Number-Average Molecular Weights of Natural Organic Matter, Hydrophobic Acids, and Transphilic Acids from the Suwannee River, Georgia, as Determined Using Vapor Pressure Osmometry

Abstract

Vapor pressure osmometry (VPO) was used to measure the number-average molecular weights (Mn) of natural organic matter (NOM) that was isolated using reverse osmosis, hydrophobic acids (HPOA) that were isolated by adsorption on XAD-8 resin, and transphilic acids (TPIA) that were isolated by adsorption on XAD-4 resin. All samples were isolated from the headwaters of the Suwannee River in southeastern Georgia, USA, in May 2012. NOM, HPOA, and TPIA samples had Mn of 634±11, 583±8, and 498±7 g/mol, respectively. Novel methodology was introduced wherein VPO measurements were made at room temperature, and Mn values were rendered from a robust analysis of data that considered not only the well-known effect of hydronium ion but also the contributions of inorganic solutes present in the samples. The method was validated by results on a known sample (benzenehexacarboxylic acid), whose molecular weight was determined accurately to within 0.5% of the true value. The VPO-determined Mn are compared to those from noncolligative methods to highlight uncertainty regarding a fundamental parameter of matter found in nearly all terrestrial waters.