Cautionary Note Regarding the Phenol Color Test by Ferric Chloride in Acidic Solution The color test by ferric chloride is a convenient method far the qualitative analysis of phenols. Salicylic acid and methyl salicylate, which are the familiar phenols and used in some drugs, form characteristic colored complexes with ferric ion in water containing methanol and show a maximum in their visible absorption spectra a t 525 and 530nm, respectively. The color developed is stahle on standing a t room temperature, hut is fleeting when sulfuric acid is added. Although the acidity of test solutions is known to he the important factor for the color test,' the factor often seems to be disregarded in qualitative interpretation of experimental results. Lack of this knowledge about the way acids prevent the color from forming may lead to a wrong conclusion. This effect of the acid on preventing color formation depends an the stability of the ferric complexes. The color of the complex from 0.05 M methyl salicylate and 0.01 M ferric chloride could not be observed with the eye a t pH 1, hut that from salicylic acid could he still recognized under tho . *,,",litinn. .w..-...-..u l'rrpnrnrinn of methyl alwylate from aspirin tablets in methanol usingsulfurlc acid os o caralssr haa been rcportcd tn thrsdournnl as demonstratingurganic synthesis uith famdlar materials, and the phrnol rnlnr rest with f r m r ehlnr,de has heen used ir, a o a n of the drmonsrrntion to d e t m the sal~cslate.' Althmrh this exrrerrment is s m d e and suceeasful and seems to a good teaching strategy for improving students' understanding in ;hemistry, the cdlor test cannot he utilized for the qualitative analysis of the salicylate in a readion mixture because of the possihlity that there is sulfuric acid present, which would prevent the color from forming. Salicylic acid must be prepared under the acidic reaction conditions in the experiment to form a stable ferric complex, which shows the reported violet color. The phenol color test by ferric ion is one of the mast commonly used methods in qualitative analysis of organic compounds. Careful attention should be paid to the acidity of test solutians when the test is applied.
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'(a) Vogel, A. Textbookof PracfcalOrganicChemi~~, 4th ed.: Longman: London, 1978:p 1065. (b)Soloway, S.: Wilen, S. H . Anal Chem. 19$2.24,979. Bunting. R; Newton, T. J. Chem. Educ 1991,68.502. Rothenberger, 0.; Tadashi Hasegawa Tokyo Gakugei University Nukuikitamachi, Koganei, Tokyo 184, Japan
Toyokazu Usui Hachiouji Higashi High School Takakuracho 68-1, Hachiouji, Tokyo 192, Japan
840
Journal of Chemical Education
