# A Polymer Inclusion Membrane for Sensing Metal Complexation in Natural Waters
## 1 Introduction  
It is well known that metals in aquatic environments are found in numerous chemical or physical forms--such as free ions, inorganic and organic complexes, and colloidal forms. The dynamic process of metal partitioning depends on several chemical and physical parameters--such as the type and concentration of organic matter, ionic strength of the medium, hardness, pH, and redox conditions of the medium, among others. The combination of these abovementioned factors favors the formation of metallic species that can affect the metal bioavailability [1, 2]. It is broadly accepted that the bioavailability and toxicity of metals cannot be predicted by total concentrations, but rather must be done by the concentration of various chemical species, in particular the free ions [3]. Electrochemical techniques--such as competing ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-AdCSV), absence of gradient and Nernstian equilibrium stripping (AGNES), and stripping chronopotentiometry--can be useful for metal speciation studies [4]. Other techniques used in this field are the Donnan technique and permeation liquid membranes (PLMs), both based on semipermeable membranes. The Donnan technique consists of a membrane-based equilibrium process that exchanges ions between two solutions separated by an ion exchange membrane [5]. In contrast, the PLM technique is based on carrier-mediated transport of the metal of interest across a hydrophobic membrane from the sample solution into a strip solution, where it is accumulated [6, 7]. The transport is based on liquid-liquid extraction coupled with diffusion. Under certain conditions, PLMs measure the free metal fraction but it has also been found that some lipophilic metal species cross the membrane and contribute to the total flux as a result of passive diffusion [4]. According to [PERSON] et al. [8], for the interpretation of PLM-based measurements in chemical speciation, it is useful to discriminate between different forms of the metal: the free form, the molecular sized complexes (where the ligand can be the proton inorganic anions or organic compounds), and colloidal or particulate species. Usually, colloidal or particulate species diffuse very slowly and cannot pass through the PLM. On the contrary, the free form of the metal, together with mobile species--including complexes and small colloidal species--contribute to a different extent to metal fluxes through the PLM and are relevant for bioaccumulation studies. In this respect, different research work has been reported in the literature for Zn, Ni, Cd, and Cu [9; 10; 6; 11], being the lability of the metal complex, the lipophilicity, and the charge, key parameters affecting the PLM behavior. Polymer inclusion membranes (PIMs) are another type of functionalized membranes where the carrier is entrapped within a polymeric matrix providing greater membrane stability than PLMs [12]. The most common polymers used to prepare PIMs are cellulose triacetate (CTA) or polyvinyl chloride (PVC). The selection of the carrier depends on the characteristics of the metal, and typical extractants used in solvent extraction are also suitable carriers for PIMs. Moreover, a plasticizer can be added to the membrane formulation both to improve membrane flexibility and to favor the diffusion of the complex formed by the metal and the carrier [13]. Various studies have used the carrier di-(2-ethylhexyl!) phosphoric acid (D2 EHPA) incorporated in PIMs for the transport of divalent metals, such as Zn and Cu. [PERSON] evaluated the simultaneous transport of Zn, Cd, Cu, and Pb through a PIM consisting of 38% CTA, 43% D2 EHPA, and 19% of the plasticizer 2-nitrophenylether (NPOE) using a 0.5 M HNO\({}_{3}\) solution as a receiving phase [14]. A PIM made of 60% PVC and 40% D2 EHPA was also the basis of a new passive sampler developed to measure Zn ion in urban waters [15]. [PERSON] et al.