Semi-micro Qualitative Analysis of Cations
Experiment 3: Semi-Micro Qualitative Analysis of Group I&II 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
Qualitative analysis refers to the identification of species in a sample. Knowing the amounts of the species in the sample is not important in this type of chemical analysis. The procedure involves separating cations in a mixture into several groups (Groups I, II, III, and IV) based on their chemical and physical properties and identifying individual cations within a group based on their chemical behaviors. Cations found in the same group display similar properties. For instance, chlorides of the cations in Group I are practically insoluble in water while chlorides of the cations in Groups II – IV are soluble in water. In other words, the cations in Group I can be separated from a mixture by adding hydrochloric acid. Hydrochloric acid is the group precipitating agent for Group I cations.
A group precipitate is further analyzed by re-dissolving it in acid or base. If a precipitate containing only one cation does not dissolve under this condition, this physical property may be used to confirm the presence of that cation. The cations in solution are further resolved in a series of steps that may include acid-base, complex ion formation, redox, or other precipitation reactions. Formation of a characteristic precipitate or colored complex ion is used to confirm the presence of a cation in the ultimate analysis.
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 Groups I, and II. The chemistry involved in the qualitative analysis of each group of cations is
iefly outlined below.
Group I (Ag+, Hg22+, Pb2+): These cations are separated from a mixture as chloride precipitates formed upon addition of dilute hydrochloric acid. Hydrochloric acid is said to be the group reagent for Group I cations. The reactions that occur are represented by Eqns. 1 -3:
It is important to add enough HCl to ensure complete precipitation, but not too large an excess. If large excess of HCl is added, some of the precipitates tend to dissolve by producing chlorocomplexes such as AgCl2- (aq).
Further analysis of the chloride precipitates begins with dissolving Lead(II) chloride, PbCl2(s), in hot water, Eqn. 4. Verification of the presence of Pb2+ ions in
XXXXXXXXXX4)
solution can be made in two ways. In the presence of Pb2+ ions, addition of a solution of potassium dichromate, K2Cr2O7 results in the formation of a yellow precipitate, Eqn. 5. The chromate ion, CrO42-, is formed by the hydrolysis of Cr2O72-.
In an alternative test, sulfuric acid is added to observe formation of a white precipitate, PbSO4 (s), Eqn. 6.
The solid remaining after hot-water treatment may contain the chlorides AgCl (s) and Hg2Cl2 (s). Treatment with aqueous ammonia results in dissolution of AgCl (s) while Hg2Cl2 (s) transforms into other solid materials. The species AgCl (s) dissolves in aqueous ammonia forming the complex ion Ag(NH3)2+, Eqn. 7.
Confirmation of the presence of Ag+ ions is made by adding nitric acid, HNO3, to the supernatant solution. Nitric acid acts on Ag(NH3)2+ to produce Ag+ and NH4+ ions allowing precipitation of AgCl(s), Eqn. 1.
Hg2Cl2 (s) dissolves in aqueous ammonia via a rather unusual oxidation-reduction reaction. The products include finely divided metallic mercury and a compound with the formula HgNH2Cl, Eqn. 8. Formation of a black-gray precipitate confirms the presence of Hg22+ ions.
Hg2Cl2 (s) + 2NH3 (aq) Hg (l) + HgNH2Cl (s) + NH4+ (aq) + Cl- (aq) (8)
XXXXXXXXXXblack white
Group II (Pb2+, Hg2+, Cu2+, Bi3+, As3+, Sb3+, Sn4+, Cd2+): The cations in this group form insoluble sulfides (PbS, HgS, CuS, Bi2S3, As2S3, Sb2S3, SnS2, and CdS) at low pH. The group precipitating reagent is hydrogen sulfide, H2S. It is generated in situ by the thermal decomposition of thioacetamide, CH3CSNH2, in solution, Eqn. 9.
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The hydrogen sulfide thus generated undergoes ionization according to Eqns. 10 – 11.
The sulfides As2S3, Sb2S3, and SnS2 form soluble sulfide complex ions when treated with excess of sulfide. This chemistry is used to separate them from the sulfides that do not dissolve in excess of sulfide. The sulfides PbS, CuS, and Bi2S3 dissolve in boiling dilute nitric acid by way of the redox reactions, Eqns. 12-14, while HgS is unaffected. HgS transforms into a white insoluble double salt, Hg(NO3)2.2HgS, if boiled with dilute HNO3 long enough.
3PbS(s) + 8HNO3 (aq) 3Pb(NO3)2 (aq) + 2 NO(g) + 4H2O(l) + 3S(s) (12)
3CuS(s) + 8HNO3 (aq) 3Cu(NO3)2 (aq) + 2NO(g) + 4H2O(l) + 3S(s) (13)
Bi2S3 (s) + 8HNO3 (aq) 2Bi(NO3)3(aq) + 2NO(g) + 4H2O(l) + 3S(s) (14)
Confirmation test for Hg2+ ions: Any precipitate remaining after the # M HNO3 treatment of the sulfides can be dissolved in aqua regia (Eq. 15).
3 HgS(s) + 2NO3-(aq) + 12Cl-(aq) + 8H+(aq)
XXXXXXXXXX3HgCl42-(aq) + NO(g) + 3S(s) + 4H2O(l) (15)
The presence of Hg2+ ions is confirmed by the reduction of Hg2+ to Hg22+ with SnCl2 (Eqns XXXXXXXXXXA white-gray precipitate forms if Hg2+ ions are present.
2HgCl42-(aq) + SnCl2(aq) Hg2Cl2(s) + SnCl62-(aq) + 2Cl-(aq) (16)
Hg2Cl2(s) + SnCl2 (aq) + 2Cl-(aq) SnCl62-(aq) + Hg(l) (17)
Confirmation test for Pb2+ ions: Formation of a white precipitate upon addition of concentrated sulfuric acid to the supernatant solution obtained from the dissolution of sulfides in nitric acid indicates the presence of Pb2+ ions, Eqn XXXXXXXXXXThe dissolution of the white precipitate upon addition of a dilute solution of ammonium acetate and formation of a yellow precipitate upon subsequent addition of potassium dichromate confirms the presence of Pb2+ ions.
Confirmation test for Bi3+ ions: Precipitation of Pb2+ ions as PbSO4 (s) leaves Cu2+ and Bi3+ ions in solution. Treatment of this solution with ammonium hydroxide results in a white precipitate, Bi(OH)3 (s), and a blue solution containing the complex ion Cu(NH3)42+ (aq). Formation of elemental bismuth, Bi(s) black, upon treatment of the white precipitate with a SnCl2/OH- mixture confirms the presence of Bi3+ ions, Eqn. 21.
2Bi(OH)3 (s) + 3HSnO2- (aq) 3HSnO3- (aq) + 2Bi (s) + 3H2O (l) (21)
Confirmation test for Cu2+: The blue solution obtained by addition of ammonium hydroxide may indicate the presence of Cu2+ ions. A confirmatory test would be to treat the blue solution with a solution of potassium fe
ocyanide, K4[Fe(CN)6]. Copper(II) ions give a reddish precipitate upon reacting with fe
ocyanide, Eqn. 23.
Cu(NH3)42+ (aq)
SESSION I & II samples:
1. At the start, you will chose which one of two samples you are analyzing, A or B, and determine which Group I and Group II cations that are in your choice of sample.
2. Both A and B will have two Group I cations and two Group II cations.
3. The possible Group I cations are: Ag+, Pb2+, and Hg22+.
4. The possible Group II cations are: Cu2+, Bi3+, and Hg2+.
SESSION I: Separation and Analysis of Group I Cations
After you have read the following procedure, view a video of:
Dr. Sehet Nauli, “Qualitative Analysis of Group I Cations”, 10/3/2018, which is like this procedure. Copy the following video link into your web
owser and play the video:
https:
www.youtube.com/watch?v=LTBEvBO4cTA
Separation:-
1. Place 1 mL (ca. 20 drops) of your unknown solution provided to you for analysis in a 10-cm test tube. Write the unknown number in your notebook.
2. Add 5 drops of 6.0 M HCl to the unknown solution and mix the content thoroughly with a glass sti
ing rod. Add a few more drops of the acid if the amount of the white precipitate formed appears to be small.
3. Take a second test tube and add water approximately the same amount as the unknown solution. Cover the test tubes with cork and place them into a centrifuge in such a way that they are balanced (opposite to each other). Turn the centrifuge on for 3 minutes and then turn it off. Wait until the centrifuge stops spinning and take your test tubes out.
4. Carefully decant the supernatant solution into a clean test tube. Label the test tube containing the solid PPt-I.
5. Add one or two drops of 6.0 M HCl to the supernatant to test completeness of precipitation. If a cloudiness develops at the top of the solution, add several more drops of 6 M HCl and stir the mixture, then separate the solid from the supernatant solution. Combine the solid with PPt-I and label the test tube containing the supernatant solution Group-II. This solution will be used for the analysis of Group II cations during Session II.
Analysis:-
6. Wash the precipitate (PPt-I) with acidified water to remove free Pb2+ ions: prepare
acidified water by adding 1 drop of 3.0 M HCl to 4 mL of water in a test tube and mix
them with a sti
ing glass rod. Add 1 mL of the acidified water to