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Summative Lab Activity:  Qualitative Analysis Introduction: In general, the system of qualitative analysis you will use involves separating groups of positive ions on the basis of the different solubilities of their compounds

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Summative Lab Activity:  Qualitative Analysis

Introduction:

In general, the system of qualitative analysis you will use involves separating groups of positive ions on the basis of the different solubilities of their compounds.  The solutions containing groups of ions are then further separated into solutions containing one kind of ion whose presence is confirmed by a specific identification test.  The overall procedure is summarized in the following flow chart: 

 

 

 

 

You will start with a known solution containing six cations. 

These are :     Ag 1+, Fe3+, Co2+, Ni2+,  Al3+, and NH41+ ions.

 

            You first add Cl- ions and precipitate an insoluble chloride.  This will be separated from the solution and identified on the basis of specific properties.  The remaining solution will then be treated with an NH3 solution.  The OH1- ions precipitate a group of slightly soluble hydroxides, while the ions which form complexes with NH3 remain in solution.  The slightly soluble hydroxides are Fe(OH)3  and Al(OH)3.  The soluble complex ions are Ni(NH3)63+ and Co(NH3)62+. 

            Again, specific tests are applied to help identify each ion in the individual groups.  You should become acquainted with specific identification or confirmatory tests by observing the reactions of known test solutions of each ion with the reagent(s) which are used to identify it.

            There are a number of advantages to using small quantities of reagents and semimicro methods as described in this procedure.  The quantities of reagents and apparatus assume that semimicro equipment such as 13 x 100 mm test tubes and centrifuges are available.  All equipment should be scrupulously clean and good laboratory technique employed.  Record all observations such as the nature and     colour of precipitates, and the colours of solutions as shown in Table 1.

 

Table I -  Analysis of a Solution Containing Known Cations

 

Appearance of solution                                                                 

 

Step	Procedure	Observations	Conclusions
1
2
3
4

etc.

 

Purpose:

To identify the ions in a known and an unknown solution.

To observe the application of equilibrium and other reaction principles.

To become acquainted with the chemistry of specific ions and other substances.

 

Materials:

6 test tubes (13 x 100 mm or smaller to fit into centrifuge) and test-tube rack (BBFJ), test tube tongs, pipet, medicine dropper, test tube brush, centrifuge, beaker, hot plate,

6 mol/L HCl,   6 mol/L NH3 (aq), 6 mol/L HNO3, 6 mol/l NaOH, 6 mol/L NH4Cl,                 0.1 mol/L KSCN, 6 mol/L KNO2, aluminon test reagent, dimethylglyoxime test reagent, litmus paper, test solution containing Ag1+, Al3+, Fe3+,  Co2+, Ni2+, NH41+, unknowns containing four of the above ions.

Procedure:

1. Place approximately 1 mL of the known test solution in a clean 13 x 100 mm test tube.  Add six to eight drops of 6 mol/L HCl.  Precipitate is AgCl.  Place test tube in a beaker of cold water for a minute.  Stir gently, centrifuge and decant the supernatant liquid into another test tube.  Set the solution aside for Procedure 2. 

Precipitate: AgCl(white).  Confirms presence of Ag+ ions. 

 

2. Neutralize half of the solution from Procedure 1 (keep one half not neutralized for step 9) by adding 6 mol/L NH3 solution until the solution is basic; then add  1 mL in excess.  Add 1 mL of 6 mol/L NH4Cl. 

            Colored precipitate: Fe(OH)3, Al(OH)3.  Centrifuge and separate the solution     into another test tube.  If the precipitate is light and flocculent, use a medicine   dropper to withdraw the solution carefully. 

            Set the solution aside for Procedure 7.  Treat the precipitate by Procedure 3.

 

3. Dissolve the precipitate from Procedure 2 by adding 6 mol/L HCl dropwise.  Make basic to litmus with 6 mol/L NaOH.  Then add 1-2 mL in excess.  The pH rises to approximately 1  Precipitate: Fe(OH)3 (rust).  Centrifuge and separate the solution into another test tube.  Set the solution aside for Procedure 5.  Treat the precipitate by Procedure 4.

 

4. Dissolve the precipitate from Procedure 3 by adding 6 mol/L HCl dropwise.  Add three drops of 0.1 mol/L KSCN solution. 

Deep red color solution: FeSCN2+.  Confirms presence of Fe3+ ions.

 

5. Acidify the solution from Procedure 3 by adding 6 mol/L HCl dropwise.  Add five drops of 6 mol/L NH4Cl and make basic by adding 6 mol/L NH3 solution.  White or grayish flocculent precipitate:  Al(OH)3.  Centrifuge and separate the solution.

Discard the solution and treat the precipitate by Procedure 6.

 

6. Dissolve the precipitate from Procedure 5 by adding 6 mol/L HCl dropwise.  Add 1 mL of the aluminon test reagent.  Add 6 mol/L NH3 solution until the solution is just basic.  Warm the solution in a water bath.  Red precipitate known as a "lake" is Al(OH)3 which has adsorbed the aluminon dye. 

Red “lake”: Al(OH)3.   Confirms presence of Al3+ ions. 

 

Show teacher two precipitates and the soluble ion complex from step 4, then discard.

 

7. Divide the solution from Procedure 2 equally into two separate test tubes.  To the first portion add 10 to 15 drops of dimethylglyoxime (DMG) and warm in a water bath. 

Precipitate:   NiC8H14 N4O4 (red).  Confirms presence of Ni2+ ions.

8. Acidify the second portion by adding 6 mol/L HNO3 dropwise until there is       1 mL in excess.  Add 1.5 mL of  6 mol/L KNO2 solution.  Gently warm mixture in water bath (not boiling water!), let stand for 15 min and then centrifuge.  Precipitate:  K3Co(NO2)6 (yellowish).  Confirms presence of Co2+ ions.

 

9. Test for NH4+ ions by taking the solution saved from step 2 and adding 2 mL of

            6 mol/L NaOH.  Heat gently and test fumes for NH3 odour and with moistened     red litmus paper which turns blue if NH3 is evolved. 

            Gas: NH3.   Confirms presence of NH4+ ions. 

 

            Show teacher two precipitates and litmus results from step 9, then discard.

 

10. After completing the analysis of a known solution, obtain an unknown from the instructor.  The solution will contain four cations.  Record all observations and conclusions in Table II.  Report the results of your analysis as directed by your instructor. 

 

 

Table II - Analysis of a Solution Containing Unknown Cations

 

 

Appearance of solution                                                     

 

Step	Procedure	Observations	Conclusions
1
2
3
4

etc.

 

 

Summary:

Ions present                                                                         

 

Ions absent                                                                           

 

 

 

 

 

 

 

 

Discussion:

            In this scheme of analysis, chloride ions are used to precipitate AgCl.  The solution is kept as cold as possible to ensure that most of the Ag1+ions precipitate as a silver halide in the first reaction.  AgCl quickly darkens on exposure to light by decomposing into elemental chlorine and metallic silver.  This reaction is used in photography and film.  Other silver halides, such as silver bromide (slightly yellowish white) and silver iodide (pale yellow) are also significantly more photosensitive than is AgCl.

            The ability of Ni2+, and Co2+ ions to form soluble complex ions with NH3 is used as a basis for separating them from Al3+ and Fe3+ which form insoluble hydroxides in NH3 solution.

 

            Ni(H2O)62+ + 6NH3    Ni(NH3)62+ + 6H2O

 

            Co(H20)62+ + 6NH3    Co(NH3)62+ + 6H2O

 

            Fe(H2O)63+ + 3NH3     Fe(H2O)3(OH)3(s) + 3NH41+

 

            Al(H2O)63+ + 3NH3   Al(H2O)3(OH)3(s) + 3NH41+

 

A low concentration of OH1- ions is essential to precipitate Al(OH)3.  A higher concentration such as that furnished by NaOH, a strong base, would change the hydroxide precipitate into a complex ion.   The equation is

 

            Al(H2O)3(OH)3(s) + OH1-  Al(H2O)2(OH)41-  +  H2O

 

The excess OH1- removes a proton from one of the H2O molecules and forms the complex ion.  It is common, however, to write the species, Al(OH)41-, without indicating the H2O molecules bonded to Al3+.  The ability of Al(OH)3 to form soluble ions in strongly basic solutions is used as a basis for separating it from Fe(OH)3.  The complex  Al(OH)41-  is destroyed by treatment with HCl.  The equation is

 

            Al(H2O)2(OH)41- + 4H3O1+  Al(H2O)63+ + 4H2O

 

Al(OH)3 is then reprecipitated by adding NH3 solution.  The aluminum test reagent "aluminon" is absorbed on the precipitate and gives the characteristic red lake.

 

           

            The tests for Co2+ and Ni2+ are specific enough so that it is not necessary to separate the ions before testing.  In the confirmatory test for Co2+ ions, nitrite ions (NO21-) oxidize Co2+ to Co3+ ions.  The Co3+ ions then react with excess NO21- and K1+ ions forming insoluble K3Co(NO2)6. The overall equation is

 

   Co2+ + 7NO21- + 3K1+ + 2HNO3    K3Co(NO2)6(s) + NO(g) + 2NO31- + H2O

 

The confirmatory test for Ni2+ consists of adding dimethylglyoxime (C4H8N2O2).  Nickel (II) ions and dimethylglyoxime react in a one-to-one ratio and yield a bright red, insoluble compound.

 

            Ni2+ + 2C4H8N2O2    Ni(C4H7N2O2)2  + 2H1+

 

The NaOH solution that is used in step 9, ionizes almost completely to form hydroxide ions.

 

            NaOH    ®  Na1++ OH1- 

 

The test for NHreleases ammonia gas by the addition of a high concentration of OH-  (the common ion effect)

            NH41++ OH1-    NH3  + H2O

The ammonia reacts with moist red litmus to affect its colour.