Ultraviolet absorption spectra of acrylic acid and its conjugate base, acrylate, in aqueous solution

Acrylic acid is an important compound widely used in industry with multiple commercial applications, and it is also a key intermediate in the marine organosulfur cycle. However, the fundamental ultraviolet (UV) absorption spectrum of acrylic acid or its conjugate base, acrylate (pKa = 4.25 at 20 degrees C) have not been determined in water. In this paper, we determined the absorption spectrum of acrylate in aqueous solution at pH 7.2 and 20 degrees C be-tween 207 and 400 nm. The molar absorptivity decreased rapidly from 3958 M-1 cm-1 at 207 nm to a non-detectable value at wavelengths greater than 330 nm, with weak absorption at wavelengths greater than 290 nm (e.g., epsilon 290nm 2.7 M-1 cm-1). No discernable absorption bands were observed in the absorption spectrum. Excellent agreement was observed when comparing absorption spectra obtained (1) with two different spec-trophotometers and (2) with standards prepared from either newly purchased sodium acrylate or from acrylate obtained from the base hydrolysis of dimethylsulfoniopropionate. Wavelength-dependent molar absorptivities were constant at pH 7.2 over a range of acrylate concentrations from 25 to 135 mu M. The absorption spectrum red shifted when the solution pH increased from 2.8 to 8.2, with an isosbestic point observed at 214 nm indicating two exchangeable species in solution. Our study provides the first detailed UV absorption spectra of acrylic acid and acrylate in aqueous solution, with important implications regarding the detection and study of these com-pounds in environmental settings and commercial applications.