Nemstian response

A Nemstian response of 59 mV per decade change in concentration is commonly observed (at 25°C). Relation to the partial pressure carbon dioxide is accomplished by the use of Henry s law. A catheter sensor configuration has been developed for in-vivo monitoring of blood carbon dioxide (61). 

From an analysis of data for polypyrrole, Albery and Mount concluded that the high-frequency semicircle was indeed due to the electron-transfer resistance.203 We have confirmed this using a polystyrene sulfonate-doped polypyrrole with known ion and electron-transport resistances.145 The charge-transfer resistance was found to decrease exponentially with increasing potential, in parallel with the decreasing electronic resistance. The slope of 60 mV/decade indicates a Nemstian response at low doping levels. 

The nnmeric solution of Eq. (32.9) for the interface A" R+X with the common cation R+ is shown in Fig. 32.1. The expected Nemstian response of 59mV per decade is observed only in a limited range of the ratio of concentrations of electrolytes R+A and R X . When this ratio is too low or too high, the equilibrium potential approaches the distribution potential of the electrolyte that is present in excess. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

The pH electrode (and its less sophisticated parent, the glass electrode) are the most commonly encountered forms of ion-selective electrodes (ISEs). Such an electrode is best defined as an electrode having a nemstian response to a single ion in solution where, by nemstian , we again mean that the Nemst equation is obeyed. The pH electrode is an ion-selective electrode since it only responds to protons in solution (with the occasional exception of cations of the alkali and alkaline-earth metals, as discussed below). 

We have briefly encountered the solid-state fluoride electrode, which has a fully nemstian response down to c. 10 mol dm . The fluoride electrode is iiiunersed in a test solution of fluoride ion (usually aqueous), and the emf is then determined. At its heart is a single crystal of lanthanum fluoride doped with erbium fluoride, (see Figure 3.10). Like the pH electrode, a full fluoride electrode also contains a small reference electrode, meaning that a fluoride electrode is in reality a cell. The fluoride electrode does not suffer from interference from CP, so an AgCl Ag reference is the normal choice owing to its convenience and compact size. 

Ion-selective electrode (ISE) In potentiometry, an electrode having a nemstian response to one ion, ideally to the exclusion of others. 

First consider the system in which no diffusion potential is formed in the membrane. The membrane potential is then determined by the conditions at the membrane/aqueous electrolyte solution boundary. In the simplest situation, a salt of a monovalent ion-exchanger ion, anion A", with monovalent determinand cation J is dissolved in the membrane. In order for this system to be the basis for a usable ISE with Nemstian response to the determinand ion in a sufficiently broad activity interval, it is necessary that the distribution coefficient kj be... 

The formation of ion-pairs between complex JX and anion A in the membrane, provided that the membrane solvent is not very polar, does not affect the Nemstian response of the ISE and appears only in the constant term in (3.3.2). If most of the complex is in the form of ion-pair JXA but the concentration of ion-pair JA is much less than the concentration of JXA, then for the activity of the free complex ions... 

The Ag2 S ISE has Nemstian response dE/d log a( = 0.0296 V in the sulphide concentration range 10" to 10" M and silver ions from 10 to 10 M if the solutions are prepared from pure salts, as a further concentration decrease is prevented by adsorption on the glass (see p. 76 and [87, 163]). After prolonged use, the limit of the Nemstian behaviour shifts to about 10" m [130] as a result of formation of mixed potentials on accumulation of metallic silver in the membrane surface. An analogous deterioration in the membrane function in the presence of iodine results from surface oxidation [23]. Cyanide interferes only at large concentrations the equilibrium constant of the reaction... 

Potentiometric determinations of H2O2 can be carried out with a CPE loaded with 4% of Mn02 and connected to the potentiometer with Cu wire. Measurements are carried out in NH4+-NH3 buffer, at pH 8.5 vs. SCE, with a Nemstian response of 19.4 mV per concentration decade. The LOD is 0.12 xM, with linear range from 0.300 p,M to... 

A poly (vinylchloride) membrane electrode was described for local anesthetics, based on dibenzo-24-crown-8 as the electroactive material, and di(2-ethyl)hexyl phthalate as the plasticizer [74]. It was reported that the electrode exhibited a Nemstian response to procaine, and other electrode properties were also presented. The analysis was performed at pH 6 to 6.5 vs. S.C.E., with a 0.2 M lithium acetate agar bridge. Less efficient crown ethers studied at this time were benzo-15-crown-5, dibenzo-18-crown-6, and dibenzo-30-crown-10. 

Shen et al. have reported studies on procaine-selective electrodes embodying PVC membranes [57]. Various ion-pair complexes (procaine derivatives with tetraphenylborate, dipicrylamine, tetraiodomecurate, and reineckate) were incorporated into platinized PVC membranes, and with dinonyl phthalate as the solvent mediator, formed procaine selective electrodes. The efficiency and performance of these were compared, and it was found that procaine picrylamine and procaine tetraphenylborate were the best electroactive materials. The procaine picrylamine electrode exhibited a Nemstian response over the range 10 pM to 0.1 M, and was used as the indicator electrode for the potentiometric determination of procaine. The method recovery was found to be 99.8%, with a standard deviation of 0.9%. 

For the alkoxy receptor 12, which can also form 1 1 complexes with guanine nucleotides, the influence of the cationic site/receptor ratio on the EMF selectivity was determined (Table 2). Electrodes based on 12 and 210 mol% 9 gave similar, Nemstian responses to both 5 -GMP and 5 -AMP, as expected for an excess of cationic sites. Due to the high concentration of cation 9, the concentration of uncomplexed nucleotides in these membrane is high and a selectivity typical for a... 

However, when the membranes were exposed to aqueous solutions of an intermediate to high concentration of primary cation thiocyanate (M SCN ), IR bands of the complex as well as the X could be observed. At very high analyte concentrations, no preferential permeation for either M or SCN occurred and the slope of the electrode response was considerably decreased as compared to a Nemstian response. Cation permselectivity for M SCN" was observed only for the 84-incorporated membranes at low analyte concentrations. 

If a deviation from the Nemstian response occurs, it can be detected in Procedures 3 and 4, especially for BH+-type acids for which homoconjugation is negligible. 

When measured with a F ion-selective electrode with a Nemstian response at 25°C, the potential due to F in unfluoridated groundwater in Foxboro, Massachusetts, was 40.0 mV more positive than the potential of tap water in Providence, Rhode Island. Providence maintains its fluoridated water at the recommended level of 1.00 0.05 mg F /L. What is the concentration of F in mg/L in groundwater in Foxboro (Disregard the uncertainty.)... 

Inner solutions that induce large super-Nemstian responses can also be used [41], since inward fluxes are created and leaching of the primary ions from the inner solution is prevented. 

Novel EPMEs based on carbon paste impregnated with (1,2-methanofullerene C60)-61-carboxylic acid (I), diethyl (1,2-methanofullerene C60)-61-61-dicarboxylate (II) and tert-butyl (1,2-methanofullerene C60)-61-carboxylic acid (III) were designed for the assay of S-clenbuterol raw material and in serum samples [52], All EPMEs showed near-Nemstian responses (56.8, 57.0 and 57.1mV/decade of concentration) for S-Clen, with correlation coefficients for the equations of calibration of 0.9996 (I), 0.9998 (II) and 0.9998 (III), respectively, and low detection limits (of lO-10 and 10 11mol/L magnitude order). R-Clen, on the other hand, showed non-Nemstian response. All electrodes displayed good stability and reproducibility over 1-month test period, when used every day for measurements (RSD <0.1%). 

It was found that the limit of detection was 1.0 x 10 5 M for both plasticizers and that linearity and Nemstian response was obtained over the range of 1.0 x 10 5-1.0 x 10 2 mol/1 (pH range of 1.5 1.0). This potentiometric method was validated for the determination of clopidogrel over the concentration range of 4.2 jig/ml to 4.2 mg/ml with an average recovery of 100.65%. Both sensors showed stable potential readings and calibration slopes for extensive time periods. 

The construction and electrochemical response characteristics of poly (vinyl chloride) membrane sensors for donepezil HC1 are described. The sensing membranes incorporate ion association complexes of donepezil HC1 cation and sodium tetraphenyl borate (sensor 1), or phospho-molybdic acid (PMA) (sensor 2), or phosphotungstic acid (sensor 3) as electroactive materials. The sensors display a fast, stable, and near-Nemstian response over a relative wide donepezil HC1 concentration... 

Because any potentiometric electrode system ultimately must have a redox couple (or an ion-exchange process in the case of membrane electrodes) for a meaningful response, the most common form of potentiometric electrode systems involves oxidation-reduction processes. Hence, to monitor the activity of ferric ion [iron(III)], an excess of ferrous iron [iron(II)] is added such that the concentration of this species remains constant to give a direct Nemstian response for the activity of iron(III). For such redox couples the most common electrode system has been the platinum electrode. This tradition has come about primarily because of the historic belief that the platinum electrode is totally inert and involves only the pure metal as a surface. However, during the past decade it has become evident that platinum electrodes are not as inert as long believed and that their potentiometric response is frequently dependent on the history of the surface and the extent of its activation. The evidence is convincing that platinum electrodes, and in all probability all metal electrodes, are covered with an oxide film that changes its characteristics with time. Nonetheless, the platinum electrode continues to enjoy wide popularity as an inert indicator of redox reactions and of the activities of the ions involved in such reactions. 

Table 2.1 summarizes a number of redox couples that are well behaved in aqueous solutions and provide a means for monitoring the indicated species by potentiometric measurements. This can be either in the form of monitoring a titration or as a direct absolute measurement of activity. Although the tabulations of standard potentials12 17 imply that the listing should be much more comprehensive, most of the couples tabulated are not well behaved in an electrochemical sense and do not provide a Nemstian response under normal laboratory conditions. The vast majority of the data tabulated is based on other than electrochemical measurements. 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

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