solution cooled in an ice-bath.
The golden brown crystal of the hexaamminecobalt(III) ion should be obtained in high
yield. Filter off the crystals on a sintered glass filter funnel and wash them with
ethanol (NO FLAMES). Transfer the crystals to a small weighed beaker and dry them in an
oven at 110-120 for 30 minutes. (NOTE that higher temperatures will decompose the
product). Allow the crystals to cool in a dessicator and then record the yield and show
the product to your demonstrator.
1g of ammonium chloride is dissolved in 9 mL of concentrated ammonia solution contained
in a 100 mL Erlenmeyer flask. The mixture is continuously agitated while 2g of finely
powdered cobalt(II) chloride hexa-hydrate is added in portions.
A yellow-pink precipitate of hexaamminecobalt(II) chloride forms on slight warming as the
reddish cobalt chloride dissolves and reacts exothermically. Any air oxidation which
occurs at this stage may be ignored.
2 mL of 30% hydrogen peroxide is then added to the warm ammoniacal slurry in a thin stream
from a burette. (HANDLE H2O2 WITH CARE. It can cause severe skin burns). Continuous
swirling is essential during the oxidation. All the cobalt(II) ammine dissolves to form a
a deep red liquid with foaming and further evolution of heat. This corresponds to the
production of the pentaammineaquacobalt(III) ion. When vigorous effervescence has
virtually ceased, 6 mL of concentrated HCl is cautiously added to the heated well-stirred
solution. This operation should be performed in the fume hood since fumes of ammonium
chloride will be produced during the neutralisation. A purple product then precipitates
from the almost boiling reaction mixture leaving a pale green-blue supernatant liquid.
Heat for fifteen minutes on a steam bath, then cool to room temperature and filter off the
product in a sintered glass crucible (#3). The mother liquor may be discarded.
When the complex salt has been drained well, it is washed, first with 4 mL of ice-cold
water in portions, followed by an equal volume of 6M HCl previously chilled to 10C or
below.
The product is the dried in an oven at 100C for one hour to complete the conversion of any
remaining pentaammineaquacobalt(III) complex. Record the yield, show the product to your
demonstrator and store the product in a dessicator.
Dissolve 1g of pentaamminechlorocobalt(III) chloride, by gently warming and stirring,
in a mixture of 20 mL water and 1.5 mL of concentrated ammonia solution (CARE! EYES!).
Filter off any slight precipitate of cobalt oxide that may form, and then cool the
filtrate in ice to about 10C. When cool, add 2M HCl dropwise until the solution is just
neutral to litmus.
Next, dissolve 1g of sodium nitrite in the solution before adding 0.5 mL 11 M HCl. Allow
the solution to stand in an ice-bath for 1 hour and then filter off the salmon-pink
crystals. Wash with 5 mL of ice-water, followed by 5 mL of alcohol, and air dry on filter
paper at room temperature. Record the yield and show the product to your demonstrator.
Dissolve 0.5g of the above nitrito-complex in 4 mL of hot water containing a few drops
of concentrated ammonia solution and then add, while cooling, 4 mL of concentrated HCl.
Cool the solution in an ice-bath and filter off the yellow-brown crystals. Wash the
product with 2 mL of alcohol and air dry on filter paper at room temperature.
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