Structure of Carboxyl Groups in Humic Acids

Schwartz and Asfeld 1 have recently reported that four moles of water were evolved to one mole of carbon dioxide when humic acids, obtained from lignite, were decarboxylated at 150° in a stream of dry nitrogen. Consequently, they suggested that the carboxyl functions of these acids were in the ionized form, —COO−H 3 O+·3H 2 O, although they were aware that this structure was contrary to that expected for relatively weak carboxylic acids. Since carboxyl groups, carboxylate ions, and hydrated hydronium ions all have characteristic absorption bands in the infrared, it was possible for us to check the validity of the proposed structure. A sample of lignite was, therefore, obtained from the same source (North Dakota lignite of the Leonardite variety, marketed as ‘Carbonox’ by the Baroid Division, National Lead Co., Houston, Texas), and humic acids were prepared and decarboxylated by the procedures of the above authors. The infrared spectra of potassium bromide pellets of the above materials (Fig. 1) were then recorded on a Perkin-Elmer 237 grating spectrophotometer.