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GEORGE L. GILBER; Denison University Granville,OH 43023
tested demonstrations Sodium D Line Emission from Pickles Submitted by Jeffrey R. ~ p p l i nand ~ ' Fredrick J. Yonke University of Kentucky Lexington, KY 40506 Richard A. Edgington Rockhurst College Kansas City, MO 64110 Steve Jacobs Mr. Wizard Studio 132 Stagecoach Road Canoga Park, CA 91307 Checked by
Richard F. Jones Sinclair Community College Dayton. OH 45402
A variety of methods are available to demonstrate the phenomenon of atomic emission, among them the simple flame test and the low-pressure gas emission tube. We present here an alternate technique to generate visible atomic emission of sodium atoms that should be useful as a classroom demonstration. This new laboratory source relies on electrically induced luminescence from pickles to produce the yellow sodium D line emission. In his investigations of solar spectra beginning in 1817, Josef Fraunhofer observed (1)many "dark" lines in addition to the exoected brieht lines due to solar emission. Fraunhofer dc'signated the more intense dark lines with letters. startine with the letter Aat the red endof the sowtrum. He notectbat the "D" dark line in the solar spectkm corresponded in wavelength to bright yellow light obsenred in the spectrum of a lamp. Soon after this discovery, these dark lines in the solar suectrum became known as the hnunhofer lines. In investigations of flame spectra of metal salts, Gustav Kirchhoff duplicated (2, Fraunhofer lines in his laboratow and concluded in 1859 that the solar "Dmline was due to sodium in the atmosphere of the sun. Thus, the yellow sodium emission near 589 n m has been referred to historically by Fraunhofer's nomenclature as the sodium D line. It is now known that the intense yellow sodium emission is actually a pair of closely spaced lines a t 589.0 n m and 589.6 nm, commonly referred to as the D line doublet. These lines arise due to transitions from the 'Pin (589.6 nm) and 'Pm (589.0 nm) excited states of sodium to the ground state, 2Sla. The lower (dashed) curve in the figure shows the D line doublet observed when sodium chloride is introduced into a natural gas flame. This spectrum was recorded using a Jarrell-Ash model 1233 spectrograph (1200 lineslmm) equipped with a diode array detector controlled by an EG&G Princeton Applied Research OMAIII optical multichannel analyzer system. Light from the flame was channeled through a fiber optic probe to the 25 'Author to whom correspondence s h o ~ abe aaaressea. Present address: Department of Chemfstry,Clemson University, Clemson, SC 29634.
250
Journal of Chemical Education
Wavelength
(nm)
Emission spectra of sodium in the range 585695 nm. The bottom (dashed) trace is due to a flame test with NaCI. The upper (solid) trace is due to sodium emission from a dill pickle. mm entrance slit, and background subtraction was employed. The top trace (solid line) in the f w r e is due to light emitted from a dill pickle that is under the influence of line current (115 V AC) passed through it via two metal electrodes plunged into both ends. Experimental conditions were identical to those used to record the NaCl calibration spectrum. This pickle emission has been demonstrated earlier to a wide audience (3), and is documented here as corresponding to the D line emission of sodium contained in the pickle. A scan of the spectrograph to longer and shorter wavelengths did not reveal additional lines due to other species. Thus, the pickle--under the conditions of the flowing current-is a source of pure atomic sodium emission. An apparatus for demonstration of this effect is constructed easily by a competent electrician. A source of line current, preferably with a switch and fuse, is needed to deliver voltage to two electrodes. A convenient line source is a power strip equipped with a circuit breaker. The authors prefer to use forks as the electrodes, with line current delivered through insulated wires terminated in alligator clips. Electrodes should be inserted into a large pickle and the pickle stabilized on an insulated support. Dill pickles work better than sweet pickles due to their higher salt content. They also emit less foul-smelling odors than the sweet varieties. The electrodes are next connected to the line current source and the switch can be thrown once the operator is sure that the apparatus is not in contact with anyone. A short time later, dependent on electrode placement, arcing will wmmence inside the pickle, generating yellow light. If a stubborn pickle refuses to glow, placing the electrodes closer together (with the line current oil?) will usually solve the problem. If possible, a sodium salt
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flame test can be Derfonned to convince observers of the cause for the glow. In an effort to understand the observed pickle emission, we have investigated electrolysis of various solutions using line current. Sodium salt solutions with water exhibit !lashes of yellow light around both electrodes. Predictahly, different metal salts and different electrode materials may yield different colored flashes as well. Clearly the presence of a relatively high level of sodium chloride in pickles, coupled with the resulting conductance, give riseto the luminescent effect.The pickle acts as a light diffuser to yield an overall glow from the sparking within the pickle. We presume that the emitting sodium atoms are present in gaseous pockets produced near the sparking electrodes. This demonstration with pickles is easy and fun. It is an effective- method to introduce atomic emission to chemistw clas~esat all levels. It can be a good addition to dcmonstra". tion shows during a focus on "household chemistw". The unexpected gene&ion of light from a pickle often initiates a livelv discussion amona students and casual obsewers ~~
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Acknowledgment Special thanks to Don Herbert and Donovan Porterfield for helpful discussions. RAE gratefully acknowledges support from NSF as a 1990 Research Experiences for Undergraduates (REU) participant. Literature Cited
Lowering the Surface Tension of Water: An Illustration of the Scientific Method Submined by
Paul G. Jasien California State University-San Marcos San Marws. CA 92096 Glenn Barnett University of San Diego San Diego, CA 92110 Checked by
David Speckhard Loras College Dubuque, IA52001
There are a countless number of demonstrations illustratine the uniaue macrosco~ic~rooertiesof water. These can'be related readil; to'the polar nature of the molecule. Many of these demonstrations illustrate the large surface tension that a water surface possesses (1, 2). One oarticular demonstration of this type that was recently in the classroom stimulatkd a good deal used of discussion, particularly among a number of colleagues. This demonstration illustrates how the scientific method can be used to stimulate student participation and also increase understanding. The demonstration consists of floating on the water surface either a nonwettable substance such as pepper or a small loop of string. The center of the water surface is then touched with a bar of soap. The result is that the pepper on the surface disperses fmm the center of the water toward 'Some brands of plastic wrap have been found to work better than others.
the outer regions of the container. In the case of the string, a circular shape is formed from the arbitrarily shaped loop. The speed a t which these changes - take place can be quite dramatic. The exolanation usuallv given for this Dhenomenon is that the soap lowers the sigace tension of the water a t the point where the soap molecules have dissolved. This results in the water molecules in the center of the surface feeling an imbalance in the forces acting upon thembv the adjacent water molecules. Since the attractive forces between the water molecules are larger than those between water and soap molecules, the central water molecules are pulled to the outer regions of the container, carrying the pepper particles or string with them. However, based on this one experiment, this explanation is not the only one possible. It is a t this point where the use of the scientific method may be integrated into the demonstration. The idea of the surface of water acting as an elastic membrane or skin is qualitatively quite satisfying. The shearing of this "membrane" by a substance such as soap that lowers the surface tension is conceptually simple to visualize, but is not the only explanation consistent with the results. An alternative hypothesis based on the results of this one simple experiment is still plausible. Soap molecules possess both a polar and nonpolar portion and are relativelv insoluble in water. Could it be that surface layer away from the center the rapid motion of is due to a soreadine of the soao film across the surface of the water? ~ c c o r d i nto~this vikw, the interaction is seen more as apush of the snrface water molecules away from the center as opposed to the other hypothesis that views the motion as a pull on the inner molecules by the outer molecules. Certainly the one demonstration experiment has not distinrm~nhedbetween these two dausible hv~otheses.Thus. according to the scientific meihod, an additional experi: ment that differentiates between the hvootheses is C of needed. Such an experiment comes from the E I ~ S S ~book Bow (3).The experiment can be done similarly to the first. although a mneh thinner layer of water is necessary for dramatic results. In this experiment, a thin layer of water that has been colored with food coloring (for enhanced visibility) is placed on a clean, nonwettable surface such as plastic wrap1, aluminum foil, etc. The water layer should be spread out as thinly as possible for best results. The thin laver. without boundaries. alleviates ~ossibleauestions conc&ning edge effects that would arise if the water was olaced in a Dan. Next. 1-2 droos of a substance that is totafiy misciblk with water is caiefully placed (to avoid splashing) in the center of the water layer. Obvious substances are methanol or ethanol. The surface immediately expands, leaving a virtually dry center. (The size of the dry central portion left behind is many times larger than the size of the dropb) that were added). The use of colored water and plastic wrap make this demonstration wellsuited for overhead projection. The results of this experiment certainly corroborate the original hypothesis of the spreading of the surface layer as being due to a force arising from an unequal pull on the water molecules in the center of the surface. The results also seem to disprove the hypothesis that the water surface movement occurs solely due to apush of a surface film. However, due to the differing miscibilities of the soap and methanol, some effect by a surface film camot be entirely ruled out in the case of the soap. The use of these two simple demonstrations provides the students with valuable insights into the nature of how the scientific method works. By attempting to explain the results of the first demonstration, the students learn how hypotheses are generated and may participate in the pro-
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Volume 70 Number 3 March 1993
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