12
I. QUALITATIVE ANALYSIS OF CATIONS
Note that the following directions are written for a “known” solution (a standard) that
contains all of the cations. An “unknown” solution (a sample) will not form all of the
products described in the procedure. Make note of any differences in the “unknown”
solution as it is analyzed. In the directions that follow, a description of the physical
properties and the chemistry of the substances appears in boxed frames:
1. Separation of the Silver from Iron, Copper, and Zinc Ions.
Most chloride salts are soluble; however, Ag+
ions form an insoluble chloride. These Ag+
ions
can be separated from the other ions present in this qualitative analysis scheme by precipitating
them as chlorides. All of the other ions will stay in solution.
Ag+
(aq) + Cl–
(aq) AgCl(s)
a. Add 8 drops of 10% HCl to approx. 5 ml of the solution to be analyzed. Stir. A white
precipitate indicates that the Ag+
ion is present.
b. Test to be sure that precipitation is complete by adding one more drop of 10% HCl. No
additional precipitate should form. If more precipitate does form, continue adding 10%
HCl until precipitation is complete.
c. Filtrate the solution to quantitatively remove AgCl.
d. Save the precipitate on the filtration paper for procedure #2.
e. Save the clear liquid (filtrate) into a second test tube for procedure# 3.
2. Confirmation of Ag(+).
Aqueous solutions of Ag+
and Zn2+
are colorless. Aqueous solutions of Fe3+
is often yellowish due
to hydrolysate of Fe(OH)3 ; Cu2+
is light blue in aqueous solutions.
When 2 M NH3 is added to AgCl, the Ag+
ion forms a colorless complex ion and goes into solution:
AgCl(s) + 2 NH3(aq)  Ag(NH3)2
+
(aq) + Cl–
(aq)
Addition of hydrochloric acid to the Ag(NH3)2+
complex ion breaks apart the ion. The NH3
combines with H+
to form NH4
+
, and the Ag+
ion recombines back with the Cl–
ion to precipitate as
white AgCl.
Ag(NH3)2
+
(aq) + Cl–
(aq) + 2 H+
(aq)  AgCl(s) + 2 NH4
+
(aq)
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a. Load (with a spatula) the precipitate from procedure 1d, which is AgCl, to a clean test
tube and add 1 mL 2 M NH3.
b. Stir until the precipitate completely dissolves.
c. Add 15 drops of 10% HCl to the solution. The solution will smoke and the reaction
between the strong acid and the base will give off heat whether or not silver is present.
The test tube may get very warm.
d. Stir and test with pH indicator paper to be sure the solution is acidic. If it is not acidic,
add more HCl. The reappearance of the white AgCl precipitate in the acidic solution
confirms the presence of Ag(+).
3. Separation of Iron and Copper from Zinc. Confirmation of ammonium.
a. Put 1 ml of the solution saved from procedure 1e in a test tube and add 2 M NaOH,
until the solution is basic and then add 3 more drops. Place a small paper plug
loosely about halfway down the test tube, but not touching the solution (see
Figure). Hang a piece of watered pH paper in the tube so that the bottom of the
paper is on the plug and not touching the solution (otherwise it will turn deep
blue!). HEAT carefully the test tube. If ammonia is present, it will be released and
color the pH paper dark green. NH4
+
(aq) + OH–
(aq)  NH3 (g) + H2O
b. To the rest of the solution saved from procedure 1e add NaOH, and observe a precipitate
indicating the presence of either copper or iron or both. Heat the testube carefully over a
burner so that the precipitation is complete.
c. Filtrate the solution to separate the clear solution from the solid. Save the clear solution,
which may contain Zn(OH)4
2–
ions for procedure #6.
d. Wash the precipitate with a mixture of 10 drops of 10% NaOH and 10 drops of water.
e. Save the precipitate for procedure #4.
4. Separation of Iron from Copper; Confirmation of Cu(2+).
In a basic solution, the amphoteric zinc will form a colorless complex ion and remain in solution,
while the hydroxides of all the other ions will precipitate. The iron will precipitate as rust colored
Fe(OH)3, and the copper as blue Cu(OH)2. The reactions are as follows:
Fe3+
(aq) + 3 OH–
(aq)  Fe(OH)3(s)
Cu2+
(aq) + 2 OH–
(aq)  Cu(OH)2(s)
Zn2+
(aq) + 4 OH–
(aq)  Zn(OH)4
2-
(aq)
Both cupric hydroxide, Cu(OH)2, and ferric hydroxide, Fe(OH)3, readily dissolve in acid solution.
Cu(OH)2(s) + 2 H+
(aq)  Cu2+
(aq) + 3 H2O(l)
Fe(OH)3(s) + 3 H+
(aq)  Fe3+
(aq) + 3 H2O(l)
Aqueous ammonia added to a solution in which Cu2+
is present, will cause the deep blue
tetraammine copper(II) complex ion to form. The presence of this deep blue color confirms the
presence of copper. At the same time, the basic ammonia solution will precipitate the hydroxides of
iron.
Cu2+
(aq) + 4 NH3(aq)  Cu(NH3)4
2+
(aq)
Fe3+
(aq) + 3 OH–
(aq)  Fe(OH)3(s)
An additional and sensitive confirmatory test for copper is to precipitate the red-brown
copper(II) hexacyanoferrate(II) [also called copper(II) ferrocyanide], Cu2[Fe(CN)6](s), from a
Cu2+
solution.
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a. To the precipitate from procedure 3e, add 5 drops of deionized water.
b. Add 35% H2SO4 dropwise until the solution is acidic when tested with pH paper (about 6
drops). Stir to dissolve precipitate.
c. To the solution, add 2 M aqueous NH3 until the solution is basic (pH paper), and then
add 1 mL extra.
d. Separate the supernatant liquid from the precipitate by filtration. Save the precipitate for
procedure #5. The presence of the deep blue [Cu(NH3)4]2+
ion is the confirmatory test
for copper.
e. For an additional confirmatory test, to the solution containing the Cu(NH3)4
2+
add 50%
CH3COOH, acetic acid, until the deep blue color fades and the solution becomes acidic.
Then add 2 drops of 10% K4[Fe(CN)6]. A red-brown precipitate of Cu2[Fe(CN)6]
reconfirms the presence of Cu(2+).
5. Confirmation of Fe(3+).
a. Wash the precipitate of iron hydroxides from procedure 4d.
b. Add 35% H2SO4 dropwise until the precipitate dissolves.
c. To the solution add 5 drops of 15% NH4SCN solution. The deep red FeSCN2+
ion
confirms the presence of Fe(3+).
6. Confirmation of Zn(2+).
a. If your solution from procedure 3c contains Zn+2
, it will be complexed with OH-
as
Zn(OH)4
2b.
Make the solution from procedure 3c slightly acidic by adding 10% HCl dropwise.
c. Add 3 drops of 10% K4[Fe(CN)6] and stir.
d. Check the confirmatory precipitate of K2Zn3[Fe(CN)6]2 which will be white to
light green or blue green in color.
e. Dispose of the zinc precipitate.
7. Repeat steps 1–6 for the cation in your unknown sample. Be sure to record the results
for each step.
SAMPLE = 2 cations
Ferric hydroxide will dissolve in sulfuric acid. Addition of the thiocyanate ion, SCN–
, forms a
deep wine-red colored complex ion with iron that is a very sensitive test for the presence of
iron.
Fe(OH)3(s) + 3 H+
(aq)  Fe3+
(aq) + 3 H2O(l)
Fe3+
(aq) + SCN–
(aq)  FeSCN2+
(aq)
The confirmatory test for zinc is the formation of a precipitate of potassium zinc
hexacyanoferrate(II), K2Zn3[Fe(CN)6]2. This precipitate is nearly white if pure, but if a trace
of iron is present, it may appear light green or blue-green in color. It might also appear as a
suspension, a jelly-like precipitate that is suspended in solution, rather than sinking to the bottom.
[Zn(NH3)4]2+
(aq) + 4 H+
(aq)  Zn2+
(aq) + NH4
+
(aq)
3 Zn2+
(aq) + 2 K+
(aq) + 2 [Fe(CN)6] 4–
(aq)  K2Zn3[Fe(CN)6]2(s)
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CATION ANALYSIS DATA TABLE
Known Solution Unknown Solution
Step Procedure Results Conclusion Results Conclusion
1
2
3
4
5
6
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