Polarographic measurement of surface-active material in natural waters

Abstract

An electrochemical method for the measurement of the effective concentration of surface-active substances of natural and synthetic origin in natural waters is described based on the suppression by surfactants of the polarographic streaming maximum occurring on the reduction wave of Hg(II) in aqueous solution. The effects of drop time, depolariser concentration, Hg reservoir height and temperature were investigated. Determination of surfactant activities in sea-water samples is made by use of a calibration graph prepared with an arbitrary standard surfactant, Triton-X-100. The overall precision of the method is 3% coefficient of variation including blank corrections. The maximum peak current decreased exponentially with salinity allowing correction of current values obtained for estuarine samples to a constant salinity of 33.43‰ at which the calibration was made. Mixing experiments indicated that the surfactant activity determined by this method is a conservative property when seawater and river water are mixed, free of flocculation effects over time scales of a few days. Samples may be stored unfiltered at 6°C in the dark for a week without significant changes in the results. Applications of the technique to samples of estuarine and coastal sea-water as well as surface microlayer samples are described. The latter were consistently enriched relative to subsurface water in surface-active matter. Apart from a number of samples with high surfactant activity possibly resulting from direct sewage input, good correlation between the results of the technique and dissolved organic carbon levels were obtained in a large number of estuarine samples. Under many conditions, the method may provide a convenient substitute for DOC determination.