Cation coextracted anion

The numbers ( ) of coextracted water molecules shown in Table 3 are from Table 2 [46]. In the case of -Pr4N, metal complex cations, larger anions of r > 0.23 nm, and polyanions, it is assumed that n = 0. Although the n value of [Fe(phen)3] or CIO4 was reported to be as small as 0.3 or 0.2 [46], these ions have been classified as nonhydrated (i.e., = 0) so that comparatively better results may be obtained. 

In addition to specific carrier features, a number of external factors may also have marked effects on transport rates. The nature of the membrane phase (in particular for liquid or supported liquid [6.10b] membranes) influences the distribution equilibria as well as the stability and selectivity of the complex in the membrane and the cation exchange rates at the interfaces. The nature of the coextracted anion affects transport via a (cationic complex-anion) pair (Fig. 10) simply by modifying the amount of salt extracted into the membrane this amount decreases with higher hydration energy and lower lipophilicity of the anion (for example, chloride compared with picrate). The concentration of salt in the aqueous phase will, of course, affect the amount extracted into the membrane and therefore the transport rates (for illustrations of these effects see for instance [6.1]). 

Of the two extracting systems, the use of an anionic extractant is more common to minimize coextraction of inorganic cations by the organic acids. 

The external factors comprise the nature of the membrane, the substrate concentration in the aqueous phase and any other external species that may participate in the process. They may strongly influence the transport rates via the phase distribution equilibria and diffusion rates. When a neutral ligand is employed to carry an ion pair by complexing either the cation or the anion, the coextracted uncomplexed counterion will affect the rate by modifying the phase distribution of the substrate. The case of a cationic complex and a counteranion is illustrated schematically in Figure 10 (centre). 

The extraction of a given metallic cation Mn+ into an organic solvent proceeds either through its coextraction with some of the anions initially present in the aqueous feed (two different mechanisms (1) and (2) are distinguished) or through its exchange with proton(s) from the organic solvent to conserve phase neutrality (mechanism (3)) ... 

DCE diluent. However, the differences between the stronger fluorinated modifiers and the weaker nonfluorinated modifiers are much more pronounced. This could be due to the fact that the stronger modifiers may be sufficiently acidic to be partly deprotonated when contacted with the alkaline simulant. The alkoxide form of the alcohol would serve as the counteranion to the extracted cesium, eliminating the need for an anion from the aqueous phase (e.g., nitrate) to be coextracted, and, in turn, making it energetically easier to transfer the cesium cation to the solvent phase, hence increasing the cesium distribution ratio. 

Pedersen and Frensdorff studied the binding and extraction of the cation by crown ethers, which require coextraction of an anion. Without the presence of an anion host, solvation directs selectivity, giving rise to Hofmeister bias selectivity favoring low anion charge density. Anions initially in aqueous solution must be dehydrated (at least partially) and are then resolvated in the solvent phase. Empirically, the HBD ability of the solvent medium is the single most important determinant of the solvation of small, inorganic anions.100... 

A more sophisticated class of optical sensors with high selectivity towards ions are the ion-selective optodes (ISOs) [21], where the matrix (hydrophobic polymer such as PVC) contains a selective lipophilic ionophore (optically silent), a chromoionophore, a plasticizer and an anionic additive. The measurement principle is based on a thermodynamic equilibrium that controls the ion exchange (for sensing cations) or ion coextraction (for sensing anions) with the sample. The source of optode selectivity is a preferential interaction between the target ion and an ionophore. For a dye to act as a chromoionophore, it must... 

For measuring a significant change of the optical signal from the optode membrane, there must be a change in the bulk composition of the membrane to influence the chromoionophore. Simultaneously, electroneutrality within the bulk membrane phase must be maintained. Therefore, the mechanism of optode response must be either an ion-exchange of two anions or two cations, or must be a coextraction of two differently charged ions where one ion of the pair alters the optical properties of the ionophore, as shown in Fig. 1. 

In the next paper (Jensen et al., 2002), EXAFS data were performed for one IL and 1-octanol, demonstrating that two nitrate ions are present in the Sr first coordination sphere when 1-octanol is the solvent, while no nitrate can be detected in the first coordination sphere when CiC5imTf2N is the solvent. However, the authors noted that it does not prove unambiguously that nitrate ions are not coextracted with because EXAFS cannot detect NO3 in an outer-sphere complex. This is perfectly right, but we would like to stress that it does not prove either that nitrates are coextracted with in an outer-sphere complex. In fact, in this case, EXAFS does not help identifying the extracted species. Consequently, Dietz and coworkers performed nitrate titration of the IL phase and wrote that the amounts of anion coextracted into the IL are vastly insufficient to produce neutral Sr complexes, so they concluded that the phase transfer reaction proceeds primarily through cation exchange as described by Eq. (17). As can be clearly seen, this model is identical to the IX model. 

An alternative ion-sensing scheme is referred to as coextraction. In this technique, a highly lipophilic anion such as chloride or salicylate is extracted into the membrane along with a cation, which is usually a proton to maintain electroneutrality. The highly lipophilic nature of the... 

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