In the Laboratory
Designing a Self-Contained Qualitative Analysis Test for Transition Metal Ions Y. S. Serena Tan and B. H. Iain Tan Raffles Junior College, 53 Mount Sinai Road, Singapore 298106 Hian Kee Lee Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260 Yaw Kai Yan* Division of Chemistry, School of Science, National Institute of Education, Nanyang Technological University, 469 Bukit Timah Road, Singapore 259756 T. S. Andy Hor* Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 119260
Although instrumental analytical methods have largely replaced qualitative analysis (Q.A.) in industrial chemical analysis, principles of qualitative analysis continue to form the basis for the development of many automated analytical techniques. Q.A. is also an effective means of fostering analytical and deductive skills in chemistry students. There is no simpler way for a student to understand and be familiarized with chemical reactions than through Q.A. (1 ). The classical Q.A. tests are based on those developed by Fresenius (2) over a century ago and rely exclusively on the use of many known external reagents to identify a few unknown chemicals. Although such tests have succeeded in familiarizing students with the basic chemical reactions of the common laboratory chemicals, they have unfortunately become predictable, with a bias towards memory work rather than analytical and deductive thinking. With the present global trend toward education that emphasizes thinking skills over knowledge drilling, a different approach to Q.A., which reflects the current demands, is pertinent. In 1940, MacWood et al. proposed an alternative method for examining a student’s skills in Q.A. (3 ). Instead of relying on external reagents to identify unknowns, students are given samples of a number of unknowns (usually 9, hence the term “9-bottle” experiment) and are challenged to identify these unknowns only by intermixing or heating these chemicals. Students are told the names of these unknown solutions but not how they correspond with the given samples. Since the publication of this “n-bottle” approach several variations of the test have been attempted (4, 5). The published works, however, invariably involve the p-block ions. Similar tests on transition metal ions are not known. We report here the first systematic approach to designing a “self-contained” test for transition metal compounds. These compounds are exciting samples for Q.A. tests because of the myriad delightful color changes they can exhibit. Clearly, there should be a separate test devoted to transition metals through which the knowledge of students on these species and their metathesis and redox reactions can be examined. *Corresponding authors.
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Advantages of the Self-Contained Approach In this test, it is virtually impossible to rely on one- or two-step deductions. Deductions and identification arise from numerous observations in the intermixing of the unknown solutions. This exercise hence demands a student to integrate knowledge of basic chemical reactions with sound logical deductions and analysis. Furthermore, variation can be created by changing the list of unknowns and by adding or removing one or more unknowns from an existing list. Such modifications could alter significantly the procedures leading to the answers. This flexibility is expected to encourage initiative and independence in thought and learning, as well as creativity. The self-contained test also minimizes the use of chemicals and hence reduces significantly the disposal problems and wastage of chemicals. In terms of organization for a big class, it is also logistically simple, as no external aids (chemicals or apparatus) are required. Problems Associated with the Design of SelfContained Q.A. Tests for Transition Metals The absence of documented work in self-contained Q.A. tests for transition metal compounds could be attributed to the more complex nature of transition metal compounds and their reactions. First, there is a lack of clear acid–base properties in many transition metal compounds. Second, their precipitation reactions may not be simple and well defined. Third, identification of transition metal compounds often relies on color changes. Although such color changes are common, they are not necessarily characteristic or well understood. Fourth, complex equilibria often exist in solutions, which could give rise to several different species. These equilibria are often concentration- or pH-dependent. A major challenge in the design of self-contained Q.A. tests for transition metals is to involve compounds and reactions that are as simple as possible without sacrificing the level of difficulty of the analysis. It is also desirable that students be provided with a theoretical framework that allows them to explain the reactions observed. This allows identification
Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
In the Laboratory
of reagents based on application of knowledge of the properties of the reagents being analyzed. The Proposed Q.A. Test A transition metal Q.A. test with the following nine solutions is proposed: K 2S2O8 /KOH colorless KI/H2SO4 colorless CrCl3 gray-blue MnCl2 pale pink/colorless FeCl3/HCl yellow CoCl 2 pink-red NiCl 2 green CuCl 2 blue ZnCl 2 colorless This list covers a wide range of transition metals. Since the test is designed for transition metals, the reactions of the transition metal compounds should be limited to the transition metal ions rather than their associated main group counterions. The inclusion of the main group species is hence restricted to only three simple ions, namely, Cl ᎑, H+, and SO42᎑, which do not play an active role in the process of deduction. To promote the characteristic redox reactions and color changes of the transition metal compounds, two main group compounds are introduced: an oxidizing agent (K 2S2O8) and a reducing agent (K I). Although these two compounds are meant to aid in the test, they remain as anonymous to the students as the other seven transition metal compounds so as to pose them a greater challenge. This Q.A. test therefore requires students to be familiar with the redox reactions and color changes of most of the colorful 3d-metals. This makes it a very useful exercise for students who are learning transition metal chemistry. A test in which students could identify transition metal compounds primarily by their colors would be of little value. This test ensures that the students cannot base their deductions purely on the colors of the solutions because half of the solutions are colorless (MnCl2, although pale pink when concentrated, looks virtually colorless at concentrations less than 0.1 M). Furthermore, students will not be allowed to conclude the identity of a solution based solely on its color, unless absolutely necessary (e.g., between NiCl2 and CoCl2, see below). A further and more conclusive observation in support of their hypothesis would have to be made before conclusions are reached. Suggested Procedure for Administering the Test All solutions are prepared at 0.1 M except for CrCl3, which is 0.02 M in order to lower its color intensity and make it more compatible with the other solutions. The lower concentration does not affect, qualitatively, its reactions with the other solutions. The FeCl3 solution is prepared with the addition of an approximately equimolar quantity of HCl to counter hydrolysis. The amount of HCl used should be just enough to keep the FeCl3 in solution. The K 2S2O8 solution is prepared with the addition of 2 molar equivalents of KOH, because we found that most of the reactions between persulfate and transition metal ions in neutral or acidic media are kinetically too slow for Q.A. purposes. The KI solution, on the other hand, is made acidic with the addition of an
equimolar quantity of H2SO4. This is to ensure that a brown coloration (of I3᎑ ) is observed on mixing the KI and K 2S2O8 solutions. Iodine is unstable in basic solution, disproportionating into the colorless iodide and iodate(V) ions (6). No external aids (acids, bases, and other chemical reagents, pH paper, litmus, gas-detection devices, heating, etc.) are allowed. One-and-a-half hours should be ample for students to complete the test. The tests are carried out in test tubes and the reagents are provided in small bottles with eyedropper tops. About 1 cm3 of each reagent is used in each test. Suggested Procedure for Solving the Test From Table 1, it is apparent that aside from NiCl2 and CoCl2, each solution has a unique set of observations when mixed with all the other solutions. NiCl2 and CoCl2 can be differentiated by their colors. Hence by starting with any of the nine unknowns, different methods can be adopted to solve the test. Identifying the redox reagents first would be very helpful, since they would help classify the various transition metal compounds as either reducing or oxidizing agents. Thus, knowing that K2S2O8 and KI are colorless, it is best to start with a colorless solution by mixing it with each of the other eight solutions. Each colorless solution would give rise to unique observations, and from there, the identity of the chosen colorless solution can be deduced. Different paths can then be taken to identify all the other solutions. The test can still be solved if any of the colored solutions is picked first. Most of the colored solutions are, however, useful only for identifying one or both of the solutions of the main group ions. Discussion This test demands from the students a good knowledge of the reactions of the transition metal compounds involved. Hence, on its own, it might be more appropriate for first-year university students or advanced students at pre-university level. However, the level of knowledge required can be easily lowered by providing students with some information, without compromising the challenging nature of the logical deduction process. For example, since most reactions in this test are redox reactions, students can be given reduction potential diagrams (6, 7) of the elements involved in the Q.A. test. From these diagrams, students can get an idea of the thermodynamic feasibility of the various possible redox reactions involved in the test and the likely reaction products. Students may also be asked to carry out experiments, in an earlier laboratory class, to construct the table of observable reactions between the ions involved in the test. The students should realize from these experiments that some of the thermodynamically feasible reactions are not observed owing to kinetic reasons. At this stage, it may also be helpful to include a solution of 0.10 M NaOH in the experiment. This would enable students to distinguish the effects due only to the precipitation of a hydroxide (1) from the effects due to oxidation. The level of difficulty of the test can also be altered by simply removing one or two solutions from the list. For example, removing CuCl2 would make the test slightly more difficult because of the distinctive reactions of Cu2+ with both KI and basic K 2S2O8. Removing one of the other transition metal compounds would make the test slightly simpler, since most of them do not serve a vital role in solving the test. However, K2S2O8 cannot be removed by itself, as it is the only
JChemEd.chem.wisc.edu • Vol. 75 No. 4 April 1998 • Journal of Chemical Education
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In the Laboratory Table 1. Observable Reactions on Intermixing Solutions K2S2O8 / KOH colorless
K I/ H2 SO4 colorless
K I/ H2 SO4 colorless
brown solution ( I3᎑ ) S2O82᎑ + 3 I ᎑ → 2SO42᎑ + I 3᎑
N.A.
CrCl3 gray-blue
grayish-green ppt [Cr(OH)3] Cr3+ + 3OH ᎑ → Cr(OH)3
no reaction
MnC l2 colorless
brown ppt (MnO2) Mn2+ + 4OH ᎑ + S2O82᎑ → MnO 2 + 2H2O + 2SO42᎑
no reaction
FeC l3 yellow
brown ppt [ Fe(OH) 3 ] Fe3+ + 3OH ᎑ → Fe(O H)3
brown solution ( I3᎑ ) 2 Fe3+ + 3 I᎑ → 2Fe2+ + I3᎑
CoC l2 red
black or very dark brown ppt [Co(O H)3 ] 2Co2+ + S2O82᎑ + 6 OH ᎑ → 2 Co(OH) 3 + 2 SO42᎑
no reaction
Ni C l 2 green
black ppt ( Ni O2 ) Ni2+ + S2O82᎑ + 4 OH ᎑ → Ni O2 + 2 H2O + 2SO42᎑
no reaction
CuCl2 blue
blue ppt [Cu(OH)2 ], turns black ( mainly CuO)a slowly Cu2+ + 2 OH ᎑ → Cu(O H)2 Cu(OH)2 → CuO + H2O
cream ppt (CuI) in brown solution ( I3᎑) 2Cu2+ + 5 I᎑ → 2Cu I + I3᎑
Z nC l2 colorless
white ppt [ Zn(OH)2 ] Zn2+ + 2OH ᎑ → Zn( OH)2
no reaction
Note: The solutions of transition metal ions do not react with each other. a The powder X-ray diffraction pattern of the black precipitate isolated from the reaction mixture is virtually identical to that of CuO. It is not known why the Cu(OH)2 → CuO conversion occurs so readily at room temperature in this reaction mixture. The presence of chemical species that catalyze this conversion cannot be ruled out.
reagent in the test that can differentiate MnCl2 and ZnCl2. A possible problem with this test is that it might not be challenging enough for students who are able to identify, by pure inspection, the five colored transition metal compounds. However, as they are not permitted to draw conclusions based solely on color inspection, these students must still work out a reasonable scheme that would allow them to identify the solutions using the minimum number of steps. This task is still formidable and would require clever experimentation. We are currently working on a different set of sample solutions that contain similar metals (as listed) but different anions. It is our objective to identify anions that would modify the colors of the unknowns to a less recognizable state, but play no active role in the characteristic reactions and hence not alter the principle and strategies of identification of the metals. The inclusion of solutions of complexing agents—for example, NH3, ethylenediamine, or EDTA—as unknowns is also being considered. Acknowledgments We are grateful to the Shaw Foundation (Singapore), which supports this work through the Science Research Programme (SRP) administered by the Ministry of Education
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(MOE) and the Faculty of Science, National University of Singapore (NUS). We thank E. L. Lim for some preliminary work and Y. P. Leong for stenographic support. Technical support from the Department of Chemistry of NUS is thankfully acknowledged. We also thank Lily Yeo, University of Birmingham, for performing the powder X-ray diffraction analysis of the CuO precipitate. Two of us (Y.S.S.T. & B.H.I.T.) are grateful to MOE and NUS for placement in the SRP program. Literature Cited 1. Svehla, G. Vogel’s Qualitative Inorganic Analysis, 6th ed.; Longman: London, 1987. Liang, M. J. Chem. Educ. 1993, 70, 666–670. 2. Fresenius, C. R. Qualitative Chemical Analysis, 10th ed.; Churchill: London, 1887; pp 291–317. 3. Macwood, G. E.; Lassettre, E. N.; Breen, G. J. Chem. Educ. 1940, 17, 520–521. 4. Steig, S. J. Chem. Educ. 1988, 65, 360–361. 5. Finholt, J. E. J. Chem. Educ. 1984, 61, 849. 6. Jolly, W. L.; Modern Inorganic Chemistry, 2nd ed.; McGraw-Hill: New York, 1991; Chapter 5. 7. Lange’s Handbook of Chemistry, 14th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1992; pp 8.124–8.139.
Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
