E8
Qualitative Analysis of Selected Group III Cations
Data, Observation and Report
Purpose
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To learn how two separate and identify Fe3+, Al3+, and Zn2+ ions as insoluble hydroxides.
To understand the role of buffers in controlling H3O+ and OH- concentrations.
To observe tendency of Zn2+ to form soluble complexes with ammonia.
To look at the amphoteric behavior of Al3+ as it forms an insoluble hydroxide that dissolves in
excess OH- to form a soluble hydroxy complex.
Discussion
Reactions of Fe3+, Al3+, and Zn2+ ions
The separation of Group III cations depends on their distinctive behavior with ammonia and with
sodium hydroxide. Zinc ion is conveniently removed and identified by its white insoluble sulfide In a
basic or slightly acidic solution. The control of pH is important in all of these separations.
Buffers
Buffers are used to control the relative hydronium ion concentration and hydroxide ion concentration
within certain limits. These consist of weak acids or weak bases together with their salts. Thus, in the
aluminum ion test that follows, a mixture of ammonia and ammonium chloride is used to produce a
basic solution that is much less basic than ammonia alone. Ammonia forms an equilibrium mixture with
its conjugate acid, ammonium ion according to
NH3 + H2O ⇐⇒ NH4+ + OHhaving an equilibrium constant, Kb, expressed as
Kb =
= 1.8 x 10-5 M
Excess NH4+ in the mixture shifts the equilibrium to the left, resulting in the lower hydroxide ion
concentration. While 1 M NH3 alone contains about 0.004 M OH-, calculations shows that if NH4Cl is
added to make the NH4+ and NH3 concentrations equal, the resulting OH- concentration then will be
only 1.8×10-5 M, a 200-fold reduction.
Likewise, if an acid solution still weaker than HC2H3O2 is desired, the addition NaC2H3O2 will decrease
the H3O+ concentration, in agreement with Le Chatelier’s principle as is evident from the equilibrium
HC2H3O2 + H2O ⇐⇒ H3O+ + C2H3O2-
Sulfide Precipitation
Data and Observations
Sulfide will be generated in the solution from thioacetamide according to the follwing reaction:
CH3CSNH2 + H2O → CH3CONH2 + H2S
The saturated solution of H2S is 0.1 M and provides a small concentration of S2- to precipitate zinc(II)
ions as ZnS(s).
Pre-Laboratory Preparation As with Group I Cation analysis study the procedure steps, part B, for
analysis below until you understand each step and the reason for adding each reagent. Then complete the
flow chart for these operations, in your notebook, in a manner similar to that in, experiment 7, Group I
Cation analysis. A skeleton flow chart is provided in part B of the report form: Insert the proper
formulas for reagents, precipitates, ions in solution, and so on. Also in your notebook and/or
experimental report, write the net ionic equations for the reactions occurring at each step of the
procedure, as numbered in the flow chart. (Your instructor may require that you complete this before an
unknown is assigned.)
EXPERIMENTAL PROCEDURE
Chemicals: 0.1 M Al(NO3)3, 0.1 M Fe(NO3)3/1 M HNO3, 0.1 M Zn(NO3)2, “aluminon reagent”-(1.0 g/L
of the ammonium salt of aurin tricarboxylic acid), thioacetamide, CH3CSNH2, 0.1 M KSCN, 6 M NH3,
6 M NaOH, 6 M HCl.
WASTE COLLECTION: None of the solutions containing the ions in this experiment should be
disposed of down the drain. Waste containers should be provided for all waste solutions.
Part A. Typical Reactions of the Ions
(1) Ammonia and hydroxide ion complexes. Place 1-mL Samples of 0.1 M Al(NO3)3, 0.1 M Fe(NO3)3,
0.1 M Zn(NO3)2, separate test tubes. To each add one drop of 6 M NH3, then continue to add more, drop
by drop with agitation, to determine whether the hydroxide precipitate first formed redissolves with
NH3. {Note: The 0.1 M Fe(NO3)3 contains excess HNO3, which must be first neutralized before any
precipitate appears.}
Repeat these tests, using 6 M NaOH instead of 6 M NH3. Summarize all the results in your notebook in
table or chart. {As provided in the report form.}
(2) Red Lake Formation with Al(OH)3. The light flocculent precipitate of aluminum hydroxide is often
difficult observe. The dye called aluminon on (ammonia aurin tricarboxylate) is absorbed by the
precipitate to form a characteristic red lake, which makes the identification of the aluminum hydroxide
easier.
To 1 mL of water (in a 10x100 mm test tube) add 3 to 5 drops of 0.1 M Al³+ solution. Add 3 drops of
aluminum reagent, then 3 to 5 drops of 6 M NH3 and shake well. Note the appearance of the precipitate.
Now make the solution acidic with ~0.5 mL 6 M HCl, and again add 6 M NH3 until just basic to Litmus.
Mix well and let the mixture stand a moment, and note the characteristic color and flocculation of the
precipitate (that now should be red due to the dye). If the pH is properly adjusted, you should see a red
precipitate that appears to be suspended “red lake” in nearly a colorless solution. A good red color is not
obtained if the solution is too basic. Explain how the preceding treatment guarantees a very slight basic
solution. {Hint: Consider the concept of buffering and apply Le Chatelier’s principle.}
Part B. Analysis of a Known Solution for Fe3+, Al3+, and Zn2+ Ions
Prepare a known solution containing Fe3+, Al3+, and Zn2+ by adding approximately 1-mL each of
0.1 M Fe(NO3)3, 0.1 M Al(NO3)3, 0.1 M Zn(NO3)2 to a test tube. Analyze according to the procedure
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Data and Observations
described below, following your completed flow chart. As you are following each step in the analysis
also complete an analysis summary in your notebook similar to Table-3 (a form for this summary is
provided in part B of the report form).
Directions for steps 1-5 were given in experiment 7, and are related to Group I Cation analysis only.
6. Precipitation of Fe(OH)3 and Al(OH)3 To 1 mL of the solution to be tested, add an excess (about
one mL) of 6 M NH3. Mix or agitate the solution, then centrifuge it. Decant supernatant solution which
contains Zn(NH3)42+ from the precipitate. Wash the precipitated Fe(OH)3 and Al(OH)3 with 1 mL water,
mix well, centrifuge, decant and discard washings.
7. Separation of Fe3+ and Al3+ Treat the mixed precipitate with about 0.5 mL of a 6 M NaOH followed
by 1 mL of water. Centrifuge decant supernatant liquid containing Al(OH)4- ion. Wash precipitate with
~1 mL of water, centrifuge, decant and discard wash.
8. Test for Fe3+ Dissolve the Fe(OH)3 precipitate by adding 1-5 drops 6 M HCl and shake to dissolve
(warm if necessary in water-bath). Add ~2 mL of H2O shake and then 1-3 drops of 0.1 M KSCN. A red
color of [Fe(SCN)]2+ ion confirms the presence of Fe3+ in test solution.
9. Test for Al3+ To the Al(OH)4- solution from step 7, add 6 M HCl until acidic to Litmus (blue → red)
then add ~0.5 mL excess. Add 1-3 drops of aluminon reagent and ~1/2-1 mL of H2O and mix. Add 6 M
NH3 until just basic to Litmus. Mix well and let it stand. A flocculent “red lake” precipitate, confirms
the presence of Al3+ in test solution. If color and precipitate are indefinite, you may again make the
solution acidic with HCl, then basic with NH3, to build up the NH4+ concentration so the solution
becomes less basic.
10. Test for Zn2+ To the filtrate from step 6, containing any zinc present as Zn(NH3)42+, add 1-5 drops
of thioacetamide, mix and place in a water-bath at 60°C under fume hood for a few minutes. A white
precipitate of ZnS is confirmatory for the presence of Zn2+ in your test solution. {Looking ahead, when
analyzing the general cation unknown use about half the filtrate to test for the Zn(II) ion and the rest for
Group IV & V cations. If you precipitated Zn2+ as ZnS, centrifuge and combine supernatant with the
other half of the solution you saved for Group IV & V.}
Part C. Analysis of Unknown Solutions for Fe3+, Al3+, and Zn2+ Ions
Obtain an unknown solution from your instructor, and analyze them by the preceding steps. Keep a
record in your notebook of each step, with your actual observations for both negative and positive tests,
by completing an analysis summary in your notebook like in part B.
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E8
Qualitative Analysis of Selected Group III Cations
Data, Observation and Report
Name:__________________________________
Partner(s) Name: _____________________
Instructor: ______________ Date: _______
Section: MW / TTH / M-TH/F
(circle)
Data and Observations
Part A. Typical Reactions of the Group III ions
(1) Ammonia and Hydroxide Ion Complexes In the spaces provided, write the formulas of the
precipitates formed, or new ions formed in solution, if any. Also indicate any characteristic colors and
the like that result when each ion is treated with the reagent in the left-hand column.
Reagent
Fe3+
Al3+
Zn2+
6 M NH3
6 M NH3 (excess)
6 M NaOH
6 M NaOH (excess)
(2) Red Lake Formation with Al(OH)3
Equation for the reaction of Al3+ with NH3: ________________________________________________
Briefly, explain how the addition, first, of an excess of HCl, then a slight excess of NH3, ensures that the
solution will not become too basic to form a satisfactory adsorption compound of the dye with the
precipitate.
Data and Observations
Part B. Analysis of a Known Solution for Fe3+, Al3+, and Zn2+ Ions
Complete the following flowchart and analysis summary for your known mixture of Fe3+, Al3+, and Zn2+
Complete the analysis summary for your known sample (as in experiment 7, Group I Cations).
Step
Sample
Reagent(s)
Observations
6
7
8
9
10
Summary Conclusions:
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Conclusions
Data and Observations
Write below, in order as numbered in the flow chart the net ionic equations for all reactions occurring in
the systematic analysis of the ions of this group.
Part C. Analysis of Unknown Solutions for Fe3+, Al3+, and Zn2+ Ions
Unknown no. _____ Ions Found: _____________________________
Analysis Summary: Report your actual observations, negative as well as positive, for each step of the
analysis and record above the ions found.
Step Sample
Reagent(s)
Observations
Conclusions
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Santa Monica College
Chemistry 12
Name: ____________________________
Date: ________________________
Lab Partner: ________________________
Lab Section: __________________
Lab Report: Qualitative Analysis of Group 1 Cations
In the space provided below construct a flow cart for the analysis of your unknown. Indicate on
the flow chart whether the test for each ion is positive or negative.
Unknown number
__________________
Ions present in your unknown
____________________________________
Qualitative Analysis of Group 1 Cations
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