LAB – QUALITATIVE ANALYSIS OF CATIONS
Name___________________
Chemicals:
Solutions of cations:
Silver nitrate, AgNO3, 0.2 M
Iron(lII) nitrate, Fe(NO3)3, 0.2 M
Copper(II) nitrate, Cu(NO3)2, 0.2 M
Lead nitrate, Pb(NO3)2, 0.5 M
Zinc nitrate, Zn(NO3)2, 0.2 M
Manganese(lI) nitrate, Mn(NO3)2, 0.2 M
Test reagents:
Hydrochloric acid, HCl, 6 M
Nitric acid, HNO3, 6 M
Ammonia, NH3, 6 M
Hydrogen peroxide, H2O2, 3%
Sodium bismuthate, NaBiO3(s)
Sulfuric acid, H2SO4, 6 M
Acetic acid, HC2H302, 6 M
Sodium hydroxide, NaOH, 6 M
Potassium thiocyanate, KSCN, 0.1 M
Potassium hexacyanoferrate(ll), K4[Fe(CN)6], 0.1 M
To neutralize spills:
Baking soda, NaHCO3(s)
Vinegar, HC2H3O2
Equipment
Test tubes, 6, 13- x 100-mm
Beaker, 250-mL for hot water bath
Beral pipets, graduated
Centrifuge
Test tube rack
ceramic hot plate
pH paper
stir rod
Procedure
Safety Alert
Most of the acids and bases used are very concentrated and can cause chemical bums if spilled.
Handle them with care. If you get acid or base on yourself, wash it off with lots of water. Small
spills (a few drops) can be cleaned up with paper towels. Larger acid spills can be neutralized with
baking soda, NaHCO3, and then safely cleaned up. Neutralize base spills with a vinegar solution
(dilute acetic acid). Some of the solutions are poisonous. Wash your hands when you are finished.
Solutions containing silver ions cause stains which do not appear for several hours. If you think
you spilled any of the solution containing silver ions on yourself, wash off with soap and water.
Wear Chemical Splash Goggles and a Chemical-Resistant Apron. Make sure all of the solutions
containing ions are disposed of in an appropriate heavy-metal waste container.
Proper Use of Centrifuge.
Before turning the centrifuge on, make sure the test tubes in the device are as evenly-distributed as
possible. If you have only one test tube to spin, balance it with a test tube filled with water.
Preparation of a Solution for Analysis.
Prepare a known solution by combining three drops of each ion to be tested. The total solution volume
will be about 2 mL.
To analyze an unknown solution, use about 1.5 mL of the solution that your teacher will give you.
1. Separation of the Silver Group (Ag+and Pb2+) from Manganese, Iron, Copper, and Zinc Ions.
Add 8 drops of 6 M HCl to the solution to be analyzed. Stir. A white precipitate indicates that one or
more of the ions Ag+or Pb2+ is present. Centrifuge the solution and test to be sure that precipitation is
complete by adding one more drop of 6 M HCI. No additional precipitate should form. If more precipitate
does form, continue adding 6 M HCI until precipitation is complete. Centrifuge and decant (pour off) the
clear liquid into a second test tube for step 4. Alternatively, you can use a Pasteur (capillary) or Beral pipet
to draw off the supernatant liquid to transfer it to another test tube. Wash the precipitate by adding 1 mL
distilled water and stirring. Centrifuge and discard the wash water.
LAB – QUALITATIVE ANALYSIS OF CATIONS
Name___________________
2. Separation and Confirmation of Lead.
Add 1 mL of distilled water to the precipitate of AgCl and PbCl2 from step #1, and heat in a boiling water
bath for about 3 minutes with occasional stirring. Centrifuge the hot solution, and quickly pour off the
supernatant liquid into another test tube for the next part of this step. Set aside the tube with the remaining
precipitate for step 3.
To the clear liquid you just decanted, add 5 drops of 6 M H2SO4. A white precipitate of PbSO4 confirms the
presence of lead. If lead ions are found to be present, check to be sure that all of the lead is removed from
the AgCl precipitate by repeating the process of adding distilled water, heating, centrifuging
and pouring off the clear liquid until the addition of H 2SO4 to the liquid no longer forms a precipitate.
Dispose of the lead sulfate precipitate by placing it in the container provided.
3. Confirmation of Silver.
To the precipitate from step 2, which is AgCl, add 1 mL 6 M NH 3. This should dissolve the AgCl
precipitate. Add 15 drops of 6 M 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. 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 silver.
Dispose of the silver compound in the container provided.
4. Separation of Manganese, Iron, and Copper from Zinc.
To the solution saved from step 1, add 10 drops of 3% hydrogen peroxide, H 2O2. With stirring, add
6 M sodium hydroxide, NaOH, until the solution is basic, and then add 3 more drops. Stir and place
the test tube in a hot water bath for 3 minutes. You should see the formation of a dark precipitate
which will be the solids listed above. Centrifuge the solution, and separate the clear solution from the
solid. Save the clear solution, which may contain Zn(OH) 42- ions for step 9. Wash the precipitate with a
mixture of 10 drops of 6 M NaOH and 10 drops of water. Centrifuge and discard the wash water.
5. Separate Manganese Ions from Iron and Copper Ions.
Add 5 drops of water to the precipitate from step 4. Then add 6 M H2SO4 dropwise until the solution
is acidic when tested with litmus paper. Centrifuge and separate the precipitate which may be dark brown
to black MnO2 from the supernatant liquid which will contain the yellow Fe3+ and blue Cu2+ ions. Save the
solution for step 7. Wash the precipitate with water, centrifuge, and discard the wash water.
6. Confirmation of Manganese Ions.
To the precipitate from step 5 add 1 mL of water and 1 mL of 6 M H2SO4. Add 1 mL of 3% H2O2
and heat in a boiling water bath with stirring. The precipitate should dissolve quickly leaving very little
residue. Add 1 mL of 6 M HNO3, and add a spatula of solid sodium bismuthate, NaBiO3. Some excess
solid sodium bismuthate should remain. Add more if needed. Stir and centrifuge to determine the color of
the supernatant liquid. The purple color of the MnO4- ion in solution confirms the presence of manganese.
Dispose of the manganese in the container provided.
7. Separation of Iron from Copper; Confirmation of Copper.
To the solution from step 5, add 6 M aqueous NH3 until the solution is basic to litmus, and then add
1 mL extra. Centrifuge and separate the supernatant liquid from the precipitate. Save the precipitate
for step 8. The presence of the blue Cu(NH3)42+ ion is the confirmatory test for copper. If you wish an
additional confirmatory test, to the solution containing the Cu(NH 3)42+ add 6 M HC2H3O2 until the blue
color fades and the solution becomes acidic. Then add 2 drops of 0.1 M K 4[Fe(CN)6]. A red-brown
precipitate of Cu2[Fe(CN)6] reconfirms the presence of copper. Dispose of the copper solution in the
container provided.
LAB – QUALITATIVE ANALYSIS OF CATIONS
Name___________________
8. Confirmation of Iron.
Wash the precipitate of iron hydroxide from step 7. Then add 6 M H2SO4 dropwise until the precipitate
dissolves. To the solution, add 5 drops of 0.1 M KSCN solution. The deep red FeSCN 2+ ion confirms
the presence of iron. Dispose of the iron solution in the container provided.
9. Confirmation of Zinc.
To the colorless solution from step 4, add enough 6M HCl dropwise to make the solution slightly acidic.
Add 3 drops of 0.1 M K4[Fe(CN)6] and stir. Centrifuge to see the confirmatory precipitate of
Zn3[Fe(CN)6]2 which will be white to light green or blue green in color. Dispose of the zinc precipitate and
solution in the container provided.
Discussion
Your laboratory report should contain all of your data from the analysis of the known solution as well as
your unknown solution. State clearly which ions were shown to be present and which ions were proven
absent.
In your report also include answers to the following questions:
1. What is the precipitating reagent for the silver group ? Would a solution of NaCl work as well? Why or
why not?
2. In the analysis scheme, Ag+ is precipitated as AgCl, the precipitate is dissolved, and then AgCl is precipitated again in the confirmatory step. Explain the chemistry of each of these steps by showing a
balanced equation for each.
3. Give the identity of the ions in your unknown. Cite specific data from your analysis that support your
conclusion.
LAB – QUALITATIVE ANALYSIS OF CATIONS
Name___________________
Qualitative Analysis of Cations
Flow Chart
Ag+ Pb+2 Mn+2 Fe+2 Cu+2 Zn+2
HCl
1
AgCl(s)
Mn+2 Fe+2 Cu+2 Zn+2
PbCl2(s)
2
Hot H2O
3
NH3
2
Ag(NH3)2+
PbSO4(s)
3
AgCl(s)
See next page
Pb+2
AgCl(s)
HCl
H2SO4
LAB – QUALITATIVE ANALYSIS OF CATIONS
Name___________________
Qualitative Analysis of Cations
Flow Chart (con’t)
Mn+2 Fe+3 Cu+2 Zn+2
H2O2
NaOH
4
Zn(OH)4-2
MnO2 (s) Fe(OH)3(s) Cu(OH)2(s)
5
H2SO4
9
K4[Fe(CN)6
]
K2Zn3[Fe(CN)6]2(s)
Fe+3 Cu+2
MnO2(s)
H2SO4
H2O2
6
Mn+2
6
7
Fe(OH)3(s)
HNO3
NaBiO3
H2SO4
8
Fe+3
MnO4-
NH3
Cu(NH3)4+2
K4[Fe(CN)6]
7
Cu3[Fe(CN)6]2(s)
KSCN
8
FeSCN+2