Slope, Nemstian

The general theoretical treatment of ion-selective membranes assumes a homogeneous membrane phase and thermodynamic equilibrium at the phase boundaries. Obvious deviations from a Nemstian behavior are explained by an additional diffusion potential inside the membrane. However, allowing stationary state conditions in which the thermodynamic equilibrium is not established some hitherto difficult to explain facts (e.g., super-Nemstian slope, dependence of the selectivity of ion-transport upon the availability of co-ions, etc.) can be understood more easily. 

FIGURE 4.5 Typical ISE response curves to monovalent cations (solid circles) with a response slope of 59.2mV decade-1 and to divalent cations (open circles) with a Nemstian slope of 29.6mV decade-1. Intercepts of the linear ranges of sensor responses define the lower and higher detection limits. 

Epoxy-based membrane of 2-[(4-chloro-phenylimino)-methyl]-phenol reveals a far Nemstian slope of 43 mV per decade for Pb+2 over a wide concentration range CIO 6 to 10 1 mol dm-3). The response time of the electrode is quite low (< 10 sec) and could be used for a period of 2 months with a good reproducibility. The proposed electrode reveals very high selectivity for Pb(II) in the presence of transition metal ions such as Cu2+, Ni2+, Cr and Cd2+at concentrations l.()xl() 3 M and 1.0><10 4 M. Effect of internal solution concentration was also studied. The proposed sensor can be used in the pH range of 2.50 - 9.0. It was used as an indicator electrode in the potentiometric titration of Pb+2 ion against EDTA. 

Satake et al. reported the use of a coated wire electrode sensitive to procaine and other local anesthetic cations, and their application to potentiometric determination [73]. Electrodes were constructed from a copper wire (0.8 mm diameter), coated with a PVC membrane comprising a mixture of the drug-tetraphenylborate ion-pair, dioctyl phthalate, polyvinyl chloride, and tetrahydrofuran. Potential measurement was made with respect to a Ag-AgCl reference electrode. The electrodes showed linear responses with a Nemstian slope for procaine over the concentration range investigated. The method was used for analyses of the drug in pharmaceutical preparations. 

The glass electrode responds especially slowly in protophilic aprotic solvents like DMSO and DMF, sometimes taking 1 h to reach a steady potential. In such cases, the use of Si3N4- and Ta205-type pH-ISFETs is very promising, because they almost respond instantaneously and with Nernstian or near-Nemstian slopes [19]. 

Four EPMEs based on cyclodextrins (a-, //-, y- and 2-hydroxy-3-trim-ethylammoniopropyl-/)-cyclodextrin (as chlorine salt) (//-derivative-cyclo-dextrins)) have been designed for the enantioanalysis of L-vesamicol [34], The linear concentration ranges for the proposed EPMEs based on cyclodextrins were from 10 9 to 10 7 mol/L for the electrodes based on a-, //- and //-derivative-cyclodextrins with detection limits of 5.7 x 10 10, 2.42 x 10 10 and 4.77 x 10 10 mol/L, respectively, while y-cyclodextrin based electrode is characterized by linear concentration range between 10-5 and 10-2 mol/L with detection limit of 7.86 x 10 6 mol/L. The electrodes displayed near-Nemstian slopes of 52-59 mV per decade of... 

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

Fig. 3.4. pH dependence of on-set potential, Uon and flat-band potential, Up, vs. NHE for various hematite materials as collected from Table 2. The solid line, with a Nemstian slope of -59 mV/pH-unit, shows the potential for hydrogen generation as a function of pH. Ebias stands for the bias, which at least is needed at pH 10 to photogenerate hydrogen at the polycrystalline hematite electrode described by McGregor et al [7]. 

Figure 8.18. Measured redox potentials in a deep groundwater. Experimental values of the measured redox potentials (recalculated to the standard hydrogen electrode scale) versus (3pH + log[Fe ]). The concentration of [Fe J has been obtained from the analytical determinations by correction for the complex formation with carbonate. The notation refers to the different test sites. The full-drawn line has been calculated using the selected value of the standard potential E. The straight line has the theoretical Nemstian slope of +0.056 V, at the temperature of measurements. (Adapted from Grenthe et al., 1992.)...

Plots of = f(log [SeOj ])pn and E = f(pH)se(iv) at 293 K were linear and had the slopes 14.5 and 87.5 mV, respectively. These values are close to the Nemstian slopes required by Reaction (A.36) and the authors concluded that the half-cell was reversible. The potential, recalculated to the standard hydrogen electrode scale, measured at [SeOj ] = 1 M and [OH ] = 1 M was - 0.364 V and assigned as the standard potential of the redox-couple in alkaline solution. The authors noted this value Is close to the value in [56LAT], but they did obviously not know that this datum is in error by about 0.03V. 

The review finds it difficult to assess this paper as experimental conditions and primary data are lacking. There can be no doubt that the authors have used concentrations instead of activities in their calculations. The activity of water was set equal to 1 despite the high concentration of solutes and any liquid junction potentials have been neglected. It appears fortuitous that Nemstian slopes were observed. The large concen-... 

Thus, the gas-sensing probe responds like any ISE with a Nemstian slope toward the analyte gas. 

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