Qualitative Analysis I - Cations
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1 1 Qualitative Analysis I - Cations Purpose: To separate and identify several metal cations from a mixture of cations, and to analyze an unknown sample of cations Introduction Qualitative analysis is the determination of what is in a sample, without any specific regard for how much of it is present (the determination of how much is referred to as quantitative analysis). In this experiment we will analyze samples for the presence of certain cations. If those cations happen to be mixed together, it will first be necessary to separate them from one another to avoid interfering reactions, then analyze for the presence of each ion. Most qualitative analysis schemes make use of selective precipitation in order to separate ions in a mixture. Selective precipitation involves addition of a reagent capable of precipitating some, but not all, ions in a mixture. If some of the ions are precipitated as solids, they can be easily separated from the ions still in solution. Further chemical tests can then confirm the presence of the suspected ions. This experiment will allow you to use selective precipitation to separate two different cations from a mixture. These three are lumped into analytical groups, which are not the same as periodic table groups (e.g. Group IA, IIB, etc.). The analytical groups are established based upon how the ions behave in the presence of certain precipitating agents. Analytical Group I cations (Ag +, Pb +2, and Hg 2 +2 ) all form insoluble chlorides. Analytical Group II cations (Bi +3, Cu +2, Sn +4, and Cd +2 ) all form insoluble sulfides that are insoluble even in strongly acidic solution. Analytical Group III cations (Fe +3, Ni +2, and Cr +3 ) all form insoluble sulfides which are soluble in acidic solution. In order to see firsthand the techniques for separating these ions, yet still finish in a reasonable timeframe, we will limit our approach to cation analysis by examining only two ions: one from group I (Ag + ), one from Group II (Cu +2 ).
2 2 Chemistry of the Analytical Group I Cation Ag + Analytical Group I cations Ag +, Pb +2, and Hg 2 +2 form insoluble chlorides, unlike most all other cations whose chlorides are water soluble. The solubility equilibrium for the Ag + /AgCl system is; AgCl (s) Ag + (aq) + Cl - (aq) (eq. 1) Caution must be exercised, however, in adding chloride ions to precipitate Ag + ion, as the presence of a large excess of chloride will cause AgCl to dissolve through formation of a complex ion; AgCl (s) + Cl - (aq) AgCl 2 - (aq) (eq. 2) It is worthy of note that silver chloride, AgCl, is a brilliant white solid which changes its color to a pale gray-lavender after a few minutes exposure to light. This reaction is unique to silver chloride; there is no other substance which behaves in this manner. Addition of aqueous ammonia, NH 3, will dissolve AgCl; AgCl (s) + 2NH 3(aq) Ag(NH 3 ) 2 + (aq) + Cl - (aq) (eq. 3) Subsequent re-acidification of the solution will cause AgCl to precipitate again; Ag(NH 3 ) 2 + (aq) + Cl - (aq) + 2H + AgCl (s) + 2NH 4 + (aq) (eq. 4) The reformation of AgCl thus confirms the presence of Ag + in the original sample. Chemistry of the Analytical Group II Cation Cu +2 The Analytical Group II cations Bi +3, Cu +2, Sn +4, and Cd +2 form insoluble sulfides which are also insoluble in a strongly acidic solution. Since a common source of sulfide ion, H 2 S, is extremely poisonous, we will generate H 2 S more safely by creating it in situ (meaning right in the reaction vessel) by reaction of thioacetamide, CH 3 CSNH 2 : CH 3 CSNH 2(aq) + 2H 2 O (l) CH 3 COO - (aq) + NH 4 + (aq) + H 2 S (aq) (eq. 5) Once produced, the H 2 S establishes the following aqueous equilibria; H 2 S (aq) H + (aq) + HS - (aq) (eq. 6) HS - (aq) H + (aq) + S -2 (aq) (eq. 7) While it is apparent that thioacetamide is a fairly complicated substance, when you encounter it in a procedural step, just think to yourself sulfide ion, as thioacetamide is the source of S -2 in solution.
3 3 The sulfide ion produced in eq. 7 then reacts with Cu +2 ion to form the sulfide, as described by the following solubility equilibrium; CuS (s) Cu +2 (aq) + S -2 (aq) (eq. 8) Note that in strongly acidic solution (i.e. high [H + ]), LeChâtelier s Principle suggests that the equilibria in eqs. 6 and 7 will be forced to the left, yielding very low concentrations of sulfide ion. This is necessary because if the sulfide concentration is too high, the Group III cations may precipitate as well, making separation of the two groups virtually impossible. The CuS precipitated in eq. 8 can be dissolved by addition of 6M HNO 3. The effect of the nitric acid is to drive eqs. 6 and 7 to the left, thereby reducing the S -2 ion concentration, which in turn forces eq. 8 to the right. The confirmation step for Cu +2 involves addition of aqueous ammonia, NH 3, to the solution, forming the absolutely gorgeous deep blue Cu(NH 3 ) 2 + complex ion; Cu +2 (aq) + 2NH 3(aq) Cu(NH 3 ) 2 +2 (aq) (eq. 9) Procedure There are certain techniques common to many qualitative analysis procedures. You will be using some of these repeatedly in the next few lab experiments in this course, so it is worth clarifying those techniques here. Mixing the Contents of a Centrifuge Tube The pointed bottom of a centrifuge tube is designed to make it easier to work with very small amounts of solids frequently encountered in qualitative analysis. However, that shape also makes it difficult to effectively stir a precipitate and its supernatant liquid, as a standard glass stirring rod will not fit all the way into the bottom of the centrifuge tube. A thin wood-shaft cotton swab may be used to stir to the bottom by using the wooden end, not the cotton end. You may sometimes get adequate mixing by sucking up most of the liquid into a disposable plastic or glass dropper, then forcibly expelling the contents directly into the bottom of the centrifuge tube, followed by a brief vortexing if necessary. Removing Supernatant Liquids Supernatant liquids can most easily be removed from precipitates by careful decanting, or pouring off the liquid so as to allow the solid to remain in the
4 4 original container. If the precipitate is unusually loose, it may be necessary to draw off the supernatant liquid carefully with a disposable plastic dropper. Checking for Completeness of Precipitation To avoid the presence of interfering cations in solutions, it is often necessary to insure that all of a particular cation has been completely precipitated. The precipitate in the tube is centrifuged (or simply allowed to settle, if appropriate) and one drop of the precipitating reagent is added. If more precipitate forms where that drop contacts the clear supernatant, then the precipitation is not complete. More drops of precipitating reagent must be added, and the solution centrifuged again and rechecked for completeness of precipitation. Washing Precipitates To remove interfering cations from a wet precipitate, it is often necessary to wash the precipitate. This involves separating the supernatant liquid from the precipitate and adding the specified amount of wash liquid (often, but not always deionized water) to the precipitate, mixing thoroughly, centrifuging and discarding the wash liquid. In this experiment you will prepare your own known solution by combining separate solutions containing Ag + and Cu +2 ions. This mixture will then be analyzed by separating the two ions, and then confirming the presence of each. By keeping detailed observations for the known analysis, you will then be able to identify an unknown solution containing one or both of the ions by carrying out the same procedure you did for the knowns, and comparing the observations you collected for the unknown with those of the known. You may wish to refer to the flow chart on page 88 while going through the procedure.
5 5 SAFETY NOTE!! Hydrochloric acid, HCl, and nitric acid, HNO 3, are strong acids. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with cold water. Notify your instructor. SAFETY NOTE!! Ammonia, NH 3, is an irritant to the skin and respiratory system. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with cold water. Notify your instructor. Precipitation and Confirmation of Group I Cation (Ag + ) (record all observations on Data Sheet 1) 1. Fill a 400 ml beaker about half full of water to use as a water bath later in the Group II analysis. Set the beaker of water on a hot plate and begin heating. Arrange the water bath and hot plate under a portable fume hood (your instructor will show you how to set this up). 2. In a clean, dry test tube, prepare a known solution by combining 1 ml each of Ag + and Cu +2 solutions. Mix gently, and label this tube knowns. If your instructor permits it, you may wish to go through all procedural steps with the known and unknown simultaneously (see Steps 14 and 15). If you do both together, you must carefully label all tubes to avoid getting them mixed up! 3. Transfer 15 drops of the known mixture to a clean dry centrifuge tube. Add 6M HCl dropwise until precipitation is complete. 4. Centrifuge the tube, making sure to use a balance tube with about the same volume of liquid, for about one minute. Do not leave an operating centrifuge unattended. Someone must be near the centrifuge at all times while it is running. 5. Decant the supernatant liquid, which contains the Group II cation, into a separate clean centrifuge tube. Label the tube and set aside for use later in step Wash the precipitate from step 3 with 2 ml deionized water, centrifuge, and discard the supernatant into the appropriate waste container. 7. Add 15 drops of 6M ammonia, NH 3, to the precipitate and vortex. If it doesn t dissolve add another drop or two of ammonia and vortex again. If it dissolves, this strongly suggests the presence of Ag +, but a confirmation is still necessary.
6 6 8. Add 6M HNO 3 dropwise just until the solution becomes acidic as tested with blue litmus paper. The presence of a white precipitate of AgCl which turns pale lavender after a few minutes confirms the presence of silver ion, Ag +, in the original sample. Precipitation and Confirmation of Group II Cation (Cu +2 ) (record all observations on Data Sheet 1) SAFETY NOTE!! Thioacetamide has been demonstrated to be a carcinogen in animal feeding studies. Avoid contact with your skin, eyes, and clothing. If you do spill some on yourself, wash immediately with cold water. Notify your instructor. 9. Add 10 drops of 1M thioacetamide solution to your supernatant from step 5. Heat the tube and its contents in a boiling water bath for 5 minutes. Be sure this is done under the portable fume hood. 10. Centrifuge the tube and its contents for one minute. Decant the supernatant liquid into another clean centrifuge tube. Label the tube containing the precipitate (which should be CuS). 11. Add 20 drops of 6M HNO 3 to the tube containing the CuS precipitate, stir gently and heat in a boiling water bath for 3 minutes to dissolve the precipitate. 12. After the liquid from step 11 has cooled, add 6M NH 3 until the solution is just basic to litmus paper. Then add 5 drops more excess NH 3. A deep blue solution confirms the presence of Cu +2 in the original sample as Cu(NH 3 ) Identification of an Unknown 13. Obtain an unknown solution from your instructor. The unknown may contain either one or both of the ions studied in the known solution. Record the unknown number on Data Sheet Go through the procedure you just completed for the known solution, recording all your observations on Data Sheet 1. If you get to a point where a precipitate was present for the known but none is present for the unknown, then the cation which caused the precipitate is absent from your unknown. You can then skip ahead to the procedure for the next analytical group.
7 7 Cation Analysis Flowchart Ag + and Cu +2 6M HCl yes ppt? no 6M HNO 3 ppt. dissolves 6M NH 3 ppt. doesn t dissolve most likely Cu +2 white ppt. confirms presence of Ag + as AgCl you ve got problems consult your instructor acidify, add thioacetamide and heat yes ppt? no ppt. dissolves 6M HNO 3 ppt. doesn t dissolve 6M NH 3 deep blue confirms Cu +2 as Cu(NH 3 ) 2 +2 you ve got problems consult your instructor no Cu +2 present
8 8 Qualitative Analysis I - Cations Data Sheet 1 step known observations unknown # observations 2 - precipitation of Group I chlorides 7 - dissolving AgCl with NH re-precipitating AgCl 9 - precipitation of Group II sulfides 11- dissolution of CuS with HNO formation of Cu(NH 3 ) 2 +
9 9 Qualitative Analysis I - Cations Post-lab Question 1. Give the identification of your unknown cation sample, and write any pertinent equations that led to the identification of the presence of the reported ion(s).
10 10 Qualitative Analysis I - Cations Pre-laboratory Assignment 1. Briefly describe how the analytical Group I, II, and III cations can be separated from one another in a mixture. 2. Explain how controlling the solution ph allows separation of Group II cations from those of Group III. 3. Describe how to check for completeness of precipitation. 4. What is the source of sulfide ion for this experiment? 5. Describe any safety precautions associated with this lab experiment.
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