CHM 152 Lab 5: Qualitative Analysis updated May, 2011 Introduction In this lab you will see how it s possible to separate a mixture using many of the common reactions you ve learned in General Chemistry (precipitation, redox, etc.). In Part I, you will separate a solution of eight known cations. The procedure you ll be following is summarized in the flow chart on page 3. In Part II, you ll be given an unknown solution containing four of these eight cations and, using your observations from Part I, you will attempt to determine their identities. Good lab technique and careful recording of your observations are essential to successfully identifying the four cations in your unknown. Concepts to Review Precipitation reactions Net ionic equations Procedure Part I: Analysis of a known mixture A. Initial Separation 1. Place 2-3mL of the known sample in a centrifuge tube and label it B. 2. In a fume hood, add 3mL of 6M NH 4 OH and five drops of 6M HNO 3, then mix with a stir rod. Centrifuge for five minutes, and then add two more drops of NH 4 OH to confirm that precipitation is complete. If additional precipitate forms, centrifuge for another couple of minutes and recheck for completeness. [Note: Remember that a centrifuge has to remain balanced to reduce the risk of an accident, using a solution of approximate equal volume directly across from it (either another group s sample or a test tube with an equal amount of water).] 3. Carefully decant the remaining solution (the supernatant) to another centrifuge labeled D. Set it aside for later analysis. B. Analysis of sample B 1. Wash sample with 5mL of deionized water, mix, and centrifuge. Decant the water into a beaker, and then discard the water by pouring down the drain. 2. Add 2mL of 1M NaOH, mix, and centrifuge. Check for completeness, and then transfer the supernatant to a test tube labeled C. Set sample C aside for later analysis. 3. Add 4mL of deionized water to sample B, then add 6M HNO 3 dropwise until the solution is acidic. Test the acidity using blue litmus paper, which turns red under acidic conditions. 4. Add a small amount of potassium thiocyanate, KSCN, to the solution. If iron (II) ion is present, the solution will turn red from the formation of Fe(SCN) 2+. C. Analysis of sample C Add 2M acetic acid, HC 2 H 3 O 2, dropwise to sample until the solution is acidic. Add 6 drops of aluminon reagent and heat in a hot bath (use a set-up similar to the one used in the Freezing-point Depression lab). If aluminum ion is present, a pink, gelatinous solid will form. D. Analysis of sample D 1. Add 4mL of 1M ammonium carbonate, (NH 4 ) 2 CO 3, mix, and centrifuge. Check for completeness, and then transfer the supernatant to a centrifuge tube labeled E. Set sample E aside for later analysis. 2. Wash the precipitate in sample D (see part B, step 1). 3. Add 2mL of water to sample D, then add 6M HCl dropwise until all of the solid has dissolved. 4. Add 2mL of 1M Na 2 SO 4 and mix. If barium ion is present, barium sulfate will form as a white precipitate.
5. Centrifuge and transfer the supernatant to a test tube. To the solution, add six drops of 6M NH 4 OH and 2mL ammonium oxalate, (NH 4 ) 2 C 2 O 4. If calcium ion is present, calcium oxalate will form as a white precipitate. E. Analysis of sample E. 1. Add 2mL of sodium hydrogen phosphate, Na 2 HPO 4, to sample and mix. If magnesium ion is present, magnesium ammonium phosphate, MgNH 4 PO 4, will form as a white precipitate. [Note: cobalt (II), copper (II) and nickel (II) form soluble ammonia complexes, M(NH 3 ) x, which prevent precipitation from occurring.] 2. Centrifuge and transfer the supernatant to a 100mL beaker. Gently heat this solution to dryness, using a hot plate, until the solid is a pale gray color. 3. Redissolve the solid using 5mL of 1M phosphoric acid, H 3 PO 4, and then add 2mL of 1M Na 2 HPO 4. Divide the solution into three test tubes labeled F1, F2, and F3. F. Analysis of Samples F1-F3 1. Add a small amount of potassium iodide, KI, to solution F1. If copper ion is present, copper (I) iodide will form as a pale tan precipitate. 2. Add 2mL of dimethylglyoxime (HDMG) to solution F2. If nickel (II) ion is present, red precipitate will form. 3. Add 2mL of 1-nitroso-2-naaphthol (NN) to solution F3. If cobalt (II) ion is present, a reddish-brown precipitate will form. Part II: Analysis of an unknown mixture You will be assigned a solution with only four of the eight cations from Part I. Repeat the above produce to determine which ions are present. Waste Disposal All solution waste should be disposed of in the Inorganic Waste bottle.
Al 3+, Ba 2+, Ca 2+, Co 2+, Cu 2+, Fe 3+, Mg 2+, Ni 2+ NH 4 OH (aq), HNO 3 (aq) Al(OH) 3 (s), Fe(OH) 3 (s) Ba 2+, Ca 2+, Co 2+, Cu 2+, Mg 2+, Ni 2+ NaOH (aq) (NH 4 ) 2 CO 3 Fe(OH) 3 (s) HNO 3 (aq) Al(OH) 4 - (aq) HC 2 H 3 O 2 (aq), aluminon BaCO 3 (s), CaCO 3 (s) HCl(aq) Co 2+, Cu 2+, Mg 2+, Ni 2+ Na 2 HPO 4 (aq) Fe(OH) 3+ 3 (s) Al(OH) 2 (aluminon), pink precipitate Ba 2+, Ca 2+ KSCN(s) Fe(SCN) 2+ (aq), dark red Na 2 SO 4 (aq) MgNH 4 PO 4 (s), white ppt Co 2+, Cu 2+, Ni 2+ 1) Dry 2) H 3 PO 4 (aq), Na 2 HPO 4 (aq) 3) Divide into 3 samples BaSO 4 (s), white ppt Ca 2+ KI(s) HDMG (aq) NN(aq) NH 4 OH(aq), (NH 4 ) 2 C 2 O 4 (a ) CaC 2 O 4 (s), white ppt CuI(s), pale tan ppt Ni(DMG) 2, red ppt Co(NN) 3, red-brown ppt
Name: Section: Data Part I: Analysis of a known mixture Record your observations from the following steps in the procedure. A. Initial Separation B. Analysis of sample B Step 4 C. Analysis of sample C D. Analysis of sample D Step 4
Step 5 E. Analysis of sample E. F. Analysis of samples F1-F3
Part II: Analysis of an unknown mixture Unknown #: Record your observations from the following steps in the procedure. If a step was skipped (e.g., a test for an ion that isn t present), simply write NA in the space provided. A. Initial Separation B. Analysis of sample B Step 4 C. Analysis of sample C D. Analysis of sample D Step 4
Step 5 E. Analysis of sample E. F. Analysis of samples F1-F3 Unknown # Ions present:
Name: Section: Post-Lab Questions 1. During your initial separation (Part A), why doesn t aluminum form Al(OH 4 ) -, as it does in part B? 2. In CHM 151, we would have predicted that all of these ions except barium and calcium would have formed insoluble hydroxide compounds, yet only two precipitated out. Given the results of your initial separation (Part A), what other factor seems to play a part in solubility. 3. What s the point of heating the solution to dryness in part E when you re redissolving it in part F? 4. A solution was analyzed with the following results. Determine which ions are present. In each case explain, which test confirms your answer. A precipitate formed after the addition of ammonium hydroxide and nitric acid. After decanting the supernatant, the solution did not dissolve after adding sodium hydroxide, but the addition of potassium thiocyanate resulted in a red solution. The addition of ammonium carbonate to the supernatant did not yield a precipitate, but one did form from the addition of sodium hydrogen phosphate. This supernatant was dried, redissolved using phosphoric acid and sodium hydrogen phosphate, and separated into three portions. One sample yielded a red precipitate from the addition of dimethylglyoxime while another gave a reddish-brown precipitate from the addition of 1-nitroso-2-naaphthol. 5. A solution contains Al 3+, Ca 2+, Fe 3+, and Ni 2+ ions. Draw a flow chart that outlines the procedure required to separate this mixture (use the back if you need more room)
Name: Section: Pre-Lab Questions 1. Each of the following compounds will be formed in this week s lab. For each one, write the net ionic equation for the reaction that will be performed to produce it a) Aluminum hydroxide b) Barium sulfate c) Calcium carbonate d) Calcium oxalate e) Fe(SCN) 2+ 2. In part E, step 1, where does the ammonia come from to produce MgNH 4 PO 4? 3. In part E, why are you splitting the sample into three portions before performing the tests for Cu 2+, Ni 2+ and Co 2+ in part F, as opposed to simply adding each indicator reagent to the same solution? 4. When a precipitate forms, why is it important to check for completeness after centrifuging the sample? 5. Why is it important to use deionized water in this lab?