Qualitative Analysis
In this experiment you will use a series of reactions to separate a mixture of cations by precipitation and
then identify those cations through a series of reactions. Your unknown contains three cations from the following
list: Ca2+, NH4+, Ag+, Fe3+, Al3+, Cr3+, Zn2+, Ni2+, Mg2+, and Na+. Your conclusion should show each reaction that
produced a result and conclusively support your identification of the cations.
Materials:
Unknown mixture
Distilled water
Nichrome wire
Solutions of:
(NH4)2C2O4
NH3
Dimethylglyoxime
HCl
HNO3
magnesium reagent
NaOH
NH4Cl
K4Fe(CN)6
Aluminon reagent
Safety:
1.
2.
3.
4.
HCl, HNO3, and NaOH are corrosive. Use them with caution.
Use NH3 in the fume hood only.
Goggles must be worn during this experiment.
Unknown aqueous waste must be disposed of in the beaker in the hood. Centrifuge tubes can be disposed of in
the trash.
Procedure:
1.
2.
3.
4.
Read the descriptions of the tests in the packet provided. Write the reactions that occur in each test in your
notebook so you know what to add and what will result. Answer the Pre-Lab Questions at the end of the
scheme.
CAREFULLY follow the cation separation scheme below, beginning with step 1. Begin by removing a small
portion of your unknown mixture and starting the scheme (don’t use it all in case you make a mistake!). After
each step, filter your sample and save the filtrate. This liquid contains all of the remaining cations. The
precipitate contains the cation you just tested for. SAVE ALL SOLIDS AND LIQUIDS. You never know
when you might have to retest.
Be extremely careful that you do not contaminate any of the test reagents. If you do, let the instructor know.
Write complete net ionic equations for each test that you do. Record all observations of every reaction in your
lab notebook. Write clearly.
Scheme:
1.
Sodium Flame Test: dip the Nichrome loop in 12 M HCl and heat it in the hottest part of the flame. Repeat
this until no color is seen in the flame. Remove a few drops of your unknown solution and test those drops in
the flame. A bright orange color indicates the presence of sodium ions. To be sure, compare your results with a
flame test of distilled water and one of NaCl solution.
Ammonium test: Place 1-3 drops of your unknown solution on a watch glass. Wet a small piece of red litmus
paper with distilled water. Add the same number of drops of 3M NaOH solution to your unknown sample and
suspend the litmus paper above it. Do not let the paper touch the liquid. If the paper turns blue, ammonium ion
is present.
2.
Silver test: Place 10 drops of the original solution to be analyzed in a test tube. Add 5 drops distilled water
and 2 drops of 6M HCl. Stir well, centrifuge, and reserve the decantate (liquid) for step 3. Wash the precipitate
with 10 drops of distilled water, centrifuge, and add the washings to the decantate. Dissolve the precipitate in 4
drops of 6M NH3, then add 6 M HNO3 to the solution until it is acidic to litmus (turns blue paper red). A white
precipitate that dissolves upon addition of HNO3 indicates Ag+.
3a. Separation of iron and aluminum: To the decantate from step 2 add 2 drops of NH4Cl solution and 6M NH3
until the solution is basic to litmus (red turns blue). Centrifuge and reserve the decantate for step 4. Wash the
precipitate with 10 drops distilled water, centrifuge, and add the washings to the decantate.
3b. Separation and detection of Iron: to the precipitate from step 3a, add 5 drops distilled water, 10 drops 3M
NaOH, and 5 drops of 3% H2O2. Stir, centrifuge, and save the decantate for step 3c. Wash the precipitate with 10
drops distilled water and add the washings to the decantate. Dissolve the reddish-brown precipitate in 2 drops of 6M
HCl and add 10 drops of K4[Fe(CN)6] solution. A blue precipitate indicates the presence of Fe3+.
3c. Separation and detection of Aluminum: If the decantate from step 3b is yellow, place it on an evaporating
dish and heat the solution on a hot plate and evaporate almost to dryness to remove excess H2O2. Add 10 drops of
distilled water and 6M HCl until acidic to litmus. Then add 6M NH3 until basic to litmus. CENTRIFUGE and
discard the decantate. Wash the precipitate with 10 drops distilled water, CENTRIFUGE and reserve the precipitate.
Dissolve the precipitate in 2 drops of 6M HCl, add 2 drops of Aluminon reagent, and 6M NH3 until basic to litmus.
CENTRIFUGE the solution. A red “lake” (layer of red that is separate) indicates Al3+.
4. Detection of Calcium: add 3 drops of the (NH4)C2O4 solution to the decantate from procedure 3a and
CENTRIFUGE. Reserve the decantate for step 5a. Wash the precipitate with 10 drops distilled water, centrifuge,
and add the washings to the decantate. Dissolve the precipitate in 2 drops 6 M HCl and perform a flame test. A red
flame indicates the presence of Ca2+. (Note: Mg2+ can be precipitated here also; test the acid solution for Mg2+ as in
step 5c)
5a. Detection of Zinc: Heat the decantate from step 4 to dryness in an evaporating dish. Add 5-6 drops
concentrated HNO3 and heat again until no more fumes are observed (you are removing excess NH4+ ions that
would interfere with the Mg+2 test). Dissolve the residue in 5 drops 6M HCl. Add 10 drops distilled water to the
HCl solution and transfer to a test tube. Rinse the evaporating dish with 10 drops distilled water and add the wash to
the acid solution. Add 3M NaOH until the solution is basic to litmus, CENTRIFUGE, and reserve the precipitate for
procedure 5b. Wash the precipitate with 10 drops distilled water and add the washings to the decantate. Add 3
drops of K4[Fe(CN)6] to the decantate and acidify with 6M HCl; a gray-blue coloration or precipitate indicates Zn2+.
5b. Detection of Nickel: Add 5 drops distilled water and 5 drops 15M NH3 to the precipitate from step 5a;
CENTRIFUGE and reserve the precipitate for step 5c. Wash the precipitate with 10 drops distilled water and add
the wash to the decantate. Add 1 drop dimethylglyoxime solution to the decantate; a strawberry-red precipitate
indicates Ni2+. This salt is Ni(C4H7N2O2)2.
5c. Detection of Magnesium: If you detected Ni2+ in step 5b, add 15M NH3 to the precipitate you saved from step
5b until no positive test for Ni2+ is observed. Dissolve the hydroxide precipitate in 2 drops of 6M HCl; add 2 drops
of the “magnesium reagent” and 3M NaOH until the solution is basic. A blue lake indicates Mg2+.
Pre-Lab Questions
1. Complete and balance the following:
NH4+(aq) + OH-(aq) 
AgCl(s) + NH3(aq) 
2. How could you separate:
(a) Fe3+ from Ag+
(b) Al3+ from Mg2+
3.
Which salt is insoluble: NaCl, FeCl3, ZnCl2, or AgCl? Which one(s) is/are colored?