Neutral-ionophore-based ISEs

When a membrane containing anionic sites is doped with a cation-selective neutral ionophore, the cationic analyte in the membrane phase forms complexes with the 

When the formation constant is large enough and the membrane contains an excess amount of the free ionophore, the complexation reaction proceeds completely so that most analytes in the membrane are in the complexed form. Therefore, the charge balance in the membrane phase can be simplified as 

Membrane compositions and selectivity coefficients of ISEs based on commercially available neutral ionophores and their complexation constants 

While selective complexation makes the membrane more permeable to the analyte ion than the co-ions, selectivity against counter ions in neutral-ionophore-based membranes is achieved by ionic sites, not by the ionophore. In fact, the ionophore-analyte complexation decreases the free analyte activity in tlie membrane to enhance the salt extraction into the membrane phase, which may result in counter-ion interference due to Donnan exclusion failure (7). Although the PVC matrix has inherent negative sites as an impurity (11), the concentration is so low that the anionic sites were initially introduced in cation-selective ISEs based on neutral ionophores to suppress the counter-ion interference (12). 

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Besides hydrophobicity of ions and stability of their ionophore complexes, the concentration and charge of the ionic sites in the membrane phase also affect the ion selectivity of ionophore-based ISEs. This effect was first found for neutral-ionophore-based ISEs (14, 43), then for charge-ionophore-based ISEs (10, 33), and most recently implemented in an equilibrium phase boundary potential model generalized for both systems with primary and interfering ions of any charges and their complexes of any stoichiometries (34). 

When zj I / Hj < zi / Wj, the selectivity can be dramatically improved by optimizing the concentration and charge of ionic sites to satisfy equation (7.3.8). Figure 7.5 shows the effect of anionic sites on the Mg + selectivity of a neutral-ionophore-based ISE as determined by the SSM (14). The selectivity coefficients strongly depend on the membrane concentration of the anionic sites and result in optimum values against most ions with 120 mol% anionic sites relative to the ionophore concentration. With 1 1 complexes between the ionophore and Mg +, a large amount of the free ionophore is available for the ion in the membrane with 120 mol% anionic sites, i.e ionophore-based mechanism. Ca + and... 

The concept of this mixed ion-transfer potential was recently extended to quantify the non-equilibrium responses of neutral-ionophore-based ISEs (48). Figure 7.11 shows the... 

Liquid membrane electrodes (1) classical ion-exchangers (with mobile positively and negatively charged sites as hydrophobic cations or hydrophobic anions), for example K+, Cl selective electrodes, (2) liquid ion-exchanger based electrodes (with positively or negatively charged carriers, ionophores), for example Ca2+, NOJ selective electrodes, (3) neutral ionophore based liquid membrane electrodes (with electrically neutral carriers, ionophores), for example Na+, K+, NI I), Ca2+, Cl selective electrodes. 

Figure 5.21 — Mechanisms involving an optical change in flow-through metal ionophore-based sensors. The ionophore (I), indicator (In) and hydrophobic counter-ion (X) are in the lipophilic phase (shaded area), which can be a solid (1), a layer (2) or a membrane (3-6). The analyte (A) can be a cation (1-4), an anion (6) or a neutral species (5). The dotted arrow indicates the origin of the optical change, which is always related to the indicator. For details, see text.

Fig. 2 The model of ion extraction/ion exchange for an optode based on a neutral ionophore and a lipophilic cationic dye in relation to the ion-optode response mechanism i+ cation to be extracted, H+ proton, S neutral ionophore, R lipophilic cationic additive, D color-changeable dye. The subscripts o and w represent the organic phase and the water phase, respectively)...

The earliest work with ISEs based on electrically neutral ionophores was inspired by the observation of Moore and Pressman that the antibiotic valinomycin (Fignre 3), K+-I, cansed the nptake of K+ into and the release of H+ from mitochondria. Simon and Stefanac showed in 1966 that thin films of water-immiscible organic solvents doped with antibiotics exhibited responses to monovalent cations with selectivities similar to those observed in biological... 

Ionized calcium and total calcium measurements are both performed with PVC-type electrodes. Membranes based on lipophilic alkyl phosphates with phosphonate plasticizers have only marginal selectivity over magnesium but have been used in the past to determine ionized calcium (i.e., free calcium) in undiluted blood samples (M8). Interference from protons at low pHs prevents such membranes from being employed for total calcium determinations on samples diluted with acid. Use of ionophore ETH 1001 (see Fig. 2) overcomes any concerns about selectivity, whether from magnesium or pH, and is now the neutral carrier system most often utilized within analyzers to detect ionized or total calcium. 

It has been shown previously, that this membrane fails at low pH values (75). This was explained as an effect of the more extensive protonation of the ionophore due to the influx of solution anions. Furtheremoie, as we showed above, the dissolution and/or decomposition of the ionophore is considerably larger at low pH values. The dramatic decrease in X 2 (decrease of the free carrier concentration), shown in Figure 4, was very similar to that noted when TDDA-based membrane was transfered from a pH=5.0 to a pH=2.0 solution (24). The depletion of the free neutral carrier in the membrane is the result of two processes (i) protonation, (ii) dissolution and or decomposition. In this case the protonation is assumed to be predominant. 

The capability of neutral-carrier-type ion selective electrodes is considered to be dependent on its functional characteristics as an ionophore of the neutral carrier itself and on the compatibility of the carrier into the PVC membrane containing membrane solvent and lipophilic salt. At first, effects of membrane solvents on the sensitivity and selectivity of the Na" " selective electrodes based on six of the calix[4]arene derivatives 1-6, were determined. The membrane solvents tested were NPOE and FPNPE as the phenyl ether type solvents and DOP as the diester type one. Selectivity coefficients for Na" " ion with respect to other alkali and alkaline-earth metal ions, NH4 and H on the electrodes based on derivative 2 are summarized in Table I. 

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