In the Laboratory
An Improved Flame Test for Qualitative Analysis Using a Multichannel UV–Visible Spectrophotometer
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Jonathan P. Blitz,* Daniel J. Sheeran, and Thomas L. Becker Department of Chemistry, Eastern Illinois University, Charleston, IL 61920; *
[email protected] Qualitative analysis schemes (1–3) are often used in undergraduate laboratory settings as a way to introduce equilibrium concepts and logical thinking. The second semester general chemistry laboratory curriculum includes a six-week “short course” in qualitative analysis. A main component of all qualitative analysis schemes is a flame test, as the color of light emitted from certain elements is distinctive. However there are recognized shortcomings in the application and interpretation of the flame test (4–6). Poor selectivity, resulting from emission by substances other than the analyte, is a most vexing problem. For instance many instructors are familiar with detecting the emission of K in the presence of Na by performing the flame test while peering through a cobalt glass filter. This is a difficult procedure requiring a large dose of physical dexterity, and the result often being little more than a guess. A flame photometer or spectrophotometer in each laboratory has been suggested as an answer to this problem (4, 7). We have recently replaced our aging set of Spectronic 20 spectrophotometers with CCD (charge-coupled device) array spectrophotometers (8, 9). When equipped with a fiber optic probe, these spectrophotometers are well suited for use in qualitative analysis for the flame test. CCD array spectrophotometers are increasingly recognized as an important new technology now available in undergraduate education as demonstrated by recent articles in this Journal (10–12). This report describes their use in our general chemistry laboratory curriculum in the context of the flame test. Experimental All spectra were acquired using Ocean Optics Inc. (Dunedin, FL) CCD array spectrophotometers (model no. UV–VIS USB2000) with an approximate wavelength range of 200–800 nm. The spectrophotometers were equipped with a UV–visible fiber optic probe (1-m length, 300-µm diameter) for the collection of emission spectra. Ocean Optics software (OOIBase32) was used to acquire and process all data. The scope mode was used for continuous viewing of spectra as acquired and the spectra were neither signal nor boxcar averaged. A 50-ms integration time was used, a compromise discussed in more detail in the Supplemental Material.W Atomic emission spectra of NaNO3 (Mallinckodt, ACS grade, 0.05 M), Ca(NO3)2 (Mallinckrodt reagent grade, 0.05 M), KNO 3 (Mallinckrodt, ACS grade, 0.09 M), K2Cu(SO4)2⭈6H2O (0.12 M, synthesized in our lab; ref 13), ZnSO3 (Sergeant, CP grade, 0.07 M in dilute HCl) and FeS (Schaer Chemical Co., ∼0.05 M in dilute HCl) were acquired. Sample introduction was achieved using a four-inch length of narrow-gauge nichrome wire looped at one end. Emission spectra were obtained utilizing the configuration shown in Figure 1.
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The hexagonal Bunsen burner base is set into a notched piece of wood, which is aligned with the fiber optic probe placed in a standard thermometer holder. This configuration is simple, inexpensive, and reliable enough for use in the general chemistry laboratory for qualitative analysis. When implemented in the general chemistry laboratory, we set up two spectrophotometer stations per lab section of 24 students, which is more than sufficient. Hazards Dissolution of ZnSO3 and FeS in dilute HCl generates SO2 and H2S, respectively. Both gases are toxic so these dissolutions should be carried out in a hood. Loose clothes should be kept away from the Bunsen burner flame, and long hair tied back. The nichrome wire (with looped end) should be approximately four-inches long such that the hand is not too close to the flame. The bare wire can be held by hand at the end without the wire becoming hot to the touch. If desired however, the end of the wire can be inserted into a cork for a thermally insulating handle. Results and Discussion The flame emission spectra of the alkalis are unique for each atom and can be used to detect any and all combinations of the alkalis, Li through Cs. In our qualitative analysis scheme Na+, K+, Mg2+, Ca2+, and NH4+ comprise the Group IV cations (see the Supplemental MaterialW). Of these sodium (yellow), potassium (violet), and calcium (red) have
Figure 1. Acquisition of a sodium flame emission spectrum.
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In the Laboratory
Figure 2. Flame emission spectra of backgrounds and selected elements.
Figure 3. Flame emission spectra of K2(CuSO4)2.6H2O, FeS, and ZnSO3.
been analyzed via the flame test. The spectra of the flame and overhead fluorescent light background, as well as emission spectra of sodium, potassium, and calcium are shown in Figure 2. Background in a normally lit classroom arises primarily from overhead fluorescent light emission. This emission minimally interferes with the analysis since the peaks are distinct from the wavelengths of interest, and the intensity is low given the configuration and spectral acquisition parameters. The ability to discriminate sodium from potassium, and more importantly detect potassium in the presence of sodium is readily demonstrated. Calcium, which provides a color that can be easily misinterpreted visually, is not readily detected in the Bunsen burner flame.1 Thus we have eliminated the calcium flame test entirely in favor of a more definitive chemical test. During the spring 2004 semester, 131 students performed the Group IV analysis. This cohort of students had no prior experience with this analysis, yet 95% came to the correct conclusion concerning the presence or absence of Na+ in their unknown on their first try. Similarly, the sole reliance of a chemical test for Ca2+ did not adversely affect and possibly enhanced student success in its analysis, since 98% of the students came to the correct conclusion concerning its presence or absence in their unknown also on their first try. Faculty teaching this course with prior experience have noted that students’ success rates appear to be higher compared to the visual flame test. Often at the end of the qualitative analysis “short course” a salt unknown is given for identification. These salts can contain a variety of cations, and some may be double salts that can be confusing to interpret with the visual flame test. One such unknown is K2Cu(SO4)2⭈6H2O. Copper emits a characteristic blue–green color, which can mask the presence of potassium. The emission spectrum in Figure 3 of this double salt readily detects the presence of potassium without any interfering copper emission. A different complication arises from impurities. Two salt unknowns with significant impurities are ZnSO3 and FeS, the former contains Na+ whereas the latter contains both Na+ and K+. While the presence of sodium and potassium in these salts can be ruled out on the basis of solubility properties,
most students fail to make this connection. A visual flame test of these unknowns often results in a false positive identification of sodium as the yellow flare is readily apparent. The emission spectra of these two unknowns, in Figure 3, indicate detectable signals from both sodium and potassium. However under the suggested experimental conditions, the intensity of these signals fall below the recommended threshold (1000 counts) for positive identification. The inherent advantage of using the spectrophotometer for the flame test is nicely demonstrated in this case.
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Conclusions Significant improvements have been made to the traditional flame test for qualitative analysis. Application of a commercially available multichannel array spectrophotometer commonly found at many college institutions eliminates potential interferences that are visually hard to discriminate. We have constructed a very simple holder to align the fiber optic probe accessory with a Bunsen burner flame that is rugged enough to use in general chemistry. Acknowledgments JPB and DJS thank Eastern Illinois University, College of Sciences for a summer stipend to carry out some of this work and Erin Blitz for taking the photograph that comprises Figure 1. WSupplemental
Material
The complete lab for the students, including a work sheet, and notes for the instructor are available in this issue of JCE Online. Note 1. The absence of a calcium, and later a copper signal we believe is a result of these elements forming oxides in the flame, which give rise to a broader molecular emission spectrum. This broader emission is not readily detectable from the baseline.
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In the Laboratory
Literature Cited 1. Sorum, S. H. Introduction to Semimicro Qualitative Analysis, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 1960. 2. McApline, R. K.; Soule, B. A. Fundamentals of Qualitative Analysis, 4th ed.; D. Van Nostrand: Princton, NJ, 1956. 3. Hahn, R. B.; Welcher, F. J. Inorganic Qualitative Analysis, 2nd ed.; D. Van Nostrand: Princeton, NJ, 1968. 4. Fragale, C.; Bruno, P. J. Chem. Educ. 1976, 53, 734–735. 5. Ager, D. J.; East, M. B.; Miller, R. A. J. Chem. Educ. 1988, 65, 545–546. 6. Bare, W. D.; Bradley, T.; Pulliam, E. J. Chem. Educ. 1998, 75, 459.
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7. Druding, C. F.; Lalancette, R. A. J. Chem. Educ. 1974, 51, 527. 8. Skoog, D. A.; Holler, F. J.; Nieman, T. A. Principles of Instrumental Analysis, 5th ed.; Saunders: Philadelphia, PA, 1998; pp 172–176. 9. Sweedler, J. V. Crit. Rev. Anal. Chem. 1993, 24, 59–98. 10. Bernazzini, P.; Pacquin, F. J. Chem. Educ. 2001, 78, 796–798. 11. Lorigan, G. A.; Patterson, B. M.; Somner, A. J.; Danielson, N. D. J. Chem. Educ. 2002, 79, 1264–1266. 12. Goode, S. R.; Metz, L. A. J. Chem. Educ. 2003, 80, 1455– 1459. 13. Snavely, F. A.; Yoder, C. H. J. Chem. Educ. 1971, 48, 621– 622
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