D. T. Haworth, R. J. Starshak ond J. G. Surak Morquette University Milwaukee, Wisconsin
I I
A Huorescent inclicator for a Confirmatory Test for the Aluminum Ion
M o s t classical schemes of qualitative analysis detect slumiuum either as a gelatinous white precipitate of Al(OH)a or as the cherry red "lake" formed by "Aluminon" reagent (ammonium salt of aurintricarboxylic acid) being adsorbed on A1(OH)s. This test is not specific, since "Alumiuon" forms red lakes with many other substances. The "Aluminon" red lake test with aluminum hydroxide can be masked by excessive amounts of Cr(OH)s and Fe(0H)s and it is recognized that silicates from the glassware may also produce a "positive" lake test. We have examined several of the newer complexiug agents and have worked out a new confirmatory test
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Journal of Chemical Educafion
for the alun~inumion using PAN [1-(2-pyridy1azo)2-naphthol] as a fluorescent indicator. PAN was found to be insoluble in aqueous solution but it was soluble in a variety of polar solvents such as the alcohols, ethers and acetone. Addition of one drop of a 0.1% PAN in 95% ethyl alcohol to a M A13+ ion in ethyl alcohol gave a n orange-red solution. A similar color was also observed in methyl, propyl, butyl, amyl, and hexyl alcohols and also in acetone. A yellow-brown solution was obtained with diethyl ether, benzene, carbon tetrachloride and chloroform. It was also observed that the alcohol and acetone solutions of the Al-PAN complex fluoresced under ultra-
violet light. It is on the basis of this fluorescence that a new test for the AIS+ion was devised. The method assumes that the currently used qualitative separation scheme is followed where A1(OH)s is separated from either Fe3+and Cr" or from Cr'+ and Zn2+.
The student can view his sample through a %in. slot in the top of the box. The dimensions of the box are 20 X 9 X 6 in. with l-in. spacing between the test tube holes. The slit is 3 in. from the tube side of the box.
The AI(OH)$ precipitate is washed with distilled water snd dissolved in a minimum amount of 6 M HNO.. The solution is centrifuged and the decantate is transferred t o a casserole where it is boiled just to dryness. Upon cooling the solid residue is dissolved in 1 ml of 95% ethyl alcohol to which is added 1 drop of 0.1% (w/v) PAN in ethyl alcohol. The solution is transferred to a semimicro test tube and exposed t o ultraviolet irradiation. The presence of aluminum is indicittedby a brilliant orange fluorescence. The intensity of the fluorescence diminishes rn the alcohol solution is diluted with water; however, the detectability of fluorescence appears to be valid over the range of 25-100Yo (v/v) ethyl alcohol.
Similar treatment of aqueous solutions of Cr3+, Few, and Zn2+ produces blue-grey, red-brown, and pink solutions respectively with PAN in 95% ethyl alcohol. None of these solutions fluoresced; however, the Zn-PAN complex will give a trace of fluorescence in acetone and ethyl ether. An inexpensive ultraviolet box which can be used to detect this fluorescence in a general chemistry laboratory is shown in the diagram. It consists of two black light blue fluorescence tubes (F15T8/BLB) and holes in the top of the box for fifteen semimicro test tubes.
Ultraviolet box in use.
With this apparatus the limit of detectability is M A13+ ion in alcohol when mixed with M PAN in ethyl alcohol in a 2: 1 ratio. This work was part of the study aided by the National Science Foundation. We are indebted to W. R. Landis for preparing the diagram of the apparatus.
Volume 41, Number 8, August 1964
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