Evaluation and implementation of methods for quantifying organic and inorganic components of biochar alkalinity

The Boehm titration, originally developed to characterize carbon blacks and activated carbons, has earned growing attention as a method for characterization of acid and base properties of biochar (Boehm, 1994; Goertzen et al., 2010). The method is based on the principle that strong acids and bases will react with all bases and acids, respectively, whereas the conjugate bases of weak acids will accept protons only from stronger acids (i.e. acids with lower pKa values). However, properties that distinguish biochar from carbon black and activated carbon, including greater carbon solubility and higher ash content, may violate integral assumptions of the Boehm titration. Three key assumptions are: (1) carbonates (as carbonate and bicarbonate) originating from atmospheric CO2 are not present during titration, (2) solid-phase reactive compounds, including oxides, hydroxides, carbonates, and other conjugate bases of weak inorganic acids such as orthophosphate and sulfate have been removed prior to equilibration with Boehm reactants, and (3) the Boehm reactants do not dissolve significant amounts of organic molecules that contain reactive functional groups. Here we use three biochars to evaluate three modified Boehm titration methods for removing carbonates and dissolved organic compounds (DOC) from Boehm extracts. Our results indicate that the original Boehm titration method for measurement of functional groups with pKa values > 10.3 is not reliable when used with biochars containing significant levels of ash or soluble organic compounds. None of the modified Bohem titration methods developed and