Spectrophotometric determination of ketones by borohydride reduction.

Relatively few spectrophotometric methods have been developed for the determination of ketones. One such has been described by Barthauer et al. (1) for the determination of acetone. This method involves measurement of the absorbance of a solution of the acetone-containing sample in 2,2-4-trimethylpentane solvent at the wavelength of maximum absorption for acetone in the solvent, 280 mt. This method has the serious limitation that no other species which absorb appreciably in the 280 m[t region can be tolerated. An indirect colorimetric determination of a wider variety of ketones has been cited by Critchfield and Hutchinson (2). This method is based on the reaction of ketones with 2,4-dinitrophenylhydrazine to form colored 2,4-dinitrophenylhydrazones. The spectra of these colored products are then utilized for the deterination of ketones for which the reaction is quantitative. Among non-colorimetric methods of determining carbonyl compounds is one derived from the discovery of Chaikin and Brown (3) that in aqueous solution sodium borohydride (NaBH4) reduces carbonyl compounds quantitatively and selectively. Thus, Jensen (4) analyzed samples for carbonyl content by adding a known excess of standard NaBH4 and determining the excess with standard iodate (5). This method has the disadvantage that it is only applicable to carbonyl compounds that react rapidly with NaBH4, such as aldehydes, alphahydroxy ketones, and 1,2-dicarbonyl compounds. Few ketones can be determined by this method, and even these must be present at high concentration to allow for adequate precision in the determination. Also, Jensen's method entails the use of standard borohydride solutions, which is somewhat of an inconvenience.