Determination of polyoxyethylene non-ionic surfactants at trace levels

Abstract

Although the effect of surfactants on the uptake of compounds into plants has been studied, very little work has been carried out on the uptake of surfactants themselves into leaves. A prerequisite for the latter is the existence of suitable methods for determining the various types of surfactant and when it was decided to investigate the uptake of a range of ethoxylated surfactants into wheat plants it was clear that an increase in sensitivity over existing methods was necessary. The objective was to develop a method that would be suitable for the determination of 5 mug of surfactant with reasonable accuracy (95per thousand), compared with the limit of about 40 mug, which would be achieved by scaling down the method based on the formation of an ammonium tetraisothiocyanatocobaltate(II) - surfactant complex. Following the completion of the work described here, Favretto and coworkers, described a method for the determination of polyoxyethylene surfactants based on complex formation with sodium picrate. Their method has a similar sensitivity to that described below but appears to have the advantage of greater simplicity. The method now described could be a useful alternative if, for example, substances absorbing in the 378-nm region prevented the use of Favretto's method. A highly sensitive method for the determination of cobalt is based on the formation of a complex of the metal ion with Nitroso-R salt, the sodium salt of 1-nitrosonaphth-2-ol-3,6-disulphonic acid. We have now shown that shaking a chloroform solution of the cobalt thiocyanate - surfactant complex with an aqueous solution of the Nitroso-R salt leads to the quantitative formation of the cobalt - Nitroso-R complex. Therefore, determination of the latter by a spectrophotometric method gives a quantitative measure of the surfactant.