The addition of MB+ is mandatory for the success of the methodolgy because it is responsible for the formation of ionic-pairs with anionic surfactants (as SDS), which are the species actually retained on the PUF surface. So, the evaluation of the concentration of dye in solution is im- portant to guarantee quantitative extraction of anionic surfactants from water by the solid-phase and ensure that a linear relationship could be observed between absorbance (related to the color intensity of foam) and the concentration of anionic surfactants. On the other hand, the ad- dition of high concentrations of MB+ must be avoided, in order to min- imize the magnitude of blank signals due to the extraction of MB+ alone by the PUF. Although the literature reports that MB+ is not extracted by the PUF, in this work it was observed that residual amounts of the dye could be retained onto PUF surface when high concentrations of MB were added to the solution. So, the influence of MB+ concentration in solution was studied in the range of 5.0 × 10− 7–2.0 × 10− 5 mol L− 1, for a constant SDS concentration of 2.0 mg L− 1 (approximately 7.0 × 10− 7 mol L− 1). The other conditions were those employed in the previous experiments.
As it can be noted in Fig. 4, absorbance signals suffered a strong increase when using the concentration of MB+ at concentrations up to 5.0 × 10− 6 mol L− 1, as expected. At concentrations higher than 5.0 × 10− 6 mol L− 1, the signals remained relatively constant, prob- ably because the excess of MB+ added to the solutions was already enough to convert all dodecylsulfate ions into the ionic-pair. In order to ensure that enough MB+ would be present in the solution even for higher concentrations of anionic surfactants, a concentra- tion of MB+ of 1.7 × 10− 5 mol L− 1 was selected for the methodology. It is important to remark that, in this condition, blank signals were very low and could be easily compensated.