Qualitative and Quantitative Analysis of Carboxylic Acids

Abstract

Ester-bound carboxylic acids, which give lipophilic, nonpolar character to certain natural products, may be analyzed qualitatively in gas-liquid chromatography or quantitatively by the hydroxylamine procedure. Glycerides, phosphatides, and simple esters can be hydrolyzed with acid or alkali and the liberated carboxylic acids can be measured by different chemical procedures. The alkaline hydroxylamine procedure of Snyder and Stephens (1959) may be directly applied to some classes of lipids for the quantitative determination of ester groups. Such an exercise (Exercise No. 63) is described in this manual. In the case of more complex glycolipids, such as bacterial polysaccharides, the alkaline hydroxylamine procedure does not give reproducible results, while total hydrolysis and liberation of the carboxylic acid content of these materials usually results in lose of some of the acids. Boron trifluoride in methanol was successfully used by Netcalfe and Schmitz (1961) for the esterification of free fatty acids. It has been shown by Duron and Nowotny (1963) that boron trifluoride in methanol or boron trichloride in methanol are able not only to esterify free fatty acids but also to transesterify certain complex lipids, yielding methyl esters of all bound carboxylic acids.