Determination of exchangeable protons in natural organic matter using a home-made hydrogen/carbon analyser

A home made hydrogen/carbon analyser was used to determine the portion of exchangeable protons in aquatic humic substances. For this purpose, equal sample amounts were dissolved in H2O and D2O, respectively, dried and combusted in a stream of oxygen. The amount of water resulting from combustion was measured by an infrared detector which recorded the OH bending vibration of H2O. The bands stemming from HOD or D2O were not registered by the detection unit. Thus, combustion of organic samples containing exchangeable protons dissolved in D2O resulted in a significantly smaller signal compared to the signal observed for the same sample dissolved in H2O. The relative intensity loss of the H2O signal observed after combustion was used to derive the portion of exchangeable protons in a standard reference material, a humic substance isolated by the International Humic Substances Society (Suwannee River fulvic acid). According to this method about 20% of the sample protons could be identified as exchangeable protons. With regard to titration data the portion of protons bound to non acidic hydroxy functions could be estimated. The validity of this procedure was proved by combustion experiments using commercially available deuterated substances as well as organic model compounds dissolved in D2O and H2O, respectively.