CHEMISTRY II - LABORATORY EXERCISE MANUAL

LABORATORY EXERCISE 8: Qualitative Analysis group III Cations:
Mn2+, Ni2+, Fe3+(Fe2+), Al3+, Zn2+


  In this last section of our Qualitative Analysis experiment, we will again be dealing with only a fraction of the total number of ions that comprise Group III.

If we were going to proceed to Groups IV and V, we would have to start our procedure by again precipitating the ions we are interested in from the solution. We would even follow a similar procedure, using thioacetamide to provide sulfide ions for precipitation. The only difference is that we would precipitate the ions in a weakly basic solution, and thereby facilitate the precipitation of aluminum as a hydroxide.

However, since this is the final step in our Qualitative analysis, we can proceed more directly by separating hydroxide precipitates from amphoteric hydroxides. With the addition of NaOH, a strong base, we can precipitate Mn2+, Ni2+, and Fe3+ as gelatinous hydroxides. Zn2+ and Al3+ are amphoteric, meaning that hydroxides of these metals behave as acids in a basic solution, and thus immediately redissolve into aluminate, Al(OH)4-, and zincate, Zn(OH)42- ions. This fact ensures complete and convenient separation to aid our analysis.

A. Separation of Hydroxide Precipitates from Amphoteric Hydroxides:

1. To the solution from Qual II, part A.2, add 15 drops 6M NaOH. Agitate thoroughly, centrifuge and decant the supernatant into a clean, centrifugable test tube. Test for complete precipitation by repeating the NaOH addition. Centrifuge if necessary, and add the supernatant to the first batch. Save this supernatant for part E.

2. Dissolve the precipitate from A.1 by adding 1mL concentrated HNO3 and heating in a hot water bath for 5 minutes.



The Manganese Ion: As with the tin ion, we observe the presence of manganese indirectly as it transforms from a free cation in solution to a complex soluble anion, the permanganate anion. This transition is accomplished by the addition of a strong oxidizing agent, in this case sodium bismuthate, NaBiO3.

14H+ (aq) + 2Mn2+ (aq) + 5BiO3- (aq) 2MnO4- (aq) + 5Bi3+ (aq) + 7H20

B. Test for Manganese Ion:

1. Decant 0. 5mL of the solution from A.2 into a clean test tube. Save the remainder for the next test in part C. Add a spatula tip full of NaBiO3 to the solution, stir, and centrifuge. A deep purple (brown) supernatant confirms the presence of manganese.



The Iron Ion: We are now left with dissolved hydroxides of iron and nickel in our test solution. To separate these, we will add an excess of concentrated ammonia, which serve to precipitate the iron and complex the nickel into a soluble hexaammine cation.

Fe3+ (aq) + 3NH3 + 3H20 --> Fe(OH)3 (s) + 3NH4+ (aq)

Ni2+ (aq) + 6NH3 (aq) <---> Ni(NH3)6 2+ (aq)

This tendency of our cations to form complex ions in certain kinds of solutions will once again be put to use in detecting iron, and overall it is very important to our determination of definitive results.

As before, we can easily dissolve the iron hydroxide precipitate with nitric acid, and then add thiocyanate ions to the solution and observe the formation of the blood red iron hexathiocyanate complex ion to confirm iron's presence.

Fe3+ (aq) + 6SCN- (aq) --> Fe(SCN)63- (aq)

C. Test for Iron Ion:

1. To the Solution from part B.1 add 10 drops of 2M NH4Cl. Take your test tube to the fume hood, and add concentrated NH3 drop by drop until the solution is basic to red litmus. At that point add one more drop of NH3 to ensure complete complexing of the Nickel ions. Centrifuge, decant the supernatant to a clean test tube, and save it for the next test in part D.

2. Dissolve the precipitate from C.1 with 1mL 6M HCl. Then add 5 drops 0. 1M NH4SCN. A blood red solution confirms the presence of iron.



The Nickel Ion: Now that we have isolated nickel ions in our test solution, it is a simple matter to confirm their presence. We will be adding dimethylglyoxime, abbreviated H2DMG to the solution of hexaammine nickel ions. Dimethylglyoxime is an organic complexing agent specific to the test for nickel.

Ni(NH3)62+ (aq) + 2H2DMG (aq) --> Ni(HDMG)2 (s) + 2NH4+ (aq) + 4NH3 (aq)

D. Test for Nickel Ion:

1. To the supernatant from part C.1 add 3 drops dimethylglyoxime solution. A brick red precipitate confirms the presence of nickel.



The Aluminum Ion: We can now return to the solution containing the aluminate and zincate ions. To separate these ions, we will first acidify the solution, which dissolves the complex ions. Then we can add ammonia, as we have done before, to precipitate the aluminum as aluminum hydroxide. Now, because the Al(OH)3 is a gelatinous precipitate, it is difficult to detect. To make this task easier, we will dissolve the precipitate with nitric acid, add aluminon, which is a red dye, and reprecipitate the Al(OH)3 with ammonia. The addition of the aluminon will give the precipitate a distinct pink color.

Al3+ (aq) + 3NH3 (aq) + 3H20 + aluminon --> [Al(OH)3*aluminon] (s) + 3NH4+ (aq)

E. Test for Aluminum Ion:

1. To the supernatant from A.1 add drops of 6M HNO3 until the solution is acid to blue litmus. At that point add 6M NH3 drop by drop until the solution is basic to red litmus. Once this happens, add 5 more drops of NH3 and heat in a hot water bath for 5 minutes. Centrifuge and decant the supernatant into a clean test tube. Save this supernatant for the next test in part F.

2. Wash the precipitate from E.1 with 1 mL hot deionized water, centrifuge, and discard the washings. Repeat the wash once more. Now, add 6M HNO3 drop by drop until the precipitate just dissolves. Agitate after each drop. Once the precipitate has dissolved, add 2 drops aluminon and stir. Then add 6M NH3 drop by drop until the solution is basic to red litmus and a precipitate reforms. Centrifuge. A pink or red precipitate in a colorless solution confirms the presence of aluminum.



The Zinc Ion: Now that we have isolated the zinc as a complex ion in solution, we can acidify the solution to dissolve the zinc tetraammine complex ion, and then add potassium hexacyanoferrate directly to precipitate the zinc.

3Zn(NH3)42+ (aq) + 4H+ --> 3Zn2+ + 4NH4+ (aq)
3Zn2+ + 2K4[Fe(CN)6] (aq)--> K2Zn3 [Fe(CN)6]2 (s) + 6K+

F. Test for Zinc Ion:

1. To the supernatant from E.1, add 6M HCl drop by drop until the solution is acid to blue litmus. At that point, add 3 drops O.2M K4[Fe(CN)6] and agitate the solution. Centrifuge if necessary. A very light green precipitate confirms the presence of zinc.



Congratulations! You have completed an introductory course in Qualitative Analysis! Check the results of your unknown analysis with your instructor to see how well you identified the composition of your unknown solution.


Equipment & Materials Needed:
Equipment & Chemicals from previous lab
NaBiO3powder
2M NH4Cl
concentrated NH3
0.1M NH4SCN
dimethylglyoxime solution (H2DMG)
aluminon
O.2M K4[Fe(CN)6

All contents copyrighted (c) 1998
Peter Jeschofnig, Ph.D., Professor of Science, Colorado Mountain College
All Rights reserved


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This page was created by Peter Jeschofnig and was last updated: 3/1/2001