Protonation Characteristics of the Dissolved Organic Matter in Seawater

Abstract

The acid-base equilibria of dissolved organic matter (DOM) in seawater must be determined before a predictive model of seawater chemistry can be made. The numbers and types of sites present on the organic matter determines the chemical behavior of DOM. Acid-base titrations of organic matter isolated from various marine environments with activated charcoal were performed in 0.7m NaC104 medium. Only two types of sites are evident in the pH range 2 to 8. The Type 1 sites, which are probably carboxylic sites, make up about 10 μmoles of sites per milligram organic carbon (μmoles/mg OC) and are quite acidic (pKa= 3.5). The less acidic Type 2 sites (pKa about 6.5) were evident in only a few samples and amounted to less than 1 μmole of sites per milligram organic carbon in those samples. The Type 1 sites show a dissociation constant which varies with the degree of dissociation, similar to soil fulvic and humic acids. This variation in the dissociation constant is most likely due to both charge interaction between sites and slight chemical differences in the sites. The protonation characteristics of the Type 1 sites vary little for organic matter isolated from different marine environments.