Polarographic determination of anionic surfactants by the desorption potentials

• Published in: BUNSEKI KAGAKU Vol. 21; no. 4; pp. 517 - 521
• Main Authors: SHINOZUKA, Noriko, SUZUKI, Hajime, HAYANO, Shigeo
• Format: Journal Article
• Language: English; Japanese
• Published: Tokyo The Japan Society for Analytical Chemistry 1972; Japan Science and Technology Agency
• ISSN: 0525-1931
• DOI: 10.2116/bunsekikagaku.21.517

Anionic surfactants such as sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), sodium di (2-ethyl-hexyl) sulfosuccinate (Aerosol OT), and sodium butylnaphthalenesulfonate (BNS) were determined by a polarographic desorption potential measurement. Surfactant solutions were prepared as follows; a weighed portion of surfactant was placed in a polarographic cell which had been deaerated by nitrogen gas and a certain volume of deaerated water or aqueous salt solution was added to the cell and a surfactant solution at a desired concentration was prepared. The solution was then stirred by a magnetic stirrer. Polarograms were recorded against a saturated caromel electrode by using Yanagimoto P-8 and Yokogawa POL-21 polarographs. The surfactants studied showed well-defined adsorption-desorption waves which were the same kind as those observed in the absence of the supporting electrolyte. In the case of the SDS solution, care must be taken for the waves due to the reorientation of the surfactant molecules on the electrode which appeared simultaneously. The values of the desorption potential were measured in the following way. The bottompoint of the oscillations on the residual current curve preceding the wave was extrapolated toward the wave and the bottompoint of the oscillations on the plateau was extrapolated toward the wave. The one-half of the rising portion of the wave between the two lines was read as the desorption potential. The values measured by this method were more reproducible and less affected by the irregularity of dropping than the ordinary half wave potential. The desorption potential shifts toward negative by an increase in the surfactant concentration and a linear relationship was obtained between the desorption potential and the logarithm of surfactant concentration. For the determination of an anionic surfactant, the linear line above mentioned was used as the calibration curve. The determination was limited by the critical micell concentration (cmc), as the desorption potential remained constant above the cmc of the surfactant. The presence of the supporting electrolyte affects the nature of the surfactant solution such that the cmc and the solubility of the surfactant decrease. The polarographic adsorption-desorption potential also changed by the change in the concentration and in the nature of the electrolyte. Thus, the same supporting electrolyte at a constant concentration should be used throughout a series of experiments. The decrease of the cmc result in lowering the determination limit, but a high concentration of salt which caused a large cmc decrement made the wave indistinct. The suitable salt concentration was 0.1.2N. Deaeration should be continued until the oxygen wave disappeares. The proposed method applied to the determination of anionic surfactants (the concentration between a few ppm and a few hundred ppm) was very simple and showed a good reproducibility.