Qualitative Analysis Procedures and Identification of Cations

Principles of Qualitative Analysis

• Primary Objectives:

  • Confirm the presence of specific cations in a known reference solution via qualitative analysis by observing physical and chemical phenomena.

  • Identify the unknown cations present in a given unknown test solution.

• Analytical Phenomena for Ion Identification:

  • Color changes in solution or precipitate

  • Formation of a precipitate (solid formation)

  • Flame test color emission

  • Gas evolution

• Sample Solution Compositions:

  • Reference Solution: Contains four specific cations: ammonium ($\text{NH}_4^+$), magnesium ($\text{Mg}^{2+}$), calcium ($\text{Ca}^{2+}$), and copper(II) ($\text{Cu}^{2+}$).

  • Unknown Solution: Contains exactly two cations selected from the four present in the reference solution.

• Analytical Methodology:

  • Perform chemical tests simultaneously on both the reference solution and the unknown test solution.

  • Direct comparison of observed phenomena between the reference and unknown samples enables systematic identification of the cations present in the unknown.

General Laboratory Procedures and Safety Protocol

• Mixing Techniques:

  • Always thoroughly mix the contents of a test tube after adding any new reagent solution.

  • Shake gently or stir thoroughly using a clean glass stirring rod.

• Glassware Maintenance and Reuse:

  • Never use paper towels to dry the inside of test tubes.

  • If test tubes must be reused during the experiment, dry them using air flow.

  • Wash all used glassware (beakers and test tubes) with soap and water, followed by thorough deionized (DI) water rinses.

• Waste Disposal Protocol:

  • Discard all used solutions and precipitates into designated liquid or solid waste containers located under the fume hood.

  • Rinse all used test tubes with DI water and dispose of the rinsing water into the corresponding waste containers.

  • Separate waste into specific streams:

  • Basic aqueous waste container

  • Acidic aqueous waste container

  • Solid waste container

  • Return all chemical reagent bottles to their original locations upon completion.

Experimental Preparation and Initial Setup

• Procedure:

  1. Obtain a reference solution (labeled test tube #REF) and an unknown test solution from the instructor. Record the exact label identifier of the unknown test solution.

  2. Pour one-third of the reference solution into a clean beaker for the $\text{NH}_4^+$ identification test. Polish a piece of zinc metal ($\text{Zn}$) and place it into test tube #REF.

  3. Pour one-third of the unknown test solution into another clean beaker for Part I. Polish a second piece of $\text{Zn}$ metal and place it in the remaining unknown solution in its test tube.

  4. Set aside both test tubes (reference and unknown containing the $\text{Zn}$ pieces) for Part II ($\text{Cu}^{2+}$ identification) after completing Part I.

Part I: Identification of Ammonium Ion ($\text{NH}_4^+$)

• Procedure:

  1. To the beaker containing one-third of the solution (or $5\,\text{mL}$ of reference solution heated in a $100\,\text{mL}$ beaker on a hot plate to a moist residue, reconstituted with $2\,\text{mL}$ DI water), add approximately 20 drops (or $2\,\text{mL}$) of $6\,\text{M NaOH}$.

  2. Moisten a piece of red litmus paper with a drop of DI water.

  3. Under a fume hood, apply gentle heat from a hot plate to warm up the mixture in the beaker.

  4. Hold the moistened red litmus paper inside the beaker above the liquid surface without touching the solution or beaker walls.

  5. Repeat steps 1 through 4 with the unknown test solution sample.

• Chemical Equation:   $\text{NH}_3(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{NH}_4^+(aq) + \text{OH}^-(aq)$

• Observations & Confirmation:

  • Evolution of ammonia gas ($\text{NH}_3$) causes the moist red litmus paper to turn blue.

• Waste Disposal:

  • Dispose of all resulting solutions in the basic aqueous waste container.

Part II: Identification of Copper(II) Ion ($\text{Cu}^{2+}$)

• Initial Observations and Zinc Reaction:

  • Record initial observations of the reference and unknown test samples containing the $\text{Zn}$ pieces in their original test tubes.

  • Chemical reaction with zinc metal:     $\text{Cu}^{2+}(aq) + \text{Zn}(s) \rightarrow \text{Cu}(s) + \text{Zn}^{2+}(aq)$

  • Observation: Solution color changes from blue to colorless as $\text{Cu}^{2+}$ is reduced, and reddish solid copper precipitates onto the zinc metal.

• Separation and Tube Setup:

  1. Pour the contents of test tube #REF into a clean, dry beaker. Use a glass stirring rod to scrape any colored solid off the zinc metal.

  2. Transfer the liquid and scraped solid into a small test tube labeled #1, leaving the zinc behind. Rinse the rod and beaker with a small amount of DI water and add the rinses to test tube #1.

  3. Clean the stirring rod and repeat step 1 for the unknown test solution using a new small test tube labeled #2.

  4. Exceptions & Caveats:

  • If no change is observed with the $\text{Zn}$ in the unknown sample, simply transfer the liquid to test tube #2 and relabel it as test tube #4. Skip steps 3 through 7 and step 10 in Part II for this sample.

  • Use small test tubes of appropriate design for centrifugation (#1 and #2).

  • If $\text{Zn}$ reacted, dispose of it in the solid waste container. If $\text{Zn}$ did not react, clean, dry, and return it to its original container.

• Centrifugation Protocol:

  1. Compare liquid levels in test tubes #1 and #2. Add DI water to the lower-volume tube until levels are equal. (If $\text{Zn}$ did not react with the unknown, use a test tube filled with tap water for balancing).

  2. Place test tubes #1 and #2 in opposite slots of the centrifuge. Spin for $30\text{--}60\,\text{seconds}$. If supernatant remains cloudy, centrifuge for a longer duration.

  3. Pour the supernatant from test tube #1 into a new small test tube labeled #3 (reserved for $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ tests).

  4. Pour the supernatant from test tube #2 (the unknown sample) into a new small test tube labeled #4.

• Dissolution of Precipitated Solid:

  1. To test tubes #1 and #2, add $6\,\text{M HNO}_3$ dropwise until all solid material reacts and disappears.

  2. Chemical reactions:      $3\text{Cu}(s) + 8\text{H}^+(aq) + 2\text{NO}_3^-(aq) \rightarrow 3\text{Cu}^{2+}(aq) + 2\text{NO}(g) + 4\text{H}_2\text{O}(l)$      $4\text{HNO}_3 + \text{Cu} \rightarrow \text{Cu}(\text{NO}_3)_2 + 2\text{NO}_2 + 2\text{H}_2\text{O}$

  3. Safety Note: Gases produced ($\text{NO}$, $\text{NO}_2$) are toxic and corrosive.

  4. Divide each solution in half:

  • Transfer half of test tube #1's content to test tube #5.

  • Transfer half of test tube #2's content to test tube #6.

• Confirmation Test A: Ammonia Complex Formation:

  1. Add concentrated $\text{NH}_3$ slowly to test tube #1 until a distinct color change occurs (deep blue tetraamminecopper(II) complex solution).

  2. Chemical reaction:      $\text{Cu}^{2+}(aq) + 4\text{NH}_3(aq) \rightarrow [\text{Cu}(\text{NH}_3)_4]^{2+}(aq)$

  3. Discard solution in the basic aqueous waste container.

  4. Repeat with test tube #2.

• Confirmation Test B: Potassium Ferrocyanide Reaction:

  1. Test acidity of solution in test tube #5 using blue litmus paper by transferring a drop via a glass rod.

  2. If the paper remains blue, add $6\,\text{M CH}_3\text{COOH}$ (acetic acid) dropwise until the solution turns the paper red.

  3. Add 3 drops of $0.2\,\text{M K}_4[\text{Fe}(\text{CN})_6]$ (potassium ferrocyanide) to test tube #5. Record observations (formation of a reddish-brown precipitate).

  4. Chemical reaction:      $2\text{Cu}^{2+}(aq) + [\text{Fe}(\text{CN})_6]^{4-}(aq) \rightarrow \text{Cu}_2[\text{Fe}(\text{CN})_6](s)$

  5. Discard mixture in the acidic aqueous waste container.

  6. Repeat acidity testing and reagent addition (steps 1–5) for test tube #6.

Part III: Identification of Calcium Ion ($\text{Ca}^{2+}$)

• Procedure:

  1. Add $6\,\text{M NH}_3$ dropwise to test tube #3 (supernatant from Part II) until the solution becomes completely clear and transparent. Mix thoroughly. Use a glass rod to transfer a drop onto red litmus paper to confirm alkalinity.

  • Warning: Do not add more than 20 drops of solution.

  1. Add 2 to 3 drops of $1\,\text{M K}_2\text{C}_2\text{O}_4$ (potassium oxalate) to test tube #3. Observe for precipitate formation.

  • If no change occurs immediately, heat the tube in a warm water bath.

  1. Chemical reaction:      $\text{Ca}^{2+}(aq) + \text{C}_2\text{O}_4^{2-}(aq) \rightarrow \text{CaC}_2\text{O}_4(s)$

  2. Centrifuge test tube #3 for $30\text{--}60\,\text{seconds}$. Record appearances of supernatant and precipitate (white solid precipitate).

  3. Pour the supernatant into test tube #7 (reserved for $\text{Mg}^{2+}$ test).

  4. Repeat steps 1 through 5 for the unknown test solution in test tube #4, collecting supernatant in test tube #8.

• Flame Test Confirmation:

  • Save the precipitated calcium oxalate ($\text{CaC}_2\text{O}_4$) solid for a flame test.

  • Characteristic flame color for Calcium: Orange-Red.

• Precipitate Disposal/Dissolution:

  • Add $6\,\text{M HCl}$ to the precipitate remaining in both test tubes. Shake to disperse and dissolve.

  • Discard in the acidic aqueous waste container.

Part IV: Identification of Magnesium Ion ($\text{Mg}^{2+}$)

• Procedure:

  1. To test tube #7 (supernatant saved from Part III), add 1 to 2 drops of $6\,\text{M NH}_3$.

  2. Add 2 to 3 drops of $1\,\text{M Na}_2\text{HPO}_4$ (disodium hydrogen phosphate).

  3. If no reaction or precipitate forms immediately, heat the solution in a hot water bath.

  4. Repeat steps 1 through 3 for the unknown sample supernatant in test tube #8.

• Waste Disposal:

  • Discard all resulting mixtures from Part IV into the basic aqueous waste container.