Response Mechanism of Ion‐Selective Electrodes Based on a Guanidine Ionophore: An Apparently ‘Two‐Thirds Nernstian’ Response Slope

Koseoglu, Secil; Lai, Chaohua; Ferguson, Chad; Bühlmann, Philippe

doi:10.1002/elan.200704066

Cited by 12 publications

(13 citation statements)

References 36 publications

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“…Such receptors often use guanidinium, amide, or urea functional groups (or their thio derivatives) as hydrogen bond donors to bind the anion of interest. , To enhance the hydrogen bond donor strength of these groups by increasing the partial positive charge on their hydrogens, these receptors are often substituted with electron-withdrawing groups that also make these receptors more acidic. This leads to the possibility of deprotonation rather than complex formation. , As shown previously in the literature, the straightforward identification of deprotonation in such systems is somewhat complicated because the titration plot of a deprotonation reaction can be satisfactorily fitted using a 1:1 association model, which can easily lead to erroneous conclusions. Therefore, one has to be careful and may have to rely upon a second, complementary technique to identify the occurrence of deprotonation.…”

Section: Resultsmentioning

confidence: 97%

Getting More out of a Job Plot: Determination of Reactant to Product Stoichiometry in Cases of Displacement Reactions and n:n Complex Formation

2011

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The method of continuous variation (often referred to as Job's method) is an easy and common method for the determination of the reactant stoichiometry of chemical equilibria. The traditional interpretation of Job plots has been limited to complex association equilibria of the type nA + mB ⇌ A(n)B(m), while little focus has been placed upon displacement type reactions (e.g., A + B ⇌ C + D), which can give Job plots that look quite similar. We developed a novel method that allows the user to accurately distinguish between 1:1 complex association, 2:2 complex association, and displacement reactions using nothing more than a pocket calculator. This method involves preparing a Job plot of the system under investigation (using regularly spaced mole fractions), normalizing the measured quantities (such as the concentration of A(n)B(m) or C for the above reactions) to their maximum value (i.e., at mole fraction 0.5), and determining the sum of the normalized values. This sum is then compared with theoretically predicted normalized sum values that depend on the nature of the equilibrium. The relationship between, on the one hand, the sum of the normalized values and, on the other hand, the reaction equilibrium constant and the concentration of the stock solutions used for the preparation of the Job plot is also explored. The use of this new technique for the interpretation of Job plots permits users to readily determine information that can be obtained otherwise only with laborious additional experiments, as illustrated by the analysis of four Job plots taken from the literature.

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Section: Resultsmentioning

confidence: 97%

Getting More out of a Job Plot: Determination of Reactant to Product Stoichiometry in Cases of Displacement Reactions and n:n Complex Formation

2011

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“…18–21 Another example illustrating an increased level of complexity is given by the apparently non-Nernstian response observed when a primary and an interfering ion form complexes of different stoichiometries and co-exist in the ISE membrane over a wide activity range. 22–26 This can result, for instance, in apparently half-, two thirds-, and twice-Nernstian responses. Note that in neither of these examples the experimentally observed ionophore forms more than one type of complex with any type of sample ion.…”

Section: Introductionmentioning

confidence: 99%

Ion-Selective Electrodes with Unusual Response Functions: Simultaneous Formation of Ionophore–Primary Ion Complexes with Different Stoichiometries

et al. 2011

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It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes, and how this results in what look like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs+ but also the smaller ions Li+, Na+, K+, and NH4+ form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 1015.0 and 1021.0 for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare, but lacking in the past an adequate discussion in the literature remained ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries.

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“…Generally, the NH hydrogen-bond donors employed in anion receptors are based on amines, guanidinium cations, pyrroles, ureas, and amides. Although routinely exploited in the fabrication of anion-selective sensors, , some of the NH donors can become protonated, which might affect the selectivity of the electrode at various pHs. − Furthermore, several NH-based ionophores experience aggregation and competition for hydrogen bond donor sites …”

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Cyanostar: C–H Hydrogen Bonding Neutral Carrier Scaffold for Anion-Selective Sensors

et al. 2018

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Cyanostar, a pentagonal macrocyclic compound with an electropositive cavity, binds anions with CH-based hydrogen bonding. The large size of the cyanostar's cavity along with its planarity favor formation of 2:1 sandwich complexes with larger anions, like perchlorate, ClO, relative to the smaller chloride. We also show that cyanostar is selective for ClO over the bulky salicylate anions by using NMR titration studies to measure affinity. The performance of this novel macrocycle as an anion ionophore in membrane ion sensors was evaluated. The cyanostar-based electrodes demonstrated a Nernstian response toward perchlorate with selectivity patterns distinctly different from the normal Hofmeister series. Different membrane compositions were explored to identify the optimum concentrations of the ionophore, plasticizer, and lipophilic additive that give rise to the best perchlorate selectivity. Changing the concentration of the lipophilic additive tridodecylmethylammonium chloride was found to impact the selectivity pattern and the analytical dynamic range of the electrodes. The high selectivity of the cyanostar sensors and their detection limit could enable the determination of ClO in contaminated environmental samples. This novel class of macrocycle provides a suitable scaffold for designing various anion-selective ionophores by altering the size of the central cavity and its functionalization.

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Response Mechanism of Ion‐Selective Electrodes Based on a Guanidine Ionophore: An Apparently ‘Two‐Thirds Nernstian’ Response Slope

Cited by 12 publications

References 36 publications

Getting More out of a Job Plot: Determination of Reactant to Product Stoichiometry in Cases of Displacement Reactions and n:n Complex Formation

Getting More out of a Job Plot: Determination of Reactant to Product Stoichiometry in Cases of Displacement Reactions and n:n Complex Formation

Ion-Selective Electrodes with Unusual Response Functions: Simultaneous Formation of Ionophore–Primary Ion Complexes with Different Stoichiometries

Cyanostar: C–H Hydrogen Bonding Neutral Carrier Scaffold for Anion-Selective Sensors

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