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

Abstract

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.