Direct determination of surface active substances in natural waters

Advantages of direct determination of surface active substances without sample pretreatment, using the electrochemical method (a.c. polarography) based on the measurement of adsorption effects on the mercury electrode, are illustrated for a number of freshwater and marine samples and phytoplankton culture media. The effects of dilution, filtration and centrifugation of samples were investigated. A high loss of organic material through filtration was observed for selected sea surface microlayer samples and phytoplankton culture media. The samples were compared with different model surfactants and were found to possess similar adsorption characteristics to hydrophobic, fatty surfactants. The freshwater samples were found to be more stable during pretreatment than the marine samples. Humic substances are the predominant surface-active compounds in freshwater systems. The use of a calibration curve for humic material is proposed for the rapid and direct determination of surfactant activity values for freshwater samples.