Determining the Composition of an Unknown Using Atomic Emission Spectroscopy and a Spreadsheet Sort
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The four columns used as the Independent variables were klsdy
hlsid y
2hk eos ylsin2r
1'
Again, lldz was the dependent variable. An initial guess of 90' was used for y, and the linear regression was run. The four parameters determined were lla , llb2, lKab), and 112. The square root of the product of the first two parameters should equal the third. The difference was used as a guide for successive guesses for y. A minimum difference was achieved for an angle of 89.444'. The linear regression results for y = 89.444 ' are listed in Table 4. Table 4. Results of Linear Regression Analysis When y = 89.444'
Constant 0.000525 Std Error of 0.001187 Y Est R Squared 0.999818 No. of 20 Observations Degrees of 15 Freedom X
0.045377
0.031078
0.037565
0.040950
0.000138
0.012454
0.000179
Again the constant was close to zero. The values for a, b, in Table 5. The product of a and b was 26.624, very close to 26.620, the inverse of the third parameter (1/0.037565). It was not possible to determine a standard deviation, and consequently a 95% confidence range, for the angle. This method has been used by our students for analysis of a broad range of unknowns by the X-ray powder diffraction method. As most of them have been introduced to spreadsheet analysis in earlier laboratory classes, the multivariate linear repression has been readilyunderstood and implemented. he results are easy to obtain and agree well with the literature values.
c, and y are listed
a (A)
b (A)
c (A)
Results
4.694
5.672
4.942
95%
0.006
0.007
0.006
Data Treament The distances (D) of all the emission lines (on the develo ~ e d~hoterauhicfilm) were measured in mm from the dno- ca&on'line at 3883.35 A. Those distances to the red side of the carbon line were considered ~ositive.and those to the blue side were negative. The distance data was then entered into a column on the spreadsheet. We used the equation 3883.35 + 5.582 * D = W to calculate the wavelength (W) in A. In a corresponding column, the label "X" was placed next to all of the unknown wavelengths. This wavelength data was converted to values (IRN) and integrated with the database of known spectral wavelengths. The range containing both the known and unknown wavelengthswas then sorted, in ascending order, using the spreadsheet sort command. Using theIDATAIQUERY commands one can easily locate the unknown X and compare its properties with those of the closest neighbors. Readers who wish to receive a copy of our database are asked to send a 3.5-in. disk and a self-addressed, stamped envelope. We will gladly supply you with this information. Literature Cited
Table 5. Values for Magnesium Tungstate Y
89.444'
confidence range
A286
Determining the qualitative composition of an unknown by atomic emission spectroscopy can be quite tedious. For example, the analyst must determine which wavelengths of the unknown emission spectra should be compared to known wavelengths. We describe here a method to simplify this process. We have used a spreadsheet to analyze and organize data on the wavelengths of an unknown. These wavelengths were calculated and listed on the basis of their distances from an emission wavelength of carbon. Then they were compared with the wavelengths of known elements listed in the database. Using the sort ca~abilitiesof the Lotus 123 spreadsheet, one can then rompare the spectra of'the unknown with known soectra in the database and identify the components in the u h o w n . Experimental The unknowns were prepared by grinding to a fine powder and thoroughly mixing with carbon. The prepared samples were analyzed using an alternating-current arc and graphite cup electrodes. AJarrell-Ash (JAModel7830) 1.5-mphotographic spectrograph with a dispersion of 5.582 Alm was used to record the spectra.
Coefficients StdErrorof 0.000230 Coefficient(s)
Literature values
Elvin Hughes, Jr., Conaetta D. Dugas, Sam Hodges, and Sheila L. Tracey Southeastern Louisianna Univesity Hammond, LA 70402
5. Johnwn, C. S.. Jr: ~
~ d L. ~G. ~&G b ~k m and ~s ,S d v t i m in Q u n t u m Chernlatry ondPhyslas;Addiaon-Wesley: Reading, MA, 1976;pp319320. 6. Maloy, J. T. Inhbordory Techniqus inEketmonolytico1 Chornisfry; Kissinger, P. T.:Heinernan,W. R., Eds.; Deker: NewYork, 1984, Chapter 16. 7. Bard, k J.;Faulkner. L. R. Elanmehemid Methods; Wiley: New York, 1980, pp CVL""?
4.690
5.680
Journal of Chemical Education
4.92
89.67'
8. Britz, D. WtalSirnulotion inEktmehernktry: SpMger-Verlag Berlin, 1981. 9. Moo=, W. J. P h y s i d Chemishy. 4th ed.; RenticeHall: New Jersey, 1972. Willard.H.H.;Merrih,Jr.,L.L.;De~e, J.&Settltl, J r . , F . A . I ~ I m m n f d M ~ f h o d s ~(Andysis, 7th ed.;Wsdsworth: California, 1988. O ~ X - c~~ Yt o ~ ~ a g m p h y ; ~ e ~ l~ae ~ w -v no ri ~ k1968. ,~ : 11. hamff, L V. 12. Swanaon, H. E.:Fuyat. R. K StandardX-my Diffraction Powder Patterns, Vok. 1, 11,and 111.National BureeuofStandards Cimrlar No. 539. 1953 and 1954. 10.
