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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 arrows. The resulting precipitate (s) and supernatant solution are shown on the lower left and right sides of the arrow. 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 briefly 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-brown), 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) (24) Cr(OH)4- (aq) + 3H2O2 (aq) + 2OH- (aq) 2CrO42- (aq) + 8H2O (l) (25) 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-brown 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-brown 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 stirring 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.libretexts.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 bubbling 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 turbidity confirms the presence of Cr3+ ions. View the following video: https://www.youtube.com/watch?v=msuQgzySxY4 May 8, 2019 - Uploaded 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 Aluminium ion test •Jun 8, 2011 11622SHARESAVE MsShanti2011 9. Wash the precipitate obtained in step 6 (Al-III) twice with 3 mL water, while warming the test tube in a water bath and stirring well. Centrifuge and discard the wash. 10. Dissolve the washed precipitate in 6 M HAc (acetic acid). Add 3 mL distilled water and 3 drops of catechol violet reagent and stir. Add drops of 6 M NH4OH until the solution turns blue which confirms the presence of Al3+ ions. Confirmed Result for A only (not for B). B. Precipitation and Analysis of Ammine Complexes:- 11. To the supernatant solution obtained in step 2 (Zn-III), add 5 – 8 drops of 1 M thioacetamide, CH3C(S)NH2, and heat in a water bath for 3 – 5 minutes while stirring with a glass rod. 12. Cool the mixture to room temperature and centrifuge for 3 minutes. Add a few more drops of CH3C(S)NH2 to the supernatant solution to test for completion of precipitation. Combine the solids and label the test tube Ni-III. You may discard the supernatant solution. Separation and Identification of Zn2+ ions:- 13. Add 1 mL of 6 M HCl to the solid from step 12. Stir the mixture, centrifuge and separate any remaining solid from the supernatant solution. Label the test tube containing the solid Co-III. 14. Make the supernatant solution from step 13 basic by adding drops of conc. NH4OH. A red litmus paper should turn blue. Add 5-8 drops of 1 M CH3C(S)NH2 and warm the mixture in water bath while stirring with a glass rod. Formation of a white precipitate at this stage confirms the presence of Zn2+ ions. Confirmed Result for B only (not for A). Separation and Identification of Co2+ ions:- 15. Dissolve the solid Co-III, from step 13, in 1 mL of freshly prepared aqua regia (3 parts conc. HCl/1 part conc. HNO3). 16. Add drops conc. NH4OH to make the solution neutral. Add 5 or more drops of 1 M KNO2 to the clear solution. Centrifuge out to separate the precipitate from the supernatant solution. Formation of a yellow precipitate confirms the presence of Co2+ ions. Save the supernatant solution for the next step. Not Confirmed for both A and B. Identification of Ni2+ ions:- 17. Make the supernatant solution from step 16 basic by adding conc. NH4OH. A blue