Ionophores concentration

FIG. 3 Dependence of the square root of the SHG intensity on the power of the irradiated fundamental light, as obtained with membrane 2 with an ionophore concentration of 3.0 x 10 M. The adjacent aqueous solution was 1.8 x 10 M KCl. Inset dependence of the SHG intensity on the input optical power. (From Ref 15.)... 

Figure 5 also shows the effect of the ionophore concentration of the Langmuir type binding isotherm. The slope of the isotherm fora membrane with 10 mM of ionophore 1 was roughly three times larger than that with 30 mM of the same ionophore. The binding constant, K, which is inversely proportional to the slope [Eq. (3)], was estimated to be 4.2 and 11.5M for the membranes with 10 mM and 30 mM ionophore 1, respectively. This result supports the validity of the present Langmuir analysis because the binding constant, K, should reflect the availability of the surface sites, the number of which should be proportional to the ionophore concentration, if the ionophore is not surface active itself In addition, the intercept of the isotherm for a membrane with 10 mM of ionophore 1 was nearly equal to that of a membrane with 30 mM ionophore 1 (see Fig. 5). This suggests the formation of a closest-packed surface molecular layer of the SHG active Li -ionophore 1 cation complex, whose surface concentration is nearly equal at both ionophore concentrations. On the other hand, a totally different intercept and very small slope of the isotherm was obtained for a membrane containing only 3 mM of ionophore 1. This indicates an incomplete formation of the closest-packed surface layer of the cation complexes due to a lack of free ionophores at the membrane surface, leading to a kinetic limitation. In this case, the potentiometric response of the membrane toward Li+ was also found to be very weak vide infra).

Introduction of ionophores into the membrane should lead to a considerable increase in the ISE selectivity (in the absence of an ionophore, the selectivity coefficient would be given by constant/Texch alone, cf. (3.3.12)) however, as (3.3.10) contains only the square root of the stability constant ratio, this increase in selectivity is not extremely high. Better results are obtained in the second system, where the membrane contains an excess constant ionophore concentration. 

Another analytically useful phenomenon in electrolysis at ITIES is ion transfer faciUtated by ionophores present in the non-aqueous phase [8]. If the ionophore is present at a low concentration in the non-aqueous phase and the aqueous phase contains a large concentration of the cation that is bound in a complex with the ionophore (for example as a component of the base electrolyte), then a voltammetric wave controlled by diffusion of the ionophore toward the ITIES or by diffusion of the complex formed away from the ITIES into the bulk of the organic phase appears at a potential lower than the potential of simple cation transfer. The peak height of this wave is proportional to the ionophore concentration in the solution and can be used for the determination (fig. 9.8). This effect has been observed with valinomycin, nonactin, cycUc polyethers and other substances [2,3,23]. The half-wave potential of these waves is... 

The agreement between the calculated and observed response slopes indicates that the primary factor determining the EMF slope is the surface-charge density which is governed by the ionophore concentration in the membrane. As a result. 

In glycerol monooleate/decane bilayers we find the steady-state conductance at zero current to be proportional to the first power of the ion concentration and to the second power of the ionophore concentration, as illustrated in Fig. 1. (The current-voltage characteristic is hyperbolic for all ionic species indicating that this molecule is in the equilibrium domain for the interfacial reactions, with the rate-limiting step being the ion translocation across the membrane interior.) The conductance selectivity sequence is seen to be Na>K>Rb>Cs, Li. 

Fig. 2.1. Zero-current ion fluxes in the ion-selective membrane. Left (A) Concentrated inner solution induces coextraction of electrolyte into the membrane increasing the primary ion-ionophore concentration within the membrane. Consequently, primary ions leach into the sample increasing the activity of primary ions at the membrane/sample phase boundary. (B) Diluted inner solution and ion exchange at the inner solution side decreases the concentration of the complex within the membrane. Primary ions are siphoned-off from the sample, and their activity at the membrane/sample phase boundary is significantly decreased. (C) Ideal case of perfectly symmetric sample and inner solution resulting in no membrane fluxes. Note that fluxes of other species (counterions or interfering ions) are not shown for clarity. Right potential responses for each case. Ideal LOD is defined by the Nikolskii-Eisenman equation (Y Kj°jaj) and is obtained only in the ideal case (C). Fluxes in either direction significantly affect the LOD.

FIG. 5 Plots of the reciprocal of the square root of the SHG intensities 1 / //(2ct>) versus the reciprocal of Li+ ion concentrations in the adjacent aqueous solutions (the Langmuir isotherm) as obtained with membrane 1. The ionophore concentrations were 3.0 x 1CT3M (A), 1.0 x 10 2M (0)> and 3.0 x 10 2 M ( ), respectively. The data points present averages for three sets of measurements. Error bars show standard deviations. (From Ref. 15.)... 

Table VII also shows the ratios of the reagent ionophore concentrations to K concentrations for the samples were >1. This is a proof that the reagent is now consiimed in stoichiometric quantities. Further, in Table VII, the ratio concentrations of Na to K for the samples varies from 3000 to 500. This confirms the fact that in methanol, MCC222 is more selective and sensitive towards K than in water.

We now demonstrate the application of this latter method to the study of the transport of sodium and lithium ions mediated by the ionophore monensln (10). The effect of adding monensln to the vesicular preparation on th linewidth of the inner signal of sodium is shown in the three traces in Fig. 7. There is a conspicuous increase in the linewidth upon increasing the ionophore concentration which we attribute to enhancement of the transport rate across the meihbrane. 

Sodium permeabilities were found to be 62, 82, 126 and 158 ni /sec for 15, 22.5, 30 and 37.5 yM monensin respectively and lithium permeabilities were 12 uid 33 ni /sec for 400 and 800 yM monensin respectively. Thus, the permeabilities extrapolated to 1 yM of monensin for Ihe same don and lipid concentration are for Na 4.0 0.4 m /sec, for Li 0.035 4 0.005 nn sec. These results show that within the concentration range studied the sodium transport rate increases fairly linearly with the ionophore concentration, indicating that the dominant transporting species is a 1 1 complex of the sodium ionophore. The much higher value obtained for sodium either indicates that the complex association-dissociation processes determine the overall rate of transport or reflects the difference in the binding constants for these two ions. 

The methods has been used to study the kinetics of ionophore mediated transport across phopholipid vesicles. In this system all the parameters affecting the transport process (i.e. ion and ionophore concentration, lipid concentration and ccnqposition, pH cuid ten rature) were quantitatively controlled and studied, thus enabling to analyze the mechanism of the transport. 

Figure 27 Dependence of the free ionophore concentration on the charge sign and concentration of ionic sites. Top row and bine line divalent target ion. Bottom row and red line monovalent interfering ion. The highest potentiometric selectivity is obtained at the ratio of ionic sites and ionophore for which there is a high concentration of free ionophore when the sensor is exposed to target ions but a very low concentration of free ionophore when the sensor is exposed to intefering ions.

Figure 32 Selectivity of an ISE for Ag+ over an interfering ion (J+) that does not bind to an electrically nentral ionophore Effect of the molar ratio of the total ionophore concentration to anionic sites. The five different cnrves show the dependence of the selectivity as calculated for ionophores that form complexes of different stoichiometries (a) ionophore does not bind Ag" " at all (b) / AgL = 10 (c) fiA L = 10 (Sa 2 = 10 (d) Pa = 10, Pa,L2 = 10 °, / AgL3 = 10 (e) = 10, SAgL2 = 10 °,...

It is noteworthy that the calculated selectivity curves intersect with one another, which might not be expected intuitively. To explain this, consider the 1.5 1 ratio. On one hand, approximately half of the Ag+ is in the form of L2Ag+ complexes if the ionophore can form such a complex, and the free ionophore concentration and potentiometric selectivity are low. On the other hand, one-third of the ionophore is in its free form if only 1 1 complexes can form, and as a result of the relatively high free ionophore concentration the selectivity is high. The direct consequence is that for each curve (i.e., each set of binding constants) there are not multiple but there is... 

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... 

In the case of ionophore determination a rather high concentration of the ion to be complexed in the aqueous phase is used while the ionophore is present at a low concentration in the organic phase. Under increasing potential scan the current peak is controlled by diffusion of the ionphore to ITIES and by diffusion of the complex formed from ITIES into the bulk of the organic phase while after scan reversal opposite processes take place. The peak currents are proportional to ionophore concentration. This method has been applied to the determination of monensin in cultures of Streptomyces cinnamonensis [33]. 

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