Non-Nernstian response

Fligh-spin iron in a nonheme environment exhibits a significant change in the thermodynamics and kinetics of protein binding on reduction from Fe to Fe . This is illustrated by the mammalian serum iron-transport protein, transferrin. The thermodynamic affinity for Fe is 10 in the presence of carbonate as a synergistic anion, and is reduced to 10 on reduction to Pg2-i- 21,22 jj-on-ligand turnover is also enhanced upon reduction. The net result is a non-Nernstian spectroelectrochemical response because of an elec-trochemically driven reduction followed by a coupled equilibrium dissociation of Fe as illustrated in eqns (2.4) and (2.5)  

The Nernst plot slope in this case doesn t yield an n value that corresponds to the true number of electrons transferred and the observed apparent midpoint potential Eijf) is also shifted positive relative to the true Nernstian value. Consequently, the observed midpoint potential, associated with the 

The Ka expression for eqn (2.5) is shown in eqn (2.6), and this equation plus the mass-balance equations described below (eqns (2.7)-(2.9)) are used to correct the observed redox potential Ei jf) to give the Nernstian E j2 for eqn (2.4). For the fully oxidised system  

The exact amount of Fe that dissociates from the reduced TfcFcc can be accounted for by the use of d, the dissociation constant, and the concentrations of intermediate Fe species involved in the reduction process can be properly determined using the matrix described below. 

A typical experiment starts with the initial concentration of TfcFcc = Co, prior to any reduction. Referring to the matrix shown below, when a potential is applied, some TfcFcc is reduced to TfcFcc. Here, we are assuming the system has not reached equilibrium, thus there is no dissociation of reduced Fe (second row of the matrix). At this point, if the concentration of TfcFcc is Cl, then the concentration of TfcFec will be Cq-Ci. However, when the system reaches equilibrium, dissociation of Fe follows the reduction. If the experimentally observed concentration of TfcFcc at equilibrium is Cl (this concentration can be calculated from the experimental absorbance reading at 465 nm using Beer s Law), then the concentration of TfcFec adjusted for the dissociation of Fe is Co-Cj-x and the concentration of Fe is x. This is shown in the third row of the matrix. The matrix below shows the calculation required to correct the concentration of various iron species at different equilibrium potentials after accounting for the dissociation  

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Unlike the midpoint slope (//1/2) of an ideal Nernstian plot, the slope of a non-Nernstian response cannot be interpreted as the number of electrons involved in the oxidation/reduction process. For the Hbs, the n parameter is influenced by site-site heterogeneity and allosteric effects.The n parameter is an indicator of the level of cooperativity that is operative high n values indicate a high level of cooperativity, while low n values indicate reduced cooperativity. The sensitivity of the n parameter to heterotropic effectors may be seen in Figure 2.11. The trend illustrated is consistent with the two-state (R and T) model for Hb. Maximum cooperativity is indicated by the highest values for max (defined in Figure 2.4) as illustrated for Hb o the absence of a heterotropic effector. The T-state is stabilised by heterotropic effectors (data points 1-4), which results in an increase in ease of reduction (increase in 1/2) and a decrease in cooperativity (decrease in max) due to a diminished ease of T R shift as a result of T-state stabilisation. R-state stabilisation occurs in HbCPA and horse Hb (data points 6-9), which is characterised by an increase in ease of oxidation (lower Eijf) and reduced cooperativity as illustrated by diminished max values. 

Casting metals consisting of Sb or Bi covered by a thin hydroxide layer and membranes of transition metal oxide bronzes show a relatively high selectivity for hydrogen ions, as recently reviewed by Vonau and Guth (2006). Metal/metal oxide electrodes display non-Nernstian responses, but applications in food control and medicine have... 

The above analysis ignores the possibility that more than one cation in the test solution can exchange with cations in the membrane. This is usually not the case so that operation of an ion-selective electrode requires that the operator be aware of interference from other species. For example, the pH electrode also responds to Na" " ions so that a non-Nernstian response is obtained when the H ion concentration is very low, that is, at high pH, and the Na" " ion concentration is high. 

In practice, a glass electrode is almost always used in place of the Pt(H2) electrode. A glass electrode has a deviation from the H+ (aq) ion response function (non-Nernstian behavior) and, therefore, should be calibrated using a set of the standard (buffer) solutions. 

Steinle, E. D. Amemiya, S. Btihlmann, P. Meyerhoff, M. E., Origin of non-nernstian anion response slopes of metalloporphyrin-based liquid/polymer membrane electrodes, Anal. Chem. 2000, 72,5766-5773... 

Spectroscopic data obtained from spectroelectrochemical experiments require careful and case-specific analysis. The Fe /Fe redox couple has a unique role in diflferent iron-containing proteins. It is hypothesised that the mammalian iron-transport protein transferrin uses the Fe /Fe redox couple as a switch that controls the time and site-specific release of iron, while other iron-containing proteins, such as myoglobin, are able to hold on to iron in both oxidation states. Therefore, it is very important to evaluate the protein and its interaction with both the oxidised and reduced states of iron and accordingly develop a data-analysis model. The spectroelectrochemical response of an iron binding protein can be ideal Nernstian, non-Nernstian resulting from coupled... 

Guillet, N., Lalauze, R. and Pijolat, C. (2004) Oxygen and carbon monoxide role on the electrical response of a non-Nernstian potentiometric gas sensor proposition of a model. Sens. Actuators B. 98 (2-3), 130-9. 

Quaternary Ammonium Ions. In a recent study (17), 1200 EW Nafion has been used to construct a membrane ion selective electrode. The electrode was placed in both the tetrabutylammonium ion and cesium ion forms, and the response characteristics of each form were measured. These electrodes show Nernstian responses, and the tetrabutylammonium ion electrode has no interference from inorganic cations such as Na" ", K" ", and Ca2" ". However, this electrode shows a marked interference with decyltri-methylammonium ion. In addition the cesium ion electrode response is sensitive to the presence of tetrabutylammonium ion and especially dodecyltrimethylammonium ion. Although membrane electrode sensitivities are not in general proportional to thermodynamic selectivity coefficients, the results do indicate that these large, hydrophobic cations are preferred over smaller inorganic cations by the polymer. The authors suggest that the surfactant character of the two asymmetric tetraalkylammonium ions may lead to non-electrostatic interactions with the fluorocarbon regions of the polymer, which would enhance their affinities (17). 

Fig. 1 Partition of metal ions and counterions at the polymer (PVC)-supported ionophore liquid-membrane/aqueous-solution interface relation with interfacial boundary potentials. C rz + and Cx- are concentrations of the ionophore-metal-ion complex and its counterions, respectively. These concentrations were measured with FT-IR-ATR to a depth of pm range in the organic phase near the interface. When CMn+>nCx, the observed membrane potential f exhibits a Nernstian response that meets Eq. 2. When the value of 11+ is small but still greater than nCx, the anion effect distorted the log C-AE relation observed for a non-Nemstian response. When...

Rosini S, Siebert E (2005) Electrochemical sensors for detection of hydrogen in air model of the non-Nernstian poten-tiometric response of platinum gas diffusion electrodes. Electrochim Acta 50 2943-2953 Schiavon G, Zotti G, BontempeUi G (1989) Electrodes supported on ion-exchemge membranes as sensors in gases and low-conductivity solvents. Anal Chim Acta 221 27-41... 

The current thus decays with time (Fig. 20.7b). An experiment in which / is monitored as a function of t in response to a potential step is termed chronoamperome-try. When electron transfer kinetics are important (i.e., a non-Nernstian regime), we obtain the Smutek equation ... 

The theoretical implications of using ultrathin sensing membranes is of utmost importance as these so-called space-charge membranes have a different behavior than conventional thick membranes with bulk electroneutrality and thin space charge regions only at their boundaries. In fact, the previously mentioned experimental studies could not benefit from the recent comprehensive theoretical description of the potentiometric behavior as well as potential and concentration profiles in ultrathin non-electroneutral membranes by Morf. This study showed clearly that only space-charge membranes contacted with aqueous solutions on both sides will exhibit a Nernstian response. While unfortunately such membranes are rather unstable mechanically, their stabilized counterparts, that is, solid-contacted thin membranes, are theoretically predicted to have a sub-Nernstian slope. 

Therefore, the ISE potential depends on the CO2 partial pressure with Nernstian slope. Contemporary microporous hydrophobic membranes permitted the construction of a number of gas probes, developed mainly by the Orion Research Company (for a survey see [143]. The most important among these sensors is the ammonia electrode, in which ammonia diffusing through the membrane affects the pH at a glass electrode. Other electrodes based on similar principles respond to SO2, HCN, H2S (with an internal S ISE), etc. The ammonia probe has a better detection limit than the ammonium ion ISE based on the non-actin ionophore. The response time of gas probes depends mostly on the rate of diffusion of the test gas through the microporous medium [77,143]. 

Recently, Ta2Os- and Si3N4-type pH-ISFETs have been used in non-aqueous systems, by preparing them to be solvent-resistant [17]. In various polar non-aqueous solvents, they responded with Nernstian or near-Nernstian slopes and much faster than the glass electrode. The titration curves in Fig. 6.5 demonstrate the fast (almost instantaneous) response of the Si3N4-ISFET and the slow response of the glass electrode. Some applications of pH-ISFETs are discussed in Section 6.3.1. 

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