Tests for Zinc Ion
Zinc is a chemical element with symbol Zn
. It is found in various salts like zinc chloride
and zinc sulphate as Zn2+ ion. In group IV
, zinc ions are precipitated as their sulphides
by passing hydrogen sulphide gas through their
solutions.
Our aim here is to test for the presence of
zinc ion in a given salt.
First, we look at the preliminary tests of
zinc ion.
Dry heating test
Materials required:
Original salt in a test tube, test tube holder
and Bunsen burner.
Procedure:
Take a small quantity of salt in a test tube.
Heat it over the Bunsen burner.
The salt becomes yellow when hot and white
when cold, indicating the presence of zinc
ion.
Flame test
Materials required:
Original salt in a watch glass, concentrated
hydrochloric acid, dropper, glass rod, platinum
wire loop and Bunsen burner.
Procedure:
Take a small quantity of the salt in a watch
glass.
To this, add a few drops of concentrated hydrochloric
acid using a dropper.
Mix the contents of the watch glass using
a glass rod to make a paste.
Take a small amount of the paste on the platinum
wire loop and introduce it into the oxidizing
flame of the Bunsen burner.
Green flashes indicate the presence of zinc
ion.
Charcoal cavity Test
Materials required:
Mixture of salt and sodium carbonate in a
watch glass, charcoal piece, distilled water,
spatula, dropper, tongs, blow pipe and Bunsen
burner.
Procedure:
Take a mixture of a small quantity of salt
and double its quantity of sodium carbonate
in a watch glass.
Take a charcoal piece with a small cavity
in it.
Using a spatula, place a small quantity of
the mixture in the cavity of the charcoal
piece.
Using a dropper, add a few drops of distilled
water to the mixture to moisten it.
Using the tongs, hold the charcoal piece in
front of the reducing flame of the Bunsen
burner.
Now, direct the reducing flame on the cavity
by means of the blow pipe and heat strongly
for some time.
A yellow residue is obtained on heating and
it becomes white on cooling, indicating the
presence of zinc ion.
Cobalt nitrate test
Materials required:
White residue obtained in the charcoal cavity
test, cobalt nitrate solution, dropper, tongs,
blow pipe and Bunsen burner.
Procedure:
Take the charcoal piece with white residue
in it.
Using a dropper, add a few drops of cobalt
nitrate solution to the white residue.
Using the tongs, hold the charcoal piece in
front of the oxidising flame of the Bunsen
burner.
Now direct the oxidising flame on the cavity
by means of the blow pipe and heat strongly
for some time.
Formation of green residue indicates the presence
of zinc ion.
Next, we look at the Analysis of Zinc Ion.
Materials required:
Salt solution in a test tube, concentrated
nitric acid, ammonium chloride, ammonium hydroxide,
hydrogen sulphide gas, spatula, droppers,
test tube holder and Bunsen burner.
Procedure:
Take a small quantity of concentrated nitric
acid using a dropper.
Add it into the salt solution taken in a test
tube.
Boil the solution for some time over the Bunsen
burner.
Using the spatula, add a small quantity of
ammonium chloride to the solution.
Boil the contents again and cool the test
tube in a beaker containing water.
Then, add excess of ammonium hydroxide solution
and shake well.
Now, pass hydrogen sulphide gas through the
solution.
Zinc (II) ions present in the salt solution
react with hydrogen sulphide to form a dull
white precipitate of zinc sulphide.
Finally we look at the confirmatory tests
of zinc ion.
Sodium hydroxide test
Materials required:
Salt solution in a test tube, sodium hydroxide
and dropper.
Procedure:
Take a small quantity of sodium hydroxide
solution using a dropper.
Add it into small quantity of salt solution
taken in a test tube.
Zinc ions present in the salt solution react
with sodium hydroxide to form white precipitate
of zinc hydroxide.
Now, add excess of sodium hydroxide solution
to the precipitate.
The white precipitate dissolves because zinc
hydroxide reacts with excess of sodium hydroxide
to form soluble sodium zincate.
Potassium ferrocyanide test
Materials required:
Salt solution in a test tube, potassium ferrocyanide
solution and dropper.
Procedure:
Take a small quantity of potassium ferrocyanide
solution using a dropper.
Add into the salt solution taken in a test
tube.
Shake the test tube well.
Zinc ions present in the solution react with
potassium ferrocyanide to form a white precipitate
of zinc ferrocyanide.
Precautions:
Handle hydrogen sulphide with proper care
because it is highly hazardous.
Handle nitric acid and hydrochloric acid proper
care because they are highly corrosive.
Heating should be done very carefully.
