To facilitate the first extraction of arsenic compounds into the organic layer, Aliquat 336, a water-insoluble quaternary ammonium chloride widely used as a phase transfer catalyst or as an extraction reagent for various metals [34] and [35], was added to the organic layer as a carrier. Aliquat 336 is known to form a hydrophobic complex in an organic phase with a negatively charged compound by ion pairing, or with an acid in the neutral form. In three-phase liquid extraction of an arsenic compound (HA), Aliquat 336 establishes the following equilibria at the interfacial regions [36]: