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Title:
Qualitative Analysis of Group III Cations.
Purpose:
To find out the presence of group II cations from the three samples given from A, B and C. To each we need to add the group reagent NH4OH in presence of NH4Cl.
In the samples Group III cations Fe3+, Cr3+, Al3+ will be present.
Need to identify the same in the three samples given for our qualitative analysis.
Procedure:
Take three different test tube containing the three different unknown solutions containing certain cations from this group. In each of the test tubes the group reagent is treated with conc. NH4OH solution in the presence NH4Cl.This would result in possible precipitation of white gelatinous precipitate to show Al(OH)3 reddish
own precipitate for Fe(OH)3 and grey green gelatinous precipitate for Cr(OH)3.
In next steps the confirmatory tests are done:
a) The white gelatinous precipitate is taken in a test tube and acetic acid is added to it. Few drops of catechol violet reagent is added which gives a blue solution. This confirm the presence of Al3+ ions.
) The reddish
own precipitate is taken in a test tube and the excess base if present is dissolved by adding dilute HCl. In the resulting solution ammonium thiocyanate is added to get the characteristic blood red colour which confirm the presence of Fe3+ ions.
c) The green gelatinous precipitate is taken in a test tube and lead acetate solution is added which forms a yellow precipitate to confirm the presence of Cr3+ ions.
DATA SHEET
Sample
Reagent(s)
added
Results
Inference
Unknown Solution A
Add NH4OH in presence of NH4Cl.
A white gelatinous precipitate is formed.
Group III ion Al3+ should be present
White gelatinous precipitate
Add acetic acid + catechol violet reagent
Blue solution is formed
This confirms the presence of Al3+ ions.
Unknown Solution B
Add NH4OH in presence of NH4Cl.
Reddish
own precipitate is formed
Group III ion Fe3+ should be present
Reddish
own precipitate
ammonium thiocyanate is added
Characteristic red colour is formed
This confirms the presence of Fe3+ ions.
Unknown Solution C
Add NH4OH in presence of NH4Cl.
grey green gelatinous precipitate is formed
Group III ion Cr3+ should be present
Grey green gelatinous precipitate
Add lead acetate solution
A yellow precipitate is formed
This confirms the presence of Cr3+ ions.
Conclusion:
From the general group reagent test in step 1 and the confirmatory tests in step 2, we can conclude as per the colour of the precipitate and the solution about the presence and type of group III cations in the unknown samples given. The ions that were identified are Al3+ ,Fe3+, Cr3+
Semi-micro Qualitative Analysis of Cations
Experiment 4: Semi-Micro Qualitative Analysis of Group III Cations
(This is derived from “Experiments 3-5: Semi-Micro Qualitative Analysis of Cations” by Kelemu Woldegiorgis, Fall 2011, for Group I, II, and III cations)
BACKGROUND
A flow chart is used to summarize the steps involved in a qualitative analysis. In the flow chart, the reagents needed for each step of the analysis are shown next to vertical a
ows. The resulting precipitate (s) and supernatant solution are shown on the lower left and right sides of the a
ow. Insoluble substances (precipitates) are shown by the formula of the precipitate followed by “(s)”, where s stands for solid. Soluble species are indicated by their formula followed by “(aq)”, where aq stands for aqueous (solution in water).
In this experiment, you will analyze unknown solutions containing cations from Group III. The chemistry involved in the qualitative analysis of the Group III cations is
iefly outlined below.
Group III (Ni2+, Co2+, Fe3+, Cr3+, Al3+, Zn2+): An unknown solution containing certain cations from this group is treated with conc. NH4OH (aq) to result in the possible precipitation of Al(OH)3 (s) (white, gelatinous), Fe(OH)3 (s) (reddish-
own), and Cr(OH)3 (s) (gray-green gelatinous). You may wish to identify the cations in the precipitate first and the cations in supernatant solution next. The precipitates Fe(OH)3 (s) and Cr(OH)3 (s) are amphoteric and dissolve if excess of base is added. The species Fe(OH)3 (s) basic and does not dissolve in excess of base. Chromium(III) is then oxidized to Cr(VI) by adding solid Na2O2 or hydrogen peroxide, Eqn. 25, leaving Al3+ unaffected. The Al3+ is separated from the
M(OH)3 (s) + OH- (aq) M(OH)4- (aq) (M = Al, Cr) XXXXXXXXXX24)
Cr(OH)4- (aq) + 3H2O2 (aq) + 2OH- (aq) 2CrO42- (aq) + 8H2O (l XXXXXXXXXX25)
chromate ion, CrO42-, by precipitating the former as Al(OH)3 (s) by adding acetic acid and ammonium hydroxide solutions, respectively.
Confirmation test for Al3+: The white precipitate, Al(OH)3 (s), can be dissolved in 3.0 M acetic acid. Addition of a few drops of catechol violet reagent gives a blue solution if aluminum ions are present, Eqn. 26.
C19H14O7S (aq) + Al3+ (aq) Al(C19H12O7S)+ (aq) + 2H+(aq) (26)
Confirmation test for Cr3+: A yellow solution resulting from the hydrogen peroxide treatment may suggest that Cr3+ ions are present in solution. To confirm the presence of this ion a solution of lead(II) acetate, Pb(CH3CO2)2 (aq) is added. Formation of a yellow precipitate, Eqn. 5, confirms the presence of Cr3+ ions.
Confirmation test for Fe3+: The red-
own precipitate Fe(OH)3 (s) left unaffected by the treatment of the hydroxide precipitates with excess of base is dissolved by adding dilute hydrochloric acid. The resulting solution is then treated with ammonium thiocyanate to give the characteristic blood red color due to formation of the complex ion, Fe(SCN)2+, confirming the presence of Fe3+ ions, Eqn. 27.
Confirmation test for Zn2+: The supernatant solution obtained after treatment of the unknown solution containing Groups III cations with NH4OH (aq) may contain the ammine complexes of Zn2+, Ni2+, and Co2+. These cations are precipitated as sulfides by adding CH3C(S)NH2 (aq). ZnS (s) dissolves in dilute HCl (aq) while the others remain unaffected. Formation of a white precipitate upon addition of NH4OH/CH3C(S)NH2 to the supernatant solution confirms the presence of Zn2+ ions, Eqn. 28.
Confirmation test for Co2+: The insoluble sulfides of Co2+ and Ni2+ can be dissolved in aqua regia (a mixture of conc. HCl and conc. HNO3 in a 3:1 volume ratio). Treating the resulting solution with potassium nitrite, KNO2, in near neutral medium leads to the formation of a yellow precipitate, if Co2+ ions are present.
Confirmation test for Ni2+: Addition of a dilute solution of dimethylglyoxime (H2dmg) to the supernatant solution results in formation of a red precipitate, confirming the presence of Ni2+ ions, Eqn. 29.
PROCEDURE: Identification of Group III cations
SESSION III samples:
1. At the start, you will chose which one of three samples you are analyzing, A or B, and determine which Group III cations that are in your choice of sample.
2. The possible Group III cations are: Ni2+, Co2+, Fe3+, Cr3+, Al3+, and Zn2+.
A. Precipitation and Analysis of Insoluble Hydroxides:-
1. Obtain 1 mL of unknown solution from your instructor. Add 1 mL distilled water and 1 mL conc. NH4OH(aq). Stir the mixture with a clean glass rod.
2. Centrifuge and separate the solid from the supernatant solution. Add more conc. NH4OH(aq) to the supernatant solution to test for completion of precipitation. Combine the precipitates and save the supernatant solution. Label the test tube containing the supernatant Zn-III.
Separation and Identification of Fe3+ ions:-
3. To the precipitate obtained in steps 1 and 2, add 6 M NaOH drop by drop to dissolve most of the solid. Centrifuge and separate the remaining reddish-
own solid from the supernatant solution. Label the test tube containing the supernatant solution Cr-III.
4. Add 6 M HCl drop by drop to the solid from step 3, while sti
ing with a glass rod, until it dissolves completely. Then add 2 or more drops of 0.5 M KSCN. Formation of a deep red solution of Fe(SCN)2+ is a definitive test for iron(III).
For the Fe3+ reactions and confirmation: https:
chem.li
etexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Qualitative_Analysis/Characteristic_Reactions_of_Select_Metal_Ions/Characteristic_Reactions_of_Iron__(Fe³⁺)
James P. Birk, Professor Emeritus (Chemistry) at Arizona State University, Characteristic Reactions of Iron (Fe³⁺)
Jun 5, 2019
Potassium Thiocyanate
KSCN will give a deep red coloration to solutions containing Fe3+:
Result for A only (not for B).
Separation and Identification of Cr3+ ions:-
5. To the supernatant solution Cr-III, add about 50 mg crystals of Na2O2(s). Warm the mixture in a water bath. You should observe bu
ling as evidence of reaction. Allow at least 5 minutes for the reaction to be complete. Then boil the mixture gently for another one minute. Cool the mixture to room temperature.
6. Add 6 M HAc drop by drop to make the solution acidic. Then, add conc. NH4OH, drop by drop, to make the solution basic again. If you see a white gelatinous precipitate beginning to form, add a few more drops of NH4OH to complete the precipitation. You may need to reduce the volume of the solution to about 2 mL by evaporation. Upon cooling the solution to room temperature, a white precipitate forms if Al3+ ions are present. Centrifuge and separate the precipitate from the supernatant solution. Label the test tube containing the solid Al-III and proceed to step 8 to analyze the supernatant solution.
7. If necessary, reduce the volume of the supernatant solution from step 7 to 2 mL by evaporation. If, after evaporation, the solution is yellow, chromium is probably present; if it is colorless, chromium is absent.
8. In the case that the solution is yellow, add 6 M HNO3 until it is acidic (blue litmus paper should turn red). Add 2 drops of 0.5 M Pb(Ac)2 (aq) (lead(II) acetate). Formation of a yellow precipitate or tu
idity confirms the presence of Cr3+ ions.
View the following video:
https:
www.youtube.com/watch?v=msuQgzySxY4
May 8, XXXXXXXXXXUploaded by FIRE BRAND Changing India
Confirmed Result, formation of yellow precipitate for B only (not for A).
Identification of Al3+ ions:-
View the following video:
https:
www.youtube.com/watch?v=D3s-_0xUS-k