Determination of Malonaldehyde by Ultraviolet Spectrophotometry ^a

Abstract

SUMMARY Malonaldehyde occurs mainly as the enol form (CHOH = CHCHO) in aqueous solution. The UV absorption spectrum of the compound is pH‐de‐pendent. Below pH 3.0, the compound is s ‐cis, planar, having an intramolecular H bond, with absorption maximum at 245 mμ and molar absorptivity (ε) = 1.34 × 10 4 . Above pH 7.0, the compound is completely dissociated and the maximum absorption of the enolate anion occurs at 267 mμ with e = 3.18 × 10 4 . The absorbance difference between acidified and basified malonaldehyde solutions at 267 mμ can be used as a measure of malonaldehyde even in the presence of other compounds that absorb in this spectral region, provided their absorption is not pH‐dependent. This difference is directly proportional to malonaldehyde concentrations from 5 × 10 ‐6 M to 3 × 10 ‐5 M. The method has been successfully applied to the assay of malonaldehyde in distillates from rancid foods. Its sensitivity is only about 40% of the 2‐thiobarhituric acid (TBA) test, but is sufficient to detect threshold levels of rancidity. The test is simpler, much more rapid, and more specific than the TBA test.