lnstituto di Chimica Analitiw Universita' deali Studi 70126 Bari, Italy
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T w o problems in flame emission analysis, encountered during t h e teaching of qualitative analysis for freshman, a r e t h e source of gaseous atoms a n d t h e filtering of t h e visible radiation t o be is01ated.l~~ These problems may be roughly resolved by using platinum wire a s a source of gaseous atoms, a n d cobalt glass a s a bandpass filter tbr e m l ~ s i o nradiation of potassium. S u c h a procedure is mainly lunited by the brevity of the emission time (few seconds) when a small crystal of the sample is introduced into t h e flame. T h e time of observation is f u r t h e r reduced when the crystal undrrgoes crarking. T h e solution u, these problems should be n flame ~ h o t o m r t e or r a spectrometer in each laboratory," h u t serious difficulties &ay arise when s t u d e n t s having n o experience in electronics a n d instrumental analysis handle such expensive a n d sophisticated apparatus. I n this note we describe a simple a n d inexpensive apparatus assembled in order t o provide our s t u d e n t s with
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1) A device far a continuous observation of the characteristic tlame colors shown by the alkali and alkali-earth metals.
Figure 1. Schematic of the apparatus.
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.. 2) An instrument providing an approach to quantitative analysis by flame emission (a commonly employed technique in chemical and clinical laboratories for determination of electrolytes like Na, Li, K). Experimental The components employed to set up the apparatus were: 1)a Teclu gas burner,% a sprayer for chromatographic reagents (Shandon), and 3) An air compressed line (in this case supplied with a compressor asoirator TMe B25 MDQU. Power $40 -- HD. . ~. r o d u dbv S.C.O. Moteun. SBgal Co, St. Denis Seine, France). The sprayer (See Fig. 1) is placed in such a way as to spray the droplets of sample solution (5) Into the flame, whose upper zone asumes a steady coloration. The students are then able to compare the differences in behavior of many metals on the platinum wire and in the continuous spray. In order to avoid the diffusion of the droplets in the room, the spray burner system should be placed in a fume hood. It is possible to get a quantitative evaluation of the phenomenon by employing an optical sensor (6 in Fig. 1): this sensor is placed a t one end of the tube (7). aligned perpendicularly to the flow of the sprayed solution, and pointing through the horizontal slit (8), onto the bright strip (4) in the flame. Proper optical filters (10) may be inserted on the rack (9). The optical sensor was a photoresisto+ inserted in an electric circuit as shown in Figure 2: we may remark that such a simple circuit ineludes no electronic device and therefore is quite understandable even to students having only a basic knowledge about direct currents. A 100 Kohm potentiometer is the Load Resistor RL. A value of 1 Kohm is selected for the Measurement Resistance RM. The measurements are performed by a Heath pen recorder ( f a variable from 10-250 mV). A common multimeter can also be alternatively employed.
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Dean, J, A,, "Flame Photometry," McGraw-Hill Book Company, Inc., New York, 1960, Chap. 3,4,6. Willard, H. H., Merritt, L. L., and Dean, J. A., "Instrumental Methods of Analysis,"4thEd., D. Van Nostrand Company, Inc.,New York, Chap. 11. Druding, L. F., and Lalancette, R. A,, J. CHEM. EDUC., 51,527 (1974). Cadmium sulfide photoresistor "Moririea" MPY-25R59 (R dark = 10 M ohm, Rlooolur= 3001500 ohm). Catalogue G.B.C. (Italy) no DFl1554-00.
Figure 2. The electrical circuit.
Figure 3. Plot of measured voltages versus lnhium wncentrations.
In order toevaluate the performances of this apparatus we made photometric measurements when solutions containing lithium, potassium, or sodium chloride, respectively, were sprayed into the flame. In Fieure 3 the measured voltaees versus lithium concentrationsgn the range 50 i1,000ppm are plotted. For this range of concentrations the exnerimental noints fall upon a s t r a-i ~ h t line passing through the origin. In Fieure 4 the standardization curve for ~otassiumsolutious in the range 100 + 5,500 ppm is plotted; the last value is eauivalent (in e iousfi) to 1.000 r.u. m of lithium. The optical filters employed were, respectively, solutions5 of 0.1 M F e i N O h in acetic acid.. 3 M.. and 0.01% w/v of methyl violet in water ?pathlength 1 cm) contained in inexpensiie ~
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Figure 4. Standardization curve far potassium solutions spectrophotometric polystyrene cells (Conacon 'isaItaly). The data obtained with Li, Na, and K solutions (all 1,000 ppm) are compared mV cation Li 32 Na 35 K
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The greater sensitivity shown by the potassium is due to the fact that the employed photoresistor has a maximum response in the near infrared zone of the spectrum. Varker, C. A,, ''Photoluminescence of Solutions," Elsevier Publishing Company, 1968, pp. 186-188.
Volume 53, Number 11. November 1976 / 735
