GROUP II – CATION ANALYSIS Group II consists of eight different cations : Hg+2, Pb+2, Bi+3, Cu+2, Cd+2, As+3, Sb+3 and Sn+4. They form very insoluble sulfides with characteristic colors at the 3-5 pH range. Thioacetamide, CH3CSNH2, is generally used to supply S2- to avoid the direct use of the very toxic, odoriferous H2S gas. At pH less than 3, thioacetamide hydrolyzes to furnish H2S; at a pH greater than 5, thioacetamide reacts directly with the cation to give a complex which subsequently hydrolyzes to acetamide and the insoluble sulfide. Analysis of Group II cations involves, its division into two subgroups by the action of the KOH solution. Consequently, Group IIA contains those cation sulfides which are insoluble in KOH, and Group IIB is composed of those which dissolve in KOH solution. PROCEDURE:
GROUP II – CATION ANALYSIS
Group II consists of eight different cations : Hg+2, Pb+2, Bi+3, Cu+2, Cd+2, As+3, Sb+3 and Sn+4. They form very insoluble sulfides with characteristic colors at the 3-5 pH range.
Thioacetamide, CH3CSNH2, is generally used to supply S2- to avoid the direct use of the very toxic, odoriferous H2S gas. At pH less than 3, thioacetamide hydrolyzes to furnish H2S; at a pH greater than 5, thioacetamide reacts directly with the cation to give a complex which subsequently hydrolyzes to acetamide and the insoluble sulfide.
Analysis of Group II cations involves, its division into two subgroups by the action of the KOH solution. Consequently, Group IIA contains those cation sulfides which are insoluble in KOH, and Group IIB is composed of those which dissolve in KOH solution.
PROCEDURE:
Precipitation of Group II Ions
- Place 10 drops each of the test solutions in a 10-mL test tube. To this, add 2 drops of 3M HNO3 and heat in a water bath for 3 minutes. Make barely alkaline with 3M NH3, then add 3M HCl until it is just acid. Concentrate to a volume of 1 mL after which, add 2 drops of conc. HCl. Add 3 drops of thioacetamide solution and heat again in boiling water bath for 5 minutes. Centrifuge and discard centrifugate.
Separation into Subgroups II-A and II-B
- Wash the precipitate from above with 10 drops of distilled water containing 1 drop of thioacetamide solution and one drop of saturated NH4Cl. Discard wash water and add 8 drops of 3M KOH. Mix thoroughly and heat in water bath for 3 minutes. Centrifuge, then separate the centrifugate.
Use the residue for Group II-A analysis and the centrifugate for II-B.
Group II-A Analysis
- Wash the residue with 10 drops of H2O and discard washings. Add 10 drops of 3M HNO3 and heat in a water bath for 3 minutes. Centrifuge. Repeat the procedure if much residue is left. Separate residue from centrifugate.
4a. Dissolve the residue from Step 3 in 4 drops of aqua regia. Add 10 drops of distilled water and boil in water bath for 2 minutes. Cool and add 2 drops of SnCl2 solution. A white or gray precipitate confirms the presence of mercuric ion.
4b. To the centrifugate from Step 3, add concentrated ammonia dropwise while stirring until the solution is basic, then add 3 drops in excess. Centrifuge. If centrifugate turns to deep blue, the presence of cupric ion is confirmed.
4c. Remove centrifugate from Step 4b and wash the precipitate with 5 drops of water. Discard washings. To this, add 10 or more drops of freshly prepared potassium stannite solution. Immediate blackening of precipitate indicates presence of bismuth ion.
Group II-B Analysis (centrifugate from procedure 2)
- Add 3M HCl to the centrifugate from Step 2 until solution is acid. Centrifuge and discard centrifugate. Add 10 drops of conc. HCl, heat in water bath for 1 minute and centrifuge. Save the centrifugate. Wash residue left with 8 drops of water and 4 drops of conc. HCl. Add washing to centrifugate of conc. HCl treatment.
5a. Wash the residue obtained from the above procedure with hot water 3 times. Add 4 drops of conc. HNO3 and heat for 5 minutes in water bath. Add 5 drops of 0.5M AgNO3 and stir. Discard residue formed. To the clear centrifugate, add 15 drops of 2.5M NaAc solution. A reddish brown precipitate confirms the presence of arsenic ion.
5b. Place 1 drop of the centrifugate from Step 5 on the surface of a clean silver coin. Place a very small piece of tin on the coin so that the solution is in contact with the tin and coin. A black deposit on the silver coin confirms the presence of antimony ion.
Note :
- Aqua Regia is 1:3 mixture of conc. nitric acid and conc. hydrochloric acid solutions – i.e., 1HNO3 : 3HCl.
- Potassium stannite solution is prepared by adding 3M KOH drop by drop to two drops of stannous chloride solution until the gelatinous precipitate of stannous hydroxide, which first forms, barely dissolves.
Experiment 2 – Group II Cation Analysis
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Answers to Questions
- What metals are members of Group II? ______________________________________________________________________________________________________________________________________________________________________________________________________________________________
- For each of the following pairs, identify a reagent that will dissolve one substance but not the other:
HgS and As2S3 _______________________________________________
HgS and Bi2S3 _______________________________________________
As2S3 and Sb2S3 _______________________________________________
PbSO4 and CuSO4 _______________________________________________
- Explain the theoretical aspect of the separation of Group A from Group B.
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