| Summary: | The complexing properties of two water-soluble resorcin[4]arenes (tetrasodium 5,11,17,23-tetrakissulfonatemethylen 2,8,14,20-tetra(butyl)resorcin[4]arene, Na<sub>4</sub>BRA, and tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(2-(methylthio)ethyl)resorcin[4]arene, Na<sub>4</sub>SRA) with polluting heavy metals such as Cu<sup>2+</sup>, Pb<sup>2+</sup>, Cd<sup>2+</sup> and Hg<sup>2+</sup> were studied by conductivity, and the findings were confirmed by using other techniques to try to apply this knowledge to removing them. The results indicate that Na<sub>4</sub>BRA is able to complex Cu<sup>2+</sup> in a 1:1 ratio and Pb<sup>2+</sup> in a 1:2 ratio, while Na<sub>4</sub>SRA complexes Hg<sup>2+</sup> in a 1:1 ratio. On the contrary, no indications have been observed that either of the resorcin[4]arenes studied complexes the Cd<sup>2+</sup> ions. The results suggest that the bonds established between the sulfur atoms located at the lower edge of the SRA<sup>4−</sup> and the solvent hydrogens could prevent the entry of the guest into the host cavity. However, in the case of Hg<sup>2+</sup> ions, the entry is favoured by the interactions between the sulfur donor atoms present on the lower edge of Na<sub>4</sub>SRA and the Hg<sup>2+</sup> ions. Therefore, it can be said that Na<sub>4</sub>BRA is selective for Cu<sup>2+</sup> and Pb<sup>2+</sup> ions and Na<sub>4</sub>SRA is selective for Hg<sup>2+</sup> ions.
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