Determination of trace surfactants by measuring the differential capacity at the mercury-electrolyte solution interface

Polarographic determination of surfactants in waters usually depends on the suppression of the maximum current of oxygen reduction. Although it is possible to determine by this method the surfactants in river waters which contain comparatively many surfactants, it is impossible to determine the trace surfactants in piped water. The purpose of the present paper is to establish a method to determine trace surfactants and to apply the method to piped water and underground water. Differential capacity was measured by a.c. polarographs. Differential capacity at a hanging mercury drop electrode decreases with time owing to the adsorption of surfactants. Thus a differential capacity-time curve was used to determine trace surfactants in waters. The detection limits of this method were 5 nM (converted to the concentration of sodium dodecyl sulfate) and 1 ppb (converted to the concentration of Triton X-100). The concentration of the surfactants in the underground water was 6.5 nM or 1.5 ppb. The concentration of the surfactants in the piped water was 17 nM or 3.2 ppb.