2615 Qualitative Analysis of Selected Cations

Qualitative Analysis of Selected Cations

1.0 INTRODUCTION

1.1 Objectives

After completing this experiment, the student will be able to:

• follow a classic analytical scheme to separate ions in a known mixture of cations.
• apply this scheme to identify the ions in an unknown mixture of cations.

1.2 Background

One common task in analytical chemistry is the identification of the various ions present in a particular sample. For example, if you are an environmental chemist your job may be to recover soil or water samples in order to determine the presence of toxic ions such as Pb2+ or Ba2+. A common experimental method used to identify ions in a mixture is called qualitative analysis.

In qualitative analysis, the ions in a mixture are separated by selective precipitation. Selective precipitation involves the addition of a carefully selected reagent to an aqueous mixture of ions, resulting in the precipitation (when a substance is deposited in solid form from a solution) of one or more of the ions, while leaving the rest in solution. Cations are separated first into five groups, then within each group cations are separated selectively. Once each ion is isolated, its identity can be confirmed by using a chemical reaction specific to that ion.

Cations are typically divided into groups, where each group shares a common reagent that can be used for selective precipitation.

• Group I Cations (Ag+, Hg22+ and Pb2+) form insoluble chlorides
• Group II Cations (As3+, Bi3+, Cd2+, Cu2+, Hg2+, Sb3+ and Sn2+) for insoluble sulfides in acidic conditions
• Group III Cations (Al3+, Co2+, Cr3+, Fe2+, Fe3+, Mn2+, Ni2+ and Zn2+) form insoluble sulfides or hydroxides in basic conditions
• Group IV Cations (Ca2+, Sr2+ and Ba2+) form insoluble carbonates or phosphates
• Group V Cations (Mg2+, Na+, K+ and NH4+) remain soluble in separation of groups I – IV ions

To illustrate quantitative analysis notice Ag+, Hg22+ and Pb2+ are called the Group I cations since they are the first group separated from the larger mixture. Since these ions all form insoluble chlorides, their separation from all other ions may be accomplished by the addition of 6 M HCl (aq) resulting in the precipitation of AgCl (s), PbCl2 (s) and Hg2Cl2 (s):

Ag+(aq) + Cl−(aq) ⟶ AgCl(s)

Hg2+2(aq) + 2 Cl−(aq) ⟶ Hg2Cl2(s)

Pb2+(aq) + 2 Cl−(aq) ⟶ PbCl2(s)

The sample must then be centrifuged (spun rapidly), which separates the solid precipitates from the ions still in solution. The solids settle to the bottom, and the solution containing the remaining ions (Groups 2 – 5) remains on top of the solid. This solution is called the supernatant and must be carefully decanted (poured off without disturbing the solid) and saved for further study. The Group I cations contained within the collected precipitate must then be separated from each other in order for the presence of each ion to be confirmed.

Lead (II) chloride can be separated from the other two chlorides based on its increased solubility at higher temperatures. This means that lead (II) chloride will dissolve in hot water, leaving the mercury (I) chloride and the silver chloride in solid form:

PbCl2(s) ⟶ Pb2+(aq) + 2 Cl− (aq)

The presence of Pb2+ in the aqueous solution can then be confirmed by the formation of a yellow precipitate ofPbCrO4 upon the addition of aqueous K2CrO4:

Pb2+(aq) + CrO42−(aq) ⟶ PbCrO4(s)

Next, Hg22+ and Ag+ cations can be separated by adding 6 M NH3(aq) to the solid mixture of the two chlorides. Silver chloride will dissolve since it forms a soluble complex ion with ammonia:

AgCl(s) + 2NH3(aq) ⟶ Ag(NH3)2+(aq) + Cl−(aq)

However, mercury (I) chloride reacts with the ammonia yielding what appears to be a gray solid which is actually a mixture of black Hg (l) and white HgNH2Cl (s). The presence of this gray solid is confirmation of the presence of Hg22+.

Hg2Cl2(s) +2NH3(aq) ⟶ Hg(l) + HgNH2Cl(s) + NH4+(aq) + Cl−(aq)

The presence of Ag+ can be confirmed by the appearance of a white precipitate upon adding 6 M HNO3 (aq) to the solution. The nitric acid reacts with the ammonia and thus destroys the complex ion containing the silver cation. Once in solution again the silver cation precipitates with the chloride as indicated by the following reactions:

Ag(NH3)2+(aq) + 2H+(aq) ⟶ Ag+(aq) + 2NH4+(aq)

Ag+(aq) + Cl−(aq) ⟶ AgCl(s)

The analysis scheme described represented in abbreviated form using a flow chart:

In this experiment you are tasked with separation and identification of group 1 cations (Ag+ and Pb2+), group 3 cations (Ni2+ and Fe3+), and group 4 cation (Ba2+). After the selective precipitation and identification of Ag+ and Pb2+, the Ba2+ ions can be precipitated with addition of 3 M H2SO4.

Ba2+(aq) + SO42−(aq) ⟶ BaSO4(s)

Finally, 15 M NH3 can be used to separate the Ni2+ from the Fe3+ ions. Once the solution has become basic, the excess ammonia will react with Ni2+ to form an aqueous complex ion while the Fe3+ will precipitate out as red Fe(OH)3 since Fe3+ does not typically form complex ions with NH3:

Ni2+(aq) + 6NH3(aq) ⟶ Ni(NH3)62+(aq)

Fe3+(aq) + 3NH3(aq)+3H2O(l) ⟶ Fe(OH)3(s) + 3NH4+(aq)

The presence of Ni2+ in the aqueous solution is confirmed by adding dimethylglyoxime (C4H8N2O2) resulting in the formation of a rose red precipitate:

Ni2+(aq) + 2C4H8N2O2(aq) ⟶ Ni(C4H7N2O2)2(s) + 2H+(aq)

In order to confirm the presence of Fe3+, the red Fe(OH)3 precipitate is dissolved in HCl, and then NH4SCN is added to the solution. A positive result is the formation of a dark red solution indicating the presence of FeSCN2+(aq):

Fe3+(aq) + SCN−(aq) ⟶ FeSCN2+(aq)

References and further reading

Technique J: Centrifuge Use

2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

3.0 CHEMICALS AND SolutionS

Chemical	Concentration	Approximate Amount	Notes
AgNO3	0.1 M	10 mL
Ba(NO3)2	0.1 M	10 mL
Fe(NO3)3	0.1 M	10 mL
Ni(NO3)2	0.1 M	10 mL
Pb(NO3)2	0.1 M	10 mL
HCl	6 M	drops
K2CrO4	2 M	drops
HC2H3O2	6 M	drops
H2SO4	3 M	drops
NaOH	6 M	drops
NH3	15 M	drops	Concentrated ammonia is stored and used in the hood. Labeled NH4OH.
NH4SCN	1 M	drops
Bromocresol blue		drops
Dimethylglyoxime	1%	drops

4.0 GLASSWARE AND APPARATUS

Beakers, varying sizes (5+)	Large beaker for hot-water bath
Medium sized test tube (5+)	Large beaker for waste
Centrifuge tubes (4)	Benchtop centrifuge
Glass stir rod	Plastic pipets (5+)
Hot plate	Test tube clamp

5.0 PROCEDURE

To get correct results in this experiment, good organizational skills and techniques are essential. Be sure to label all tubes and solutions because these accumulate rapidly, and it is very easy to get tubes and/or solutions mixed up. It is also suggested that students keep a “waste” beaker near their work area in which to discard all solutions locally, and then pour their combined waste solutions into the large waste container for the class at the end of the lab.

Finally, be sure to clean out and thoroughly rinse all glassware (including stirring rods!) with deionized water between uses. Cross-contamination is one of the most common causes for false observations leading to incorrect conclusions.

5.1 CHEMICAL TESTS OF INDIVIDUAL CATIONS

1. Obtain about 10 mL of the following solutions in separate clean, dry, labeled beakers: 0.1 M AgNO3, 0.1 M Ba(NO3)2, 0.1 M Fe(NO3)3, , 0.1 M Ni(NO3)2 and 0.1 M Pb(NO3)2.

5.1.1 Precipitation with Cl–

2. Transfer 10 – 15 drops of each solution to separate clean, dry, labeled test tubes. Record observations of the solutions on data table 1.

3. To each test tube add 5 drops of 6M HCl. Mixing each solution by flicking the test tube with finger. Record observations in data table 1.

4. If a precipitate forms, transfer the mixture to a centrifuge tube then centrifuge the mixture. Make sure that you balance the centrifuge before you start. Balancing is done by placing centrifuge tubes of approximately equal weight opposite each other. If no precipitate forms, the mixture can be discarded in your waste container.

5. Pour the supernatant solution into your waste container and save the precipitate for further analysis.

5.1.2 Heat and precipitation with CrO42–

6. Wash the precipitate from Step 5 to remove contaminates that may be trapped within the solid. This can be accomplished by adding 1.0 mL of deionized water, mixing, and then centrifuging. The wash supernatant can be discarded in your waste container. Wash the precipitate twice saving the precipitate.

7. Prepare a hot water bath by adding tap water to a 250 mL beaker. Add water until the beaker is about one half full. Heat the water on a hot plate until it is boiling.

8. Add 1.5 mL of deionized water to the centrifuge tubes containing the clean precipitate. Place it in the hot water bath. Periodically stir the mixture in the test tube for 3 – 4 minutes. Record observations in data table 1.

9. Remove the centrifuge tube from the hot water bath and immediately centrifuge the hot mixture. Carefully decant the hot supernatant solution into another fresh labeled test tube.

10. Add 1 drop of 2 M K2CrO4 to the hot liquid. Record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.3 Precipitation with CrO42–

11. To 10 – 15 drops of fresh solution from step 1 in separate clean, dry, labeled test tubes, add 1 drop of 6 M HC2H3O2 to the supernatant solution. Next add 3-4 drops of 2 M K2CrO4. Record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.4 Precipitation with SO42–

12. To 10 – 15 drops of fresh solution from step 1 in separate clean, dry, labeled test tubes, add 5 drops of 3 M H2SO4. Record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.5 Precipitation with OH–

13. To 10 – 15 drops of fresh solution from step 1 in separate clean, dry, labeled test tubes, add 10 drops 6M NaOH. Record observations in data table 1.

14. If a precipitate forms, transfer the mixture to a centrifuge tube then centrifuge the mixture. The supernatant can be discarded in your waste container. Save the precipitate for further analysis. If no precipitate forms, the mixture can be discarded in your waste container.

15. Wash the precipitate once with deionized water.

5.1.6 Reaction of hydroxide precipitate with NH3 and dimethylglyoxime

16. To the precipitate from Step 15 add 10 drops of 15M NH3 (labeled NH4OH) and mix vigorously for one minute. Record observations in data table 1.

17. Add eight drops of dimethylglyoxime (dmg) to the solutions. Record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.7 Reaction of hydroxide precipitate with acid and SCN–

18. For solutions that formed a hydroxide precipitate in Step 13, prepare fresh test tubes: 10 – 15 drops fresh solution from step 1 in separate clean, dry, labeled test tubes.

19. Obtain and wash hydroxide precipitates again by adding 10 drops 6M NaOH to the test tubes, transferring to centrifuge tubes, centrifuging, and washing the precipitate once with deionized water (repeating Steps 13 – 15).

20. To the precipitate from Step 18, add 5 drops of 6M HCl and mix vigorously. Record observations in data table 1.

21. Add two drops of 1M NH4SCN and record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.8 Reaction of fresh solution with NH3

22. To 10 – 15 drops of fresh solution in separate clean, dry, labeled test tubes, add 10 drops 15M NH3. Record observations in data table 1. Discard solutions and precipitates from your tests.

5.1.9 Reaction of fresh solution with SCN–

23. To 10 – 15 drops of fresh solution in separate clean, dry, labeled test tubes, add two drops of 1M NH4SCN. Record observations in data table 1. Discard solutions and precipitates from your tests.

All supernatant and precipitates from tests can be discarded in your waste container. Save original cations solutions and test reagents for further analysis. Be sure to clean out and thoroughly rinse all glassware (including stirring rods!) with deionized water before proceeding to the next section.

5.2 ANALYSIS OF A KNOWN MIXTURE OF CATIONS

As the section is preformed add observations to data table 2. Be mindful to include observations of the solution, precipitate and supernatant. Include interpretations of the observations. For example, what ions are contributing to the observations observed.

1. Prepare a mixture of cations by adding 1.0 mL of each of the following aqueous solutions to a small test tube: 0.1 M AgNO3, 0.1 M Ba(NO3)2, 0.1 M Fe(NO3)3, , 0.1 M Ni(NO3)2 and 0.1 M Pb(NO3)2. Note that 1.0 mL is generally between 10-15 drops. Swirl to thoroughly mix.

5.2.1 Separation of Ag+ and Pb2+ from Ni2+, Fe3+ and Ba2+

2. Transfer 1.0 mL of the mixture into a second small test tube and add 5 drops of 6 M HCl to this test tube.

3. Transfer the mixture to a centrifuge tube then centrifuge the mixture. Make sure that you balance the centrifuge before you start. Balancing is done by placing centrifuge tubes of approximately equal weight opposite each other.

4. Pour the supernatant solution into a fresh labeled test tube. Save the precipitate; it contains the Ag+ and Pb2+ ions as AgCl(s) and PbCl2(s).

5. To the supernatant solution, add one more drop of 6M HCl to ensure the precipitation is complete. If more precipitate is observed, centrifuge the mixture again. Save the supernatant solution; it contains the Ni2+, Fe3+ and Ba2+ cations.

5.2.2 Separation and identification of Pb2+

6. Wash the precipitate from Step 4 to remove contaminates that may be trapped within the solid. This can be accomplished by adding 1.0 mL of deionized water, mixing, and then centrifuging. The wash supernatant can be discarded in your waste container. Wash the precipitate twice saving the precipitate.

7. Prepare a hot water bath by adding tap water to a 250 mL beaker. Add water until the beaker is about one half full. Heat the water on a hot plate until it is boiling.
8. Add 1.5 mL of deionized water to the centrifuge tubes containing the clean precipitate. Place it in the hot water bath. Periodically stir the mixture in the test tube for 3 – 4 minutes.

9. Remove the centrifuge tube from the hot water bath and immediately centrifuge the hot mixture. Carefully decant the hot supernatant solution into another fresh labeled test tube; it contains the Pb2+ cations. Save the precipitate for further analysis; it contains Ag+ ions as AgCl(s).

10. In order to confirm the presence of Pb2+, add 1 drop of 6 M HC2H3O2 to the supernatant solution. Next add 3 – 4 drops of 2 M K2CrO4. The formation of a bright yellow precipitate is confirmation of the presence of the Pb2+ ion.

5.2.3 Identification of Ag+

11. Wash the precipitate from Step 9 containing the Ag+ cations once with deionized water.

12. Add 10 – 15 drops of 15 M NH3 (labeled NH4OH) to dissolve the precipitate. It now contains the Ag(NH3)2+ ion.

13. To the Ag(NH3)2+ solution, slowly add 6M HCl until the solution is acidic. The acidity can be tested by dipping a stirring rod into the solution and then touching it (with a drop of solution) to a piece of blue litmus paper resting on a clean, dry watch glass. If the solution is acidic, it will turn blue litmus paper red. Formation of a white precipitate of AgCl in the acidic solution is confirmation of the presence of Ag+.

5.2.4 Separation and identification of Ba2+ from Ni2+ and Fe3+

14. To the supernatant solution containing the Ni2+, Fe3+ and Ba2+ cations from Step 4, add 5 drops of 3 M H2SO4. Formation of a white precipitate of BaSO4 in the solution is confirmation of the presence of Ba2+ as all of the Ag+ and Pb2+ ions have been removed from the supernatant.

15. Transfer the mixture to a centrifuge tube then centrifuge the mixture.

16. Pour the supernatant solution into a fresh labeled test tube and add one more drop of 3 M H2SO4 to ensure the precipitation is complete. If more precipitate is observed, centrifuge the mixture again, decanting the supernatant to a fresh labeled test tube. Save the supernatant for further analysis; it contains the Ni2+and Fe3+ cations.

5.2.5 Separation Ni2+ from Fe3+

17. To the supernatant solution containing the Ni2+and Fe3+ from Step 16, slowly add 15 M NH3 to the solution dropwise until it is basic (test with red litmus paper). You should see a red-brown precipitate of Fe(OH)3 appear when the solution turns basic.

18. Add an additional 10 drops of 15 M NH3 and flick to mix well. At this point the nickel should be dissolved in the solution as [Ni(NH3)6]2+.

19. Centrifuge the mixture and decant the supernatant solution into another fresh labeled test tube. Save both the precipitate and supernatant.

5.2.6 Identification of Fe3+

20. Wash the precipitate from Step 19 once with deionized water.

21. To the Fe(OH)3 precipitate add twelve drops of 6 M HCl. Next, add 4 mL of deionized water and stir well until the precipitate is completely dissolved. Finally, add two drops of 1 M NH4SCN. The formation of a dark red solution confirms the presence of Fe3+ in the original solution.

5.2.7 Identification of Ni2+

22. To the [Ni(NH3)6]2+ solution from Step 19, add eight drops of dimethylglyoxime to the solutions. The formation of a red precipitate confirms the presence of Ni2+ in the original solution.

5.3 ANALYSIS AND IDENTIFICATION OF AN UNKNOWN SAMPLE

1. Obtain a test tube which contains a mixture of cations. Record the identification of the sample on your data sheet.

2. Transfer 1.0 mL of the unknown mixture into a test tube and repeat the procedure used in section 5.2.

3. On your data sheet, record observations and construct a flow chart similar to the one shown in the Background. Indicate on the flow chart whether the test for each ion is positive or negative. Then, in space provided, indicate which ions are present in your unknown sample.

6.0 DATA RECORDING SHEET

Last Name

First Name

Partner Name(s)

Date

Table 1. Chemical Tests

Section Test and Reagents	Ag+	Ba2+	Fe3+	Ni2+	Pb2+
Solution appearance
5.1.1 Cl–
5.1.2 heat
CrO42-
5.1.3 CrO42-
5.1.4 SO42-
5.1.5 OH-
5.1.6 NH3
dmg
5.1.7 H+
SCN-
5.1.8 NH3
5.1.9 SCN-

Table 2. Separation of a known mixture

	Observations & Interpretations (Why?)
5.2.1 Separation of Ag+ & Pb2+
5.2.2 Separation & identification of Pb2+
5.2.3 Identification of Ag+
5.2.4 Separation & identification of Ba2+
5.2.5 Separation Ni2+
5.2.6 Identification of Fe3+
5.2.7 Identification of Ni2+

Analysis and Identification of Unknown

In the space provided below construct a flow cart for the analysis of your unknown. Indicate on the flow chart whether the test for each ion is positive or negative:

Unknown identifications: __________________

Ions present in your unknown: ____________________________________