Contribution of organic acids to the pool of surface active substances in model and marine samples using o-nitrophenol as an electrochemical probe

Voltammetric prepeak height of the of o-nitrophenol (ONP) probe is very sensitive to the presence of acidic organic matter adsorbed on the mercury electrode. The ONP prepeak height increases with increasing acidity of organic substances, depending also on their concentrations in solution and on the adsorption time. For a better interpretation of real marine samples, the adsorption behaviour was studied of a few model substances which may contribute to the acidity of the sample. Also, in phytoplankton monoculture media of diatom Phaeodactilum tricornutum, it was found that major acidic organic substances originated from hydrophilic fraction while acidity of organic substances excreted by microflagellate Emiliana huxleyi was mostly ascribed to hydrophobic acid organic substances. Seawater samples from two stations in the northern Adriatic sea were investigated monthly during a one-year period. The highest acidity values of the present organics were noticed during the winter period, corresponding to the dominant humic type organic material, while the obtained acidity values were lower in periods of higher phytoplankton activities. In two fractionated northern Adriatic seawater samples, the results obtained with the ONP probe demonstrated well how the composition of organic matter is influenced by dominant phytoplankton populations.