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EXPERIMENT C6: QUALITATIVE ANALYSIS OF CATIONS
Learning Outcomes
Upon completion of this lab, the student will be able to:
- Analyze a given sample of an ionic compound and identify which of the following cations are present: silver, lead (II), bismuth (III), iron (III), manganese (II), aluminum, chromium (III), barium, strontium, calcium, nickel (II), copper (II), magnesium, zinc
Introduction
The cations being tested in the qualitative analysis scheme are organized into groups labeled A through D (Table 1) Group Cations A Ag+, Pb2+ B Bi3+, Fe3+, Mn2+, Al3+, Cr3+ C Ba2+, Sr2+, Ca2+ D Ni2+, Cu2+, Mg2+, Zn2+ TABLE 1 This grouping of ions was arrived at based on the differences in the solubility of these ions. GROUP A: Ag+, Pb2+ The KSP of the chloride salts of the two ions in this group is given in Table 2. Salt KSP AgCl 1.7 × 10 -^10 PbCl 2 1.6 × 10 -^5 TABLE 2 The chloride salts of all the other cations that will be analyzed in this experiment are highly soluble. Therefore, addition of aqueous HCl will result in a precipitate containing chlorides of the Group A cations. Ag+(aq) + Cl−(aq)! AgCl(s) Pb2+(aq) + 2Cl−(aq)! PbCl2(s)
The chlorides of all other cations will remain in solution. When this mixture is centrifuged: the precipitate will contain the group A cations and the supernatant will contain the cations from the other groups. Of the two chloride precipitates, PbCl 2 is soluble in hot water and AgCl is soluble in aqueous ammonia. Test for Pb2+ When the chloride precipitates of the Group A cations are mixed with hot water, PbCl 2 will dissolve. This is separated from AgCl by centrifuging the mixture. The PbCl 2 in the supernatant can be confirmed by reacting the supernatant with a solution of KI, which results in a yellow precipitate of PbI 2. PbCl 2 (s) + H 2 O(l) + heat! Pb2+(aq) + 2Cl−(aq) Pb2+(aq) + 2 KI(aq)! PbI2(s) + 2K+(aq) Test for Ag+ The AgCl precipitate dissolves in aqueous ammonia due to the formation of a complex ion. AgCl(s) + 2NH3(aq)! Ag(NH 3 )2(aq) + Cl−(aq) Addition of HNO 3 results in the decomposition of the silver ammonium complex. The free silver ions combine with the chloride ions from the above reaction to form the AgCl precipitate again. Groups A, B, C, D Add 6M HCl Precipitate: Group A- AgCl, PbCl 2 Supernatant: Groups B, C, D
NOTE: The Mn2+^ is oxidized to Mn4+by the H 2 O 2 The B1 cations are all soluble in HCl. The precipitate containing the B1 cations is dissolved in hot HCl solution. This solution is then tested for each of the three B cations. Test for Mn2+ In an acidic environment the Mn4+^ is reduced to Mn2+^ using H 2 O 2. The Mn2+^ is converted to MnO 4 −^ using bismuthate, BiO 3 −. The formation of a purple colored solution (from the MnO 4 −) confirms the presence of Mn2+. The bismuthate is a strong oxidizing agent and causes a vigorous reaction and must therefore be added in small portions. Mn4+(aq) + H 2 O 2 (l) € ⇔ Mn2+(aq) + O2(aq) + H+(aq) Mn2+(aq) + H+(aq) + BiO 3 −(aq) € ⇔ (^) Bi3+(aq) + MnO 4 −(aq) Test for Bi3+ Bi3+^ has a higher reduction potential (E°RED = 0.286 V) than Sn2+^ (E°RED = - 0.136 V). Therefore Sn2+^ reduces Bi3+. The reaction happens under basic conditions. The formation of a black precipitate, which is the element Bi, confirms the presence of Bi3+. Bi3+(aq) + Sn2+(aq) + 7OH−(aq)! Bi(s) + Sn(OH) 72 - (aq) Groups B, C, D Add aqueous NH 3 Precipitate: Group B- Mn(OH) 2 , Fe(OH) 3 , Bi(OH) 3 , Al(OH) 3 , Cr(OH) 3 Add 6M NaOH + 3% H 2 O 2 Precipitate: Mn4+, Fe3+, Bi3+^ Supernatant: Al(OH) 4 - , CrO 4 2- Supernatant: Groups C, D
Test for Fe3+ Fe3+^ forms a complex with thiocyanate, SCN−. Addition of potassium thiocyanate to Fe3+^ produces a reddish-brown color due to the formation of this complex. The formation of the reddish-brown color confirms the presence of Fe3+. Fe3+(aq) + KSCN(aq) € ⇔ [FeSCN]2+(aq) + K+(aq) B2 cations Treatment with 6 M NaOH and 3% H 2 O 2 converts the B2 cations to Al(OH) 4 −^ and CrO 42 -. Under acidic conditions the following reactions occur. Al(OH) 4 −(aq) + 4 H+(aq)! Al3+(aq) + 4H 2 O(l) 2CrO 42 - (aq) + 2H+(aq)! Cr 2 O 72 - (aq) + H 2 O(l) Test for Al3+ The solution containing the Al3+^ is treated with aqueous NH 3. This results in the formation of a white aluminum hydroxide precipitate. The dye aluminon when added to the precipitate, changes the color of the precipitate. The appearance of a red color confirms the presence of Al3+. Al3+(aq) + 3NH3(aq) + 3H 2 O(l)! Al(OH)3(s) + 3NH 4 +(aq) Al(OH)3(s) + aluminon dye! red precipitate Test for Cr3+ Any Cr3+^ present in the test solution is found as Cr 2 O 72 -^ at this point. In an acidic medium the dichromate is oxdized by H 2 O 2 to peroxychromate, CrO 5. The appearance of a dark blue colored CrO 5 confirms the presence of Cr3+. However, the CrO 5 is unstable and decomposes quickly. This causes the color to disappear. Cr 2 O 72 - (aq) + 4 H 2 O2(aq) + 2H+(aq) € ⇔ 2CrO5(aq) + 5H 2 O(l) GROUP C: (Ba2+, Sr2+, Ca2+) The similarity between cations of Groups C and D is that they both form insoluble salts with oxalate anion. However, the Group D oxalate salts are soluble in aqueous ammonia whereas the Group C oxalate salts are insoluble in aqueous ammonia. Note that after separating the Group B cations, the solution is already in a medium containing aqueous ammonia and at a high pH. Therefore, when ammonium oxalate
dryness. Oxalates are removes as CO 2 gas and ammonium ions are removed as N 2 O gas. The residue at this stage is dissolved in HCl which results in a solution that may be bluish green in color and will be used for analysis of the Group D cations. Test for Ni2+ Under basic conditions, Ni2+^ forms a cherry rd color precipitate with dimethylglyoxime (DMG). The formation of the red color precipitate confirms the presence of Ni^2 +. [Ni(H 2 O) 6 ]2+(aq) + 6NH3(aq)! [Ni(NH 3 ) 6 ]2+(aq) + 6H 2 O(l) [Ni(NH 3 ) 6 ]2+(aq) + 2DMG! Ni(DMG)2(s) + 6NH3(aq) Test for Cu2+ Under slightly acidic condition, Cu2+^ forms a maroon precipitate with hexacyanoferrate. The formation of the maroon precipitate confirms the presence of Cu^2 +. 2[Cu(NH 3 ) 4 ]2+(aq) + 4H+(aq) + [Fe(CN) 6 ]^4 - (aq)! Cu 2 Fe(CN) 6 + 4NH 4 +(aq) Removal of Ni2+^ and Cu2+ Sulfides of Ni2+^ and Cu2+^ are black insoluble solids. These precipitates are centrifuged and removed prior to the analysis for Mg2+^ and Zn2+ Test for Mg2+ The test solution containing Mg2+^ and/or Zn2+^ is treated with aqueous NH 3 and Na 2 HPO 4. Zn2+, if present precipitates as Zn 3 (PO 4 ) 2 and Mg2+, if present precipitates as MgNH 4 PO 4. Mg2+(aq) + NH 4 +(aq) + HPO 42 - (aq)! MgNH 4 PO4(s) 3Zn2+(aq) + 2 NH3(aq) + 2HPO 42 - (aq)! Zn 3 (PO 4 )2(s) + 2 NH 4 +(aq) The precipitates are dissolved in NaOH; only the Zn 3 (PO 4 ) 2 dissolves. Presence of a white precipitate after addition of NaOH confirms the presence of Mg^2 +. Zn 3 (PO 4 )2(s) + 12OH−(aq)! 3Zn(OH) 42 - (aq) + 2PO 43 - (aq) Test for Zn2+
The supernatant solution at the end of the last test is analyzed for the presence of Zn2+. Potassium hexacyanoferrate is added to a slightly acidic test solution. Formation of a white precipitate confirms the presence of Zn2+. 2Zn(OH) 42 - (aq) + 8H+(aq) + K 4 Fe(CN) 6 ! Zn 2 Fe(CN)6(s) + 4K+(aq) + 8H 2 O(l)
solid must be broken up and mixed well with the wash liquid. After thorough stirring, centrifuge the sample and decant the wash solution.
4. Adjusting and testing the pH When directed to check the pH of a solution, stir the solution thoroughly with a clean glass stirring rod and then touch the tip of the rod to a piece of pH paper. Several such tests may be performed on each strip of paper. Never insert the test paper into the test tube, since the chemicals on the paper could contaminate the contents. 5. Heating Due to the small quantity of material being heated, test tubes containing samples should NEVER be heated directly in a flame. A solution in test tube can reach its boiling point within a few seconds, and may be ejected violently from the test tube. All heating should be done using a water bath. Be careful that the tops of the test tubes are well above the water. The water may be boiling at times and could spatter into the test tubes, contaminating the contents. Labeling tape will fall off in a boiling water bath; therefore, it is best to label test tubes that are to be placed in a water bath with a sharpie marker. 6. Performing a flame test When exposed to a flame, certain elements emit light of a characteristic color. Individual known sample solutions can be flame tested directly or the cation can be precipitated and a flame test performed on the precipitate. In either case, a wire loop is used to introduce the sample into the flame. The loop of the flame test wire must first be thoroughly cleaned of any trace contamination. Begin by lighting a Bunsen Burner and adjusting the flame so that it burns hot; that is it appears blue, not yellow. Insert the wire loop into the hottest part of a Bunsen Burner flame; the tip of the inner blue cone. If the wire is contaminated, the flame will exhibit a color characteristic of the contaminant. Repeat the process of dipping the wire loop into DI water and then into the hot flame until no contamination is evident. Note that upon sufficient heating the test wire itself will turn the flame orange. After cleaning the wire, it can be used to test a solution or a precipitate. To test a solution, use a clean dropper to remove one drop of the rest solution and place this drop in the wire loop then insert it into the flame, observing the color that is emitted. For the known solutions, DO NOT insert the wire loop directly into the reagent bottle. Instead, place a small amount of the solution into a test tube for use. Each metal cation may not emit at the same burner temperature. Therefore, when performing flame tests best results are often obtained by slowly bringing the wire loop containing the sample into the flame
from the side. As the wire moves into the flame, it is subjected to a range of temperatures within the flame. This method of bringing the loop slowly into the flame is more important for the unknown samples, where concentrations of the emitting elements tend to be lower than in the known samples. To flame test a precipitate, centrifuge and decant the supernatant. Wet the precipitate slightly with DI water. Dip a clean wire loop in the mixture containing the sample. If solid is present, attempt to get some of it to stick to the loop. Insert the wire loop into the flame, bringing the loop slowly into the flame from the side.
7. Known sample preparation: Group A Combine 10 drops each of 0.1 M solutions of Ag+^ and Pb2+^ in a centrifuge tube. Add three drops of 6 M HCl into the tube. Stir/shake the contents of the tube and wait for two minutes. Centrifuge the tube for two minutes. Discard the supernatant and test the precipitate for the Group A cations. 8. Known sample preparation: Group B Obtain 10 drops each of 0.1 M solutions of Bi3+, Fe3+, Mn2+, Al3+, Cr3+^ in a centrifuge tube. Add four drops of 6 M HCl followed by 6 M NH 3 until the pH of the solution is between 9 and 10. Centrifuge the tube for two minutes. Discard the supernatant. Wash the precipitate twice with 10 drops of deionized water. Add 10 drops of 6 M NaOH and two drops of 3% H 2 O 2 and place the mixture in a boiling hot water bath for two minutes. Centrifuge the mixture for two minutes. Analyze the precipitate for the B1 cations and the supernatant for the B2 cations. 9. Known sample preparation: Group C Obtain 10 drops each of 0.1 M solutions of Ba2+, Ca2+, Sr2+^ in a centrifuge tube. Add four drops of 6 M HCl followed by 6 M NH 3 until the solution is neutral. Add an equal amount of NH 3 in excess. Centrifuge the mixture for two minutes. Analyze the supernatant solution for the Group C cations. 10. Known sample preparation: Group D Obtain 10 drops each of 0.1 M solutions of Ni2+, Cu2+, Mg2+, Zn2+^ in a centrifuge tube. Add four drops of 6 M HCl followed by 6 M NH 3 until the solution is neutral. Add an equal amount of NH 3 in excess. Add five drops of 0.5 M ammonium oxalate. Stir the mixture and centrifuge for two minutes. Analyze the supernatant solution for the Group D cations.
Procedure
PART 1: GROUP A (Ag+, Pb2+)
- Obtain 10 drops of the unknown solution in a centrifuge tube.
- Add three drops of 6 M HCl into the above centrifuge tube.
- If a precipitate forms, then one of the group A cations in present in the unknown. In this case proceed to the next step. If no precipitate forms, then none of the group A cations are present and in this case proceed to Part 2.
- Centrifuge the mixture containing the precipitate for two minutes. Decant the precipitate and save the supernatant for analysis in Part 2.
- Wash the precipitate with 10 drops of deionized water two times.
- Add 20 drops of deionized water to the precipitate and heat the mixture in a boiling hot water bath for three minutes.
- Centrifuge the mixture again. If there is no precipitate, then confirm the presence of Pb2+^ using the test described in Step 8. If there is a precipitate, then confirm the presence of Ag+^ using the test described in Step 9.
- Test for Pb2+: Add three drops of 1 M KI to the clear solution from Step 7. The formation of a yellow precipitate confirms the presence of Pb2+.
- Test for Ag+: Add 10 drops of 6 M NH 3 to the precipitate in Step 7. Stir the mixture. If Ag+^ is present, the precipitate will dissolve. Add 6 M HNO 3 to the same solution until a white precipitate of AgCl forms to confirm the presence of Ag+.
PART 2: GROUP B (Bi3+, Fe3+, Mn2+, Al3+, Cr3+)
- Obtain the supernatant from Step 3 of Part 1. If additional test sample is needed, then repeat Steps 1-3 of Part 1.
- Add 6 M NH 3 to the above sample until the pH is between 9 and 10. Centrifuge the mixture for two minutes. The precipitate will contain the Group B cations and the supernatant will contain the cations from Groups C & D. Save the supernatant for analysis in Part 3.
- Wash the precipitate from Step 2 above twice with deionized water.
- Add 10 drops of 6 M NaOH and two drops of 3% H 2 O 2 to the precipitate, mix thoroughly and place in a boiling hot water bath for two minutes.
- Centrifuge the above mixture for two minutes. The precipitate will contain the cations from Group B1 and the supernatant will contain the cations from Group B2. Test for the B1 cations according to procedures described in Steps 6-10. Test for the B2 cations according to procedures described in Steps 1 1 - 14.
- Test for B1 cations: Add 2 mL of deionized water to the precipitate from Step 5. Mix thoroughly and heat for 10 minutes in a boiling hot water bath. This will remove any excess H 2 O 2 from Step 4. Centrifuge the mixture and discard the supernatant.
- Test for B1 cations: Add 10 drops of 6 M HCl to the precipitate. Mix and heat for 10 minutes in a boiling hot water bath. Centrifuge the mixture. The B1 cations will be found in the supernatant. Use the supernatant in the next three steps to test for each of the B1 cations. Discard the precipitate.
- Test for Mn2+: To five drops of the supernatant solution from Step 7, add several small amounts of NaBiO 3 until no further reaction is observed. Centrifuge the mixture and observe the color of the supernatant solution. A purple supernatant solution confirms the presence of Mn2+.
- Test for Bi3+: To five drops of the supernatant solution from Step 7, add three drops of 6 M NaOH followed by a small amount of SnCl 2. Formation of a black precipitate confirms the presence of Bi3+.
- Test for Fe3+: To five drops of the supernatant from Step 7, add three drops of 0. M KSCN. Formation of a reddish brown complex confirms the presence of Fe3+.
- Test for B2 cations: Obtain the supernatant solution from Step 5 above and heat the solution for 10 minutes in a boiling hot water bath. Now, cool the solution in a cold bath. Add 6 M HCl until the solution is acidic and observe any color
PART 3: GROUP C (Ba2+, Sr2+, Ca2+)
- Obtain the supernatant solution from Step 2 of Part 2. This contains the cations from Groups C & D. If additional test solution is required, then repeat Step 2 of part 2.
- To the solution from Step 1 above, add five drops of 0.5 M ammonium oxalate and mix. The cations from Group C will precipitate and the cations from Group D will stay dissolved. Centrifuge this mixture for two minutes. Save the supernatant solution for analysis in Part 4.
- Wash the precipitate with 1 mL of deionized water.
- Transfer the precipitate into a crucible. Use concentrated HNO 3 to do the transfer. Cover the crucible partially with a lid. Heat the crucible over a low flame and evaporate the liquid.
- Cool the crucible for five minutes and add six drops of concentrated HNO 3. Cover the crucible partially with a lid. Heat the crucible over a low flame and evaporate the liquid.
- Dissolve the precipitate in four drops of 6 M HCl. Transfer the solution to a clean test tube.
- Test for Ba2+: Perform the flame test with the solution from Step 6. Emission of yellow-green light confirms the presence of Ba2+.
- Test for Sr2+: Perform the flame test with the solution from Step 6. Emission of a crimson light confirms the presence of Sr2+.
- Test for Ca2+: Perform the flame test with the solution from Step 6. Emission of a brick-red light confirms the presence of Ca2+.
PART 4 : GROUP D (Ni2+, Cu2+, Zn2+, Mg2+)
- Obtain the supernatant solution from Step 2 of Part 3. This will contain the Group D cations.
- Pour the supernatant liquid containing the Group D cations into a clean crucible. Place the lid on the crucible slightly ajar, and heat the solution to dryness over a low flame.
- Cool the crucible for five minutes, and then add six drops of concentrated nitric acid, washing the inside of the crucible. Replace the lid and heat to dryness once again.
- Cool the crucible for five minutes, repeat the addition of concentrated nitric acid and heat to dryness.
- Cool the crucible for 5 minutes, and dissolve the residue by adding 5-10 drops of 6 M HCI. (It is alright if all of the solid does not dissolve.) Using a clean Pasteur pipet, transfer the solution to a clean test tube, label the test tube as “D”.
- Rinse the crucible with 5 drops of deionized water. Add this rinse to the test tube labeled “D”.
- Test for Ni2+: Transfer one drop of the solution from test tube “D” to a clean test tube. Add 6 M NH 3 , with stirring, until the solution tests basic to litmus paper. Add one drop of 1% dimethylglyoxime. The appearance of a cherry red precipitate confirms the presence of Ni2+.
- Test for Cu2+: Transfer one drop of the solution from test tube “D” to a clean test tube. Add 6 M NH 3 until the solution tests only weakly acidic. If the solution becomes basic, use 6 M acetic acid to make it slightly acidic. Add 3-4 drops of 0. M K 4 [Fe(CN) 6 ], solution. The appearance of a maroon precipitate confirms the presence of Cu2+.
- Separation of Cu2+/Ni2+^ from Mg2+/Zn2+: Add 6 M NH 3 to the remainder of the solution in test tube D until it tests slightly basic (pH 8-9). Then add 6 M acetic acid until the solution is weaklv acidic (pH 4-5). Add about 0.2 g of solid sodium thiosulfate, Na 2 S 2 O 3 , and heat for five minutes in a boiling water bath. If Cu2+ and/or Ni2+^ are present, then a black precipitate should form. If a black precipitate does not form under these conditions, add one to two more drops of 6 M acetic acid and continue heating a few more minutes. (Formation of the black precipitate is pH sensitive.) Cool for 1 minute by swirling the test tube in cold tap water. Centrifuge and decant the supernatant into a clean test tube labeled D1. The precipitate may be discarded.
Data Table
In each instance, clearly describe the observations and the appropriate inferences. PART 1: GROUP A (Ag+, Pb2+) Cation Analyzed Observations from known Observations from unknown Inference about unknown Pb2+ Ag+
PART 2: GROUP B (Bi3+, Fe3+, Mn2+, Al3+, Cr3+) Cation Analyzed Observations from known Observations from unknown Inference about unknown Bi^3 + Fe^3 + Mn2+ Al3+ Cr3+
