Acid buffering capacity of basic carbons revealed by their continuous pK distribution

In a previous paper [Carbon, 1997, 35, 83] it was shown for a series of activated carbons with acidic behavior that surface characterization with a continuous pK distribution was possible based on potentiometric titration provided that several experimental precautions were taken during data collection. The number and strength of acid functionalities calculated from the proton affinity distribution (PAD) were in quantitative agreement with those measured by Boehm titration of the same samples. In this work the surface basicity of an activated carbon was investigated using the same analytical procedures. The factors that may denature the results of potentiometric titration have been analyzed; it is found that incremental addition controlled by a pH stability criterion (the "fully automatic" titration) provides reproducible results even for large particle granulation. From a practical point of view this shows that the results of this optimized analytical method are reliable. The acid buffering capacity of basic carbon, with the quality and quantity of proton binding groups, reveals that the most important proton binding processes occur in the ranges of pH 4-7, 8.4-8.6 and >9.5; the second process accounts for most of the acid buffering capacity of this carbon. The basic properties of the carbon surface are proposed to be due to the combination of redox reactions and proton transfer to/from the surface and the supporting electrolyte. (C) 1998 Elsevier Science Ltd. All rights reserved.