Evaluation of Modified Boehm Titration Methods for Use with Biochars

The Boehm titration, originally developed to quantify organic functional groups of carbon blacks and activated carbons in discrete pK(a) ranges, has received growing attention for analyzing biochar. However, properties that distinguish biochar from carbon black and activated carbon, including greater carbon solubility and higher ash content, may render the original Boehm titration method unreliable for use with biochars. Here we use seven biochars and one reference carbon black to evaluate three Boehm titration methods that use (i) acidification followed by sparging (sparge method), (ii) centrifugation after treatment with BaCl2 (barium method), and (iii) a solid-phase extraction cartridge followed by acidification and sparging (cartridge method) to remove carbonates and dissolved organic compounds (DOC) from the Boehm extracts before titration. Our results for the various combinations of Boehm reactants and methods indicate that no one method was free of bias for all three Boehm reactants and that the cartridge method showed evidence of bias for all pK(a) ranges. By process of elimination, we found that a combination of the sparge method for quantifying functional groups in the lowest pK(a) range (similar to 5 to 6.4), and the barium method for quantifying functional groups in the higher pK(a) ranges (similar to 6.4 to 10.3 and similar to 10.3 to 13) to be free of evidence for bias. We caution, however, that further testing is needed and that all Boehm titration results for biochars should be considered suspect unless efforts were undertaken to remove ash and prevent interference from DOC.