LACQUER COLOR FILTERS FOR QUALITATIVE FLAME TESTS' JOHN A. BROWN Georgia Institute of Technology, Atlanta, Georgia
T m s paper describes the manufacture of certain transparent colored lacquers and their use as color filters in the semispectroscopic detection of the alkaline metals. In theory, it should be easy to see one colored flame in the presence of another by observing the combined flame thru a color filter which transmits the spectral lines due to one flame and blocks those of the other. This principle is employed in the familiar flame test for potassium, wherein a cobalt glass is used to filter out the yellow light of sodium. The filters described herein extend this application to include barium and strontium. An examination of the visible flame spectra of barium and strontium salts reveals that each has bright lines in a region of the spectrum where the other has none. (This is not true of calcium, all of whose bright lines are nearly duplicated by either barium or strontium.) Therefore, a color filter which blocks red and yellow light, but passes green, should enable one looking through it to see a barium flame, but not a strontium flame. Similarly, a filter passing red, but blocking yellow and green, should enable one to see a strontium flame, but not a barium flame. For the past several terms, we have been using such filters as the sole means of detection of barium, strontium, sodium, and potassium in an abbreviated course in qualitative analysis a t Georgia Tech. The analytical procedure is simple: First barium and strontium are separated from sodium and potassium by precipitation of the barium and strontium with (NH&C03. Then the precipitate, after washing, is dissolved in a few drops of concentrated HCI. A platinum wire is dipped into this solution, and held in the hottest part of a Bunsen flame and the resulting colored flame is ohserved 6rst through a red-transmitting "Strontium Filter," and then through a green-transmitting "Barium Filter." Similarly, the sodium-potassium flame is observed with the naked eye for sodium, and through a red-and-blue-transmitting filter for potassium. If any flame is visible through the "Strontium Filter" strontium is reported; if any flame is visible through the "Barium Filter" barium is reported; and similarly for sodium and potassium. Actually, since the filters are seldom perfect, a glimmer of the wrong flame may sometimes be seen; but these glimmers can easily be distinguished from the strong, steady flames of positive tests. Presented to the Joint Meeting of the Georgia Academy of Science and the Georgia Section of the American Chemical Society, Atlanta, Georgia, April 1&19, 1952.
40W
5WO
wave length, k.
6WO
7MH1
In the case of barium and strontium, the strength of the flame is not dependent upon the concentration of the metal ion, but is reasonably constant for any concentration which is sufficient to yield a precipitate with Preparation of the Filtersa
Lacquer Base Acetone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 ml. 5-6 Sec. nitrocellulose.. . . . . . . . . . . . . . . . . . . . . . . . . . 30 gm. Cellomlve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 ml. Methyl amyl acetate.. . . . . . . . . . . . . . . . . . . . . . . . . . . 200 ml.
Ba~iurnFiller Lacquer base.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 ml. Dyb soh. (140 mg. DuPant Luxol Green BL in 5 ml. 95%ethanol). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ml. Cdco acetate green 2B.. . . . . . . . . . . . . . . . . . . . . . . . . 210 mg. Polassium Filler Laoquerbase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 ml. Calcosine pure blue BO. . . . . . . . . . . . . . . . . . . . . . . . . 460 mg. St~ontiumFilter Lacquer base.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 ml. Cellanthrene brilliant blue (DuPont). . . . . . . . . . . . . . 70 mg. Calcasine green V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 mg. Cdco acetate teed.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 mg. Calcozine violet ex. conc.. . . . . . . . . . . . . . . . . . . . . . . . . 50 mg. Caloo diazo black.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 mg. a
Formulas courtesv of The Sunfilm Corr,
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(NH&C03, since the precipitate obtained is dissolved in the minimum amount of HCI, leading to a reasonably ronstant concentration in the flame solution. The sodium and potassium flames do vary in strength with the conrentration since no precipitate is ever obtained, and this variation may be used to estimate concentrations. Each student prepares his own set of filters by painting inch-square spots of special lacquers onto glass plates and allowing them to dry. The lacquers themselves are prepared by coloring nitrocellulose solutions with appropriate textile dyes, as shown in the table. The nitrocellulose is rather slow to dissolve, and the mixture should be stirred in a closed vessel-or better, shaken-until the solution is completely smooth. Then the dyes are added to portions
JOURNAL OF CHEMICAL EDUCATION
of the lacquer base and agitated until solution is complete. Since most organic dyes are somexvhat fugitive to light, the finished lacquers should be stored in brown bottles and the finished filters kept in the dark when not in actual use. The figure shows the transmission spectra of typical lacquer films on glass. The exact transmission values will, of course, vary somewhat depending on the thickness of individual filters. The filters used to make these spectra were entirely satisfactory on actual flame tests. ACKNOWLEDGMENT
The author wishes to express his appreciation to Dr.
W. C. Carter of the Georgia Tech. Textile School for his help in suggesting likely dyes.
