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Determination of Clay Surface Acidity by Infrared Spectroscopy¹

ABSTRACT

A new and simple method for estimating pH at the surface of colloidal particles has been devised. By use of infrared spectroscopy the effect that proximity to a colloidal surface has on an organic acid molecule can be ascertained. An intensity ratio of the carboxylate to carbonyl infrared absorption bands (1,550 to 1,610 cm^-1 and 1,690 to 1,760 cm^-1, respectively) of organic acids can be used as an indicator of hydrogen ion activity in the compound's environment. Benzoic acid seems to be the best choice for this type of study, since it is not adsorbed by the clay surface in the presence of water. The surface of Wyoming bentonite seems to be about 100 times more acid than the solution at a pH of about 7.0. As the pH decreases, there is a decreasing difference between surface and solution pH.

¹ Journal Paper 2692, Purdue University Agr. Exp. Sta., Lafayette, Ind.; contribution from the Department of Agronomy, presented before S-2, Soil Science Society of America, Kansas City, Mo., Nov. 1964.

² Graduate Teaching Assistant and Associate Professor, respectively.

Received for publication June 7, 1966. Accepted for publication September 12, 1966.