Separation and Identification of Group C Cations (Ca 2+, Sr 2+ and Ba 2+ )

Transcription

1 Name: Objectives Separation and Identification of Group C Cations (Ca 2+, Sr 2+ and Ba 2+ ) To develop a separation and identification scheme for the Group C cations: calcium, strontium and barium. To complete a flow diagram summarizing the qualitative analysis scheme for the Group B ions. To successfully identify the Group C cation present in an unknown solution. Background Chemistry and Discussion After removing the Group A and Group B cations from your unknown by precipitation, the Group C and D cations remain in solution. The three ions; Ca 2+, Sr 2+ and Ba 2+, that make up Group C have soluble chlorides and hydroxides that are reasonably soluble in ammonia solutions. These three ions can be separated from the Group D ions by selective precipitation. Identification of your Group C cation based solely upon the color of the precipitate formed is not possible since the precipitates formed by Ca 2+, Sr 2+ and Ba 2+ do not have colors unique from each other. Identification can be achieved by using a combination of solubility behavior and flame tests. You are required to develop a scheme that will allow you to separate and correctly identify the Group C cation present in your unknown solution. About Flame Tests When exposed to a flame, certain elements emit light of a characteristic color. As part of our analysis, flame tests of known samples of the Group C cations are necessary in order to become familiar with the characteristic color emitted by each one. 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. Then dip the wire loop in concentrated HCl (in a test tube) and insert the 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 the HCl and then inserting it into the flame until no more contamination is apparent. 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, dip the wire loop into the solution and 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 across the flame from the side. As the wire moves across the flame, it is subjected to a range of temperatures within the flame. This method of bringing the loop slowly across 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 1-2 drops of either concentrated HCl or concentrated HNO 3. You may need to test to see which acid works best for the flame tests. The precipitate may not necessarily dissolve in the acid and that is all right. 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 across the flame from the side. BE ALERT AND WATCH CLOSELY!!!! Washing Precipitates Before Flame Testing Your Group C unknown precipitate should be white, if it is not you must wash it prior to flame testing to remove contaminants that could interfere with the flame test. To wash a precipitate that is a sulfate, use a few drops of 6 M HCl (insoluble sulfates do not dissolve in 6 M HCl). To wash a precipitate that is an oxalate, carbonate or hydroxide, use a few drops of 6 M NH 3 ; the NH 3 will remove the contaminants by forming complex ions with them, but will not dissolve the Group C precipitate. Calcium oxalate may also be washed with a few drops of 6 M acetic acid. Dr. L.J. Larson 1 Revised/Printed 6/27/08

2 Reagents Available 1-M (NH 4 ) 2 SO 4 1-M (NH 4 ) 2 CO 3 Saturated Ammonium Oxalate solution Concentrated HCl 6-M NaOH 6-M NH 3 Concentrated HNO 3 6-M acetic acid (CH 3 COOH) Dilute, Known Solutions of Ca 2+, Sr 2+ and Ba 2+ Safety and Waste Disposal Some of Group C cations are toxic. The HCl, HNO 3 and NaOH are irritants. Avoid contact and wash immediately if any is spilled or splashed on you. Wear eye protection at all times. As you perform the experiment, collect all waste solutions in a waste beaker. This mixture should then be discarded in the appropriate waste container. DO NOT POUR ANY OF THE SOLUTIONS DOWN THE DRAIN. Unknowns and Knowns Separate known samples containing Ca 2+, Sr 2+ and Ba 2+ are provided for your use. Testing known samples is helpful in this analysis since doing so will allow you to observe what a positive test looks like. The known Group C samples are best tested separate from each other. To prepare a known sample for testing, add 2 to 3 drops of the solution containing the cation you wish to test to about 0.5 ml of water. Remember that experimental conditions, such as ph, for the known tests must be the same as that for the unknown. Outline of Procedure: Use the template given below for a written summary of your proposed procedure and use a flow diagram in your notebook to record all observations for each step in your procedure. Part of your procedure should include flame tests of the known samples. It is likely that your initial procedure will need to be modified, depending on your results in lab, as you progress through the analysis. Chemistry and Relevant Background Information: Experimental Procedure: Dr. L.J. Larson 2 June 27, 2008

3 Prelab Exercise: 1. Explain why flame tests are necessary in the Group C analysis. Name: 2. To help you determine a proposed separation and identification scheme for the Group C cations, answer the following questions: a) Refering to Table 2 in the Introduction to Qualitative Analysis, list K sp values for the hydroxide, oxalate, sulfate and carbonate salts of the Group C cations. Organize your values in table form below. b) Now study the solubilites of the Group D cations, referring again to Table 2 of the Introduction to Qualitative Analysis. Which of the anion(s) listed in part (a) could possibly be used to selectively precipitate the Group C cations while leaving the Group D cations in solution? c) Now write the solubility rule given for sulfates in Table 3 of the Introduction to Qualitative Analysis. d) Now write the solubility rule given for oxalates in Table 3 of the Introduction to Qualitative Analysis. Dr. L.J. Larson 3 June 27, 2008

4 3. Complete a proposed flow diagram below for Group C analysis. For each step, indicate the chemical form of each Group C cation that is expected at the given step. Also indicate where in the analysis you will perform a flame test. Ca 2+, Sr 2+ and Ba 2+ and Group D Dr. L.J. Larson 4 June 27, 2008

5 Follow-up Questions: Name: For Numerical Problems, you must show all work for credit! 1. Describe the colors emitted by calcium, strontium and barium ions when placed in a flame. 2. Many elements emit light of specific wavelengths when they are placed in a flame. Historically, chemists used such emission fingerprints to determine whether certain elements were present in a sample. Some characteristic wavelengths of emission for several elements are: Ag nm Fe nm Au nm K nm Cu nm Mg nm Ni nm Na nm a) Which of these wavelengths are in the visible part of the spectrum? (Refer to your textbook or another reference source to help you determine this.) b) Which of the above wavelengths has the highest energy per photon? Which has the lowest energy? c) When burned, a sample of an unknown substance is found to emit light of frequency s -1. Which of the above elements is probably in the sample? Show work to support your answer. d) When placed in a flame, copper emits light that appears green to our eyes, evidence that copper exhibits at least one more emission line than the one that is listed above. Referring to your textbook, roughly estimate the wavelength in nanometers of the green emission from copper. 3. A solution that has a ph of 3 is known to contain barium chloride. Ammonium oxalate is added, and no precipitate forms. Ammonia is then added until the solution tests basic to ph paper and a precipitate is observed to form. Explain these observations. Dr. L.J. Larson 5 June 27, 2008

6 4. Complete a flow diagram showing the actual steps you used to identify the Group C cation in your unknown solution. For each step, indicate the chemical form of your Group C cation and the color of the solution and/or precipitate. Also indicate the flame test color where applicable. Dr. L.J. Larson 6 June 27, 2008