Lab 8Qualitative Analysis of CationsConfirmation of Fe3+We can confirm Fe3 by dissolving the precipitate of Fe(OH)3 in HCl (Eq. 8-13) and adding KSCN solution. Ifiron(III) is present, the blood-red FeSCN2+ complex ion will form:Fe3 (aq)+SCN (aq) FeSCN2 (aq)Eq. 8-17blood-redProcedureCaution: Wear eye protection and gloves while performing this experiment.Note: Make sure to clearly label all test tubes. When you are finished with the experiment, dispose of thecontents of the test tubes into a waste container, unless otherwise directed by your instructor. Nosolutionsbe poured down the drain.mayNote: See Appendices 1-4 for information applicable to qualitative analysis.Preparation of a Known SolutionTo learn what a positive result looks like for each of the cations, we will analyze a known solutionconcurrently with an unknown solution. Prepare the known solution containing the five cations by mixingtogether 0.5 mL portions of each of the appropriate solutions containing those cations (1 mL 1 cm depthin the test tube). In addition, obtain an unknown, which contains some number of the five cations, frominstructor. For each the following steps, perform it on both of the solutions (known and unknown).yourSeparation of Ag and Pb2+ from Cr3+, Ni2, and Fe3+1.Transfer 1 mL of the solution to a small test tube. Add two drops of 6 M HCl to 1 mL and mix with yourstirring rod.Note: Always rinse your stirring rod between uses to prevent contamination of the unknown.Centrifuge the mixture, making sure there is a test tube containing about the same amount of liquid in theopposite opening in the centrifuge for balance.Add additional 1 drop of 6 M HCl to ensure complete precipitation of Ag and/or Pb2. Mix and centrifugeagain.2.A white precipitate indicates the presence of Ag and/or Pb2. Decant the supernatant liquid into anothertest tube and save it for Step 7.Separation of Pb2* from Ag and Confirmation of Pb2+Wash the precipitate from Step 2 with 1-2 mL of water. Stir, centrifuge, and decant the liquid, which may3.be discarded.Add 2 mL deionized water from your wash bottle to the precipitate in the test tube, and place the test tubein a 250 mL beaker that is about half full of boiling water. Leave the test tube in the bath for a minute ortwo, stirring oc casi onally with a glass rod. This will dissolve most of the PbCl2, but not the AgCl.Centrifuge the hot mixture, and decant the hot liquid into a test tube to be used in Step 4. Set aside theremaining precipitate for Step 5.Add one drop of 6 M acetic acid and a few drops of 0.1 M K2CrO4 to the solution from Step 3. If Pbpresent, a bright yellow precipitate of PbCrO4 will form.is4.Confirmation of Ag*Returning to the precipitate from Step 3, add 1 mL 6 M NHs and stir thoroughly.5.Note: If a white precipitate remains, it is likely that some of the solid PbCl2 did not dissolve in Step 3.Centrifuge the mixture and decant the liquid into a test tube.6.acidity by dipping the end of your stirring rod in the solution then touching it to a piece of blue litmus paper(blue litmus turns red in acidic solution). A white precipitate (AgCl) will form if Ag is present in theAdd 6 M HNO3 to the solution from Step 5 until it is acidic to litmus paper. It will take about 1 mL. Test forsolution.8-3 Lab 8Qualitative Analysis of CationsSeparation of Ni and Fe from Cr7. Returning to the solution from Step 2, add 6 M NaOH dropwise with stirring until the solution is basic tolitmus paper (red litmus turns blue in basic solution). It will likely take just a few drops. Add 1 mL more of6 M NaOH to the solution and place the test tube in a 250 mL beaker that is about half full of gently boilingwater. Leave the test tube in the bath for two minutes, stirring occasionally with a glass rod.Remove the test tube from the water bath and slowly add 1 mL of 1 M NaOCl, sodium hypochlorite. Mixwith your stirring rod for 30 seconds, then return the test tube to the water bath for two minutes.Remove the test tube from the water bath and add 0.5 mL of 6 M NHs. Let stand for 30 seconds, then returnthe test tube to the water bath for two minutes.Centrifuge the mixture. A precipitate indicates the presence of Ni and/or Fe3. Decant the solution, whichcontains chromium (as CrO42), into a test tube to be used in Step 8.Combine 2 mL of water and 0.5 mL 6 M NaOH in a clean test tube. Wash the precipitate with half of thissolution. Mix, centrifuge, and discard the wash. Repeat with the other half of the solution. Add 1 mL ofwater and 1 mL of 6 M H2SO4 to the precipitate and set the test tube aside for use in Step 9.Confirmation of Cr38. Add 0.5 mL of 1 M BaCl2 to the solution from Step 7. In the presence of chromium, a finely divided yellowprecipitate of BaCrO4 will form. It may be mixed with a white precipitate of BaSO4. The presence of awhite precipitate does not indicate the presence of chromiumPut the test tube in a boiling water bath for a few minutes, Centrifuge the solid and discard the liquid.Wash the solid with 2 mL of water; centrifuge and discard the wash.To the washed solid, add 0.5 mL of 6 M HNO3 and stir to dissolve the BaCrO4.Add 1 mL of water, stir the orange solution, and add two drops of 3% H2O2. A deep blue solution, whichmay fade quite rapidly, is confirmatory evidence for the presence of chromium.Separation of Ni from Fe3Returning to the precipitate from Step 7, stir to dissolve the solid in the H2SO4. If necessary, warm the testtube in the water bath to complete the dissolution9.process.Add 6 M NHs until the solution is basic to litmus. At this point, iron will precipitate as brown Fe(OH)s.Add 1 mL more of the NHs and stir to form the Ni(NHs)6 complex ion.Centrifuge and decant the liquid into a test tube to be used in Step 10 (if the solution is blue, nickel isprobably present). Save the precipitate for Step 11.Confirmation of Ni2+10. To the solution from Step 9 add 0.5 mL dimethylglyoxime reagent. Formation of a rose-red precipitateindicates thepresence of nickel.Confirmation of Fe3+11. Dissolve the precipitate from Step 9 in 0.5 mL of 6 M HCl. Add 2 mL water and stir.Add 2 drops of 0.5 M KSCN. Formation of a deep red solution of FESCN is a definitive test for iron.For Your ConsiderationWhat hazards are associated with this lab and what steps are taken to avoid them?Why does adding HNO; to the solution containing [Ag(NH3)2] make AgCl precipitate? Where does thechloride come from?How could the procedure be simplified if you knew from the beginning that some of the cations wereabsent?8-4

Question
Asked Nov 13, 2019
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  • What hazards are associated with this lab and what steps are taken to avoid them?
  • Why does adding HNO3 to the solution containing [Ag(NH3)2]+ make AgCl precipitate? Where does the chloride come from?
  • How could the procedure be simplified if you knew from the beginning that some of the cations were absent?
Lab 8
Qualitative Analysis of Cations
Confirmation of Fe3+
We can confirm Fe3 by dissolving the precipitate of Fe(OH)3 in HCl (Eq. 8-13) and adding KSCN solution. If
iron(III) is present, the blood-red FeSCN2+ complex ion will form:
Fe3 (aq)+SCN (aq) FeSCN2 (aq)
Eq. 8-17
blood-red
Procedure
Caution: Wear eye protection and gloves while performing this experiment.
Note: Make sure to clearly label all test tubes. When you are finished with the experiment, dispose of the
contents of the test tubes into a waste container, unless otherwise directed by your instructor. No
solutions
be poured down the drain.
may
Note: See Appendices 1-4 for information applicable to qualitative analysis.
Preparation of a Known Solution
To learn what a positive result looks like for each of the cations, we will analyze a known solution
concurrently with an unknown solution. Prepare the known solution containing the five cations by mixing
together 0.5 mL portions of each of the appropriate solutions containing those cations (1 mL 1 cm depth
in the test tube). In addition, obtain an unknown, which contains some number of the five cations, from
instructor. For each the following steps, perform it on both of the solutions (known and unknown).
your
Separation of Ag and Pb2+ from Cr3+, Ni2, and Fe3+
1.
Transfer 1 mL of the solution to a small test tube. Add two drops of 6 M HCl to 1 mL and mix with your
stirring rod.
Note: Always rinse your stirring rod between uses to prevent contamination of the unknown.
Centrifuge the mixture, making sure there is a test tube containing about the same amount of liquid in the
opposite opening in the centrifuge for balance.
Add additional 1 drop of 6 M HCl to ensure complete precipitation of Ag and/or Pb2. Mix and centrifuge
again.
2.
A white precipitate indicates the presence of Ag and/or Pb2. Decant the supernatant liquid into another
test tube and save it for Step 7.
Separation of Pb2* from Ag and Confirmation of Pb2+
Wash the precipitate from Step 2 with 1-2 mL of water. Stir, centrifuge, and decant the liquid, which may
3.
be discarded.
Add 2 mL deionized water from your wash bottle to the precipitate in the test tube, and place the test tube
in a 250 mL beaker that is about half full of boiling water. Leave the test tube in the bath for a minute or
two, stirring oc casi onally with a glass rod. This will dissolve most of the PbCl2, but not the AgCl.
Centrifuge the hot mixture, and decant the hot liquid into a test tube to be used in Step 4. Set aside the
remaining precipitate for Step 5.
Add one drop of 6 M acetic acid and a few drops of 0.1 M K2CrO4 to the solution from Step 3. If Pb
present, a bright yellow precipitate of PbCrO4 will form.
is
4.
Confirmation of Ag*
Returning to the precipitate from Step 3, add 1 mL 6 M NHs and stir thoroughly.
5.
Note: If a white precipitate remains, it is likely that some of the solid PbCl2 did not dissolve in Step 3.
Centrifuge the mixture and decant the liquid into a test tube.
6.
acidity by dipping the end of your stirring rod in the solution then touching it to a piece of blue litmus paper
(blue litmus turns red in acidic solution). A white precipitate (AgCl) will form if Ag is present in the
Add 6 M HNO3 to the solution from Step 5 until it is acidic to litmus paper. It will take about 1 mL. Test for
solution.
8-3
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Lab 8 Qualitative Analysis of Cations Confirmation of Fe3+ We can confirm Fe3 by dissolving the precipitate of Fe(OH)3 in HCl (Eq. 8-13) and adding KSCN solution. If iron(III) is present, the blood-red FeSCN2+ complex ion will form: Fe3 (aq)+SCN (aq) FeSCN2 (aq) Eq. 8-17 blood-red Procedure Caution: Wear eye protection and gloves while performing this experiment. Note: Make sure to clearly label all test tubes. When you are finished with the experiment, dispose of the contents of the test tubes into a waste container, unless otherwise directed by your instructor. No solutions be poured down the drain. may Note: See Appendices 1-4 for information applicable to qualitative analysis. Preparation of a Known Solution To learn what a positive result looks like for each of the cations, we will analyze a known solution concurrently with an unknown solution. Prepare the known solution containing the five cations by mixing together 0.5 mL portions of each of the appropriate solutions containing those cations (1 mL 1 cm depth in the test tube). In addition, obtain an unknown, which contains some number of the five cations, from instructor. For each the following steps, perform it on both of the solutions (known and unknown). your Separation of Ag and Pb2+ from Cr3+, Ni2, and Fe3+ 1. Transfer 1 mL of the solution to a small test tube. Add two drops of 6 M HCl to 1 mL and mix with your stirring rod. Note: Always rinse your stirring rod between uses to prevent contamination of the unknown. Centrifuge the mixture, making sure there is a test tube containing about the same amount of liquid in the opposite opening in the centrifuge for balance. Add additional 1 drop of 6 M HCl to ensure complete precipitation of Ag and/or Pb2. Mix and centrifuge again. 2. A white precipitate indicates the presence of Ag and/or Pb2. Decant the supernatant liquid into another test tube and save it for Step 7. Separation of Pb2* from Ag and Confirmation of Pb2+ Wash the precipitate from Step 2 with 1-2 mL of water. Stir, centrifuge, and decant the liquid, which may 3. be discarded. Add 2 mL deionized water from your wash bottle to the precipitate in the test tube, and place the test tube in a 250 mL beaker that is about half full of boiling water. Leave the test tube in the bath for a minute or two, stirring oc casi onally with a glass rod. This will dissolve most of the PbCl2, but not the AgCl. Centrifuge the hot mixture, and decant the hot liquid into a test tube to be used in Step 4. Set aside the remaining precipitate for Step 5. Add one drop of 6 M acetic acid and a few drops of 0.1 M K2CrO4 to the solution from Step 3. If Pb present, a bright yellow precipitate of PbCrO4 will form. is 4. Confirmation of Ag* Returning to the precipitate from Step 3, add 1 mL 6 M NHs and stir thoroughly. 5. Note: If a white precipitate remains, it is likely that some of the solid PbCl2 did not dissolve in Step 3. Centrifuge the mixture and decant the liquid into a test tube. 6. acidity by dipping the end of your stirring rod in the solution then touching it to a piece of blue litmus paper (blue litmus turns red in acidic solution). A white precipitate (AgCl) will form if Ag is present in the Add 6 M HNO3 to the solution from Step 5 until it is acidic to litmus paper. It will take about 1 mL. Test for solution. 8-3

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Lab 8
Qualitative Analysis of Cations
Separation of Ni and Fe from Cr
7. Returning to the solution from Step 2, add 6 M NaOH dropwise with stirring until the solution is basic to
litmus paper (red litmus turns blue in basic solution). It will likely take just a few drops. Add 1 mL more of
6 M NaOH to the solution and place the test tube in a 250 mL beaker that is about half full of gently boiling
water. Leave the test tube in the bath for two minutes, stirring occasionally with a glass rod.
Remove the test tube from the water bath and slowly add 1 mL of 1 M NaOCl, sodium hypochlorite. Mix
with your stirring rod for 30 seconds, then return the test tube to the water bath for two minutes.
Remove the test tube from the water bath and add 0.5 mL of 6 M NHs. Let stand for 30 seconds, then return
the test tube to the water bath for two minutes.
Centrifuge the mixture. A precipitate indicates the presence of Ni and/or Fe3. Decant the solution, which
contains chromium (as CrO42), into a test tube to be used in Step 8.
Combine 2 mL of water and 0.5 mL 6 M NaOH in a clean test tube. Wash the precipitate with half of this
solution. Mix, centrifuge, and discard the wash. Repeat with the other half of the solution. Add 1 mL of
water and 1 mL of 6 M H2SO4 to the precipitate and set the test tube aside for use in Step 9.
Confirmation of Cr3
8. Add 0.5 mL of 1 M BaCl2 to the solution from Step 7. In the presence of chromium, a finely divided yellow
precipitate of BaCrO4 will form. It may be mixed with a white precipitate of BaSO4. The presence of a
white precipitate does not indicate the presence of chromium
Put the test tube in a boiling water bath for a few minutes, Centrifuge the solid and discard the liquid.
Wash the solid with 2 mL of water; centrifuge and discard the wash.
To the washed solid, add 0.5 mL of 6 M HNO3 and stir to dissolve the BaCrO4.
Add 1 mL of water, stir the orange solution, and add two drops of 3% H2O2. A deep blue solution, which
may fade quite rapidly, is confirmatory evidence for the presence of chromium.
Separation of Ni from Fe3
Returning to the precipitate from Step 7, stir to dissolve the solid in the H2SO4. If necessary, warm the test
tube in the water bath to complete the dissolution
9.
process.
Add 6 M NHs until the solution is basic to litmus. At this point, iron will precipitate as brown Fe(OH)s.
Add 1 mL more of the NHs and stir to form the Ni(NHs)6 complex ion.
Centrifuge and decant the liquid into a test tube to be used in Step 10 (if the solution is blue, nickel is
probably present). Save the precipitate for Step 11.
Confirmation of Ni2+
10. To the solution from Step 9 add 0.5 mL dimethylglyoxime reagent. Formation of a rose-red precipitate
indicates the
presence of nickel.
Confirmation of Fe3+
11. Dissolve the precipitate from Step 9 in 0.5 mL of 6 M HCl. Add 2 mL water and stir.
Add 2 drops of 0.5 M KSCN. Formation of a deep red solution of FESCN is a definitive test for iron.
For Your Consideration
What hazards are associated with this lab and what steps are taken to avoid them?
Why does adding HNO; to the solution containing [Ag(NH3)2] make AgCl precipitate? Where does the
chloride come from?
How could the procedure be simplified if you knew from the beginning that some of the cations were
absent?
8-4
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Lab 8 Qualitative Analysis of Cations Separation of Ni and Fe from Cr 7. Returning to the solution from Step 2, add 6 M NaOH dropwise with stirring until the solution is basic to litmus paper (red litmus turns blue in basic solution). It will likely take just a few drops. Add 1 mL more of 6 M NaOH to the solution and place the test tube in a 250 mL beaker that is about half full of gently boiling water. Leave the test tube in the bath for two minutes, stirring occasionally with a glass rod. Remove the test tube from the water bath and slowly add 1 mL of 1 M NaOCl, sodium hypochlorite. Mix with your stirring rod for 30 seconds, then return the test tube to the water bath for two minutes. Remove the test tube from the water bath and add 0.5 mL of 6 M NHs. Let stand for 30 seconds, then return the test tube to the water bath for two minutes. Centrifuge the mixture. A precipitate indicates the presence of Ni and/or Fe3. Decant the solution, which contains chromium (as CrO42), into a test tube to be used in Step 8. Combine 2 mL of water and 0.5 mL 6 M NaOH in a clean test tube. Wash the precipitate with half of this solution. Mix, centrifuge, and discard the wash. Repeat with the other half of the solution. Add 1 mL of water and 1 mL of 6 M H2SO4 to the precipitate and set the test tube aside for use in Step 9. Confirmation of Cr3 8. Add 0.5 mL of 1 M BaCl2 to the solution from Step 7. In the presence of chromium, a finely divided yellow precipitate of BaCrO4 will form. It may be mixed with a white precipitate of BaSO4. The presence of a white precipitate does not indicate the presence of chromium Put the test tube in a boiling water bath for a few minutes, Centrifuge the solid and discard the liquid. Wash the solid with 2 mL of water; centrifuge and discard the wash. To the washed solid, add 0.5 mL of 6 M HNO3 and stir to dissolve the BaCrO4. Add 1 mL of water, stir the orange solution, and add two drops of 3% H2O2. A deep blue solution, which may fade quite rapidly, is confirmatory evidence for the presence of chromium. Separation of Ni from Fe3 Returning to the precipitate from Step 7, stir to dissolve the solid in the H2SO4. If necessary, warm the test tube in the water bath to complete the dissolution 9. process. Add 6 M NHs until the solution is basic to litmus. At this point, iron will precipitate as brown Fe(OH)s. Add 1 mL more of the NHs and stir to form the Ni(NHs)6 complex ion. Centrifuge and decant the liquid into a test tube to be used in Step 10 (if the solution is blue, nickel is probably present). Save the precipitate for Step 11. Confirmation of Ni2+ 10. To the solution from Step 9 add 0.5 mL dimethylglyoxime reagent. Formation of a rose-red precipitate indicates the presence of nickel. Confirmation of Fe3+ 11. Dissolve the precipitate from Step 9 in 0.5 mL of 6 M HCl. Add 2 mL water and stir. Add 2 drops of 0.5 M KSCN. Formation of a deep red solution of FESCN is a definitive test for iron. For Your Consideration What hazards are associated with this lab and what steps are taken to avoid them? Why does adding HNO; to the solution containing [Ag(NH3)2] make AgCl precipitate? Where does the chloride come from? How could the procedure be simplified if you knew from the beginning that some of the cations were absent? 8-4

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