A Questionable Method of Distinguishing Maleic and Fumaric Acids Aa pan of a rcgularlv scheduled urganlc iaboracor). expenmenr, we were asked to find similarities (such as rquivnlent or molcculnr weighi) and diffrrenres (such as ir spccrrum or m e l t r n ~point, between maleic and fumaric ac:ds {rts- and ,rnni.hnr~nrdinir aridso Ou m - !he oi ~- the emerment. the iortuitous dmrovrrv u,ns made that a mud method of . , ..r.. .~ro~!rre .~. distinguishing the two acids was to spill aqueous solutions onto the dittoed set of liboratory instructions. Fumarie acid causes no change in the blue color of the ditto print, while maleic acid turns it a bright green. Curiosity as to the cause of this phenomenon led us to further investigation. The color change in the ditto dye was found to he dependent on t h e p K of the acid. Some of the dye was scraped off the hack of a ditto master and dissolved in water. Titration of this solution revealed that the pK, was about 2. The literature values' for the pK's of maleie and fumarie acids are 1.8 and 3.0, respectively. From these values it can be seen that maleic is the stranger of the two acids and in solution in moderate concentration will have a p H belowpK,, thus causing the color change. Fumaric acid in solution will give a p H ahave pKI and thus will not cause the ditto dye t o change color. ~
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'"Handbook of Chemistry and Physics," (42nd ed.), Chemical Rubber Publishing Co., 1960. Carroll College Weukesha, Wisconsin 53186
Lawrence R. Noldan Bruce Pallard
Volume 50, Number 7, July 1973
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