Deviation from Nernstian behavior

A reference electrode which can directly be dipped in the organic phase is not available, except the AgPh4B/Ag electrode [44]. It is customary to use a nonpolarized liquid-liquid interface, i.e., a reference ITIES. The potential drop across this interface is primarily determined by an ionic species distributed commonly in both aqueous and organic phases. There are two points to be taken into account in using this reference ITIES the deviation from nernstian behavior and limited reversibility. 

Real systems deviate frequently from ideal behavior due to large uncompensated electrolyte resistance, slow kinetics of electron transfer, and site-to-site interactions (Ilangovan and Chandrasekara Pillai, 1997). Such deviations from Nernstian behavior can be expressed by an interaction term, r, which can be estimated from the variation of peak potential with potential scan rate ... 

At the same time, the response deviates more from the theoretical value the fewer proton-binding sites are present. On the other hand, both the adsorption effects and the sub-Nernstian behavior vanish if the thickness of the hydrated layer is allowed to increase up to 800 nm (Fig. 6.24). It is seen from this model that as the thickness of the hydrated layer exceeds the thickness of the space the adsorption effects and the sub-Nernstian behavior disappear. 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

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. 

This section describes the various calibration and evaluation methods which can be used with ion-selective electrodes. The specific procedure chosen determines whether the activity or concentration of the indicated ion is obtained as a result of the measurement Fundamental to all analytical methods is the Nernst equation. We have seen in Chap. 1.2 that the Nernst equation is a theoretically derived relationship. In practice it is usually the case that an electrode deviates somewhat from the Nernst equation. As long as this deviation is reproducible, the electrode can still be used for analytical applications. Testing an electrochemical cell for exact Nernstian behavior is quite painstaking, and is often impossible. Standard solutions of precisely known activities are needed. This, in turn, requires the individual activity coefficients of the corresponding measured ions, and these are not always known with sufficient accuracy. Instead, one usually uses an empirical form of the Nernst equation for analytical applications ... 

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