Nernstian slope

In case b) a potentiometrically measured calibration curve for ion M+ would show a sigmoidal form, starting with a sub-Nernstian slope leading to a super-Nemstian... 

We recently synthesized several reasonably surface-active crown-ether-based ionophores. This type of ionophore in fact gave Nernstian slopes for corresponding primary ions with its ionophore of one order or less concentrations than the lowest allowable concentrations for Nernstian slopes with conventional counterpart ionophores. Furthermore, the detection limit was relatively improved with increased offset potentials due to the efficient and increased primary ion uptake into the vicinity of the membrane interface by surfactant ionophores selectively located there. These results were again well explained by the derived model essentially based on the Gouy-Chapman theory. Just like other interfacial phenomena, the surface and bulk phase of the ionophore incorporated liquid membrane may naturally be speculated to be more or less different. The SHG results presented here is one of strong evidence indicating that this is in fact true rather than speculation. 

Useful experimental parameters in cyclic voltammetry are (i) the value of the separation of the potentials at which the anodic and cathodic peak currents occur, A = Pia — PiC, and (ii) the half wave potential, 1/2, the potential mid-way between the peak potentials. A value of AE of c. 0.057 V at 25°C is diagnostic of a Nernstian response, such as that shown in Figure 2.87. More generally, if n electrons are transferred from R, then the separation will be 0.057/n V. It should be noted that the expected value for AE of 0.57/nV has no relationship to the usual Nernstian slope of RT/nF = 0.059/n V at 25UC. 

If the second term on the right-hand side of Equation 29 is dominant, then Equation 30 yields a Nernstian slope if the first term on the right-hand side of Equation 29 is significant, then the slope given by Equation 30 will be sub-Nernstian. 

It is clear that small values of K, as are obtained from Method I, favor the predominance of the first term over the second, and a sub-Nernstian slope furthermore, if is constrained to be small, and the proportionality constant /3 is to be consistent with the experimental value, then it is necessary that C is large. 

Keywords Electroactive sensor, indicator electrode, selectivity coefficient, topography, Nernstian Slope... 

The usefulness of the ion-selective electrode for determining ion J is Umited by the distribution equilibrium for salt JA between the membrane and solution 1. If the activity of ion J which is present in solution 1 only as a result of this distribution equihbrium, is much smaller than the overall activity aj+(l), then the dependence of the membrane potential on the activity of determinand J has Nernstian slope. If this condition is not fulfilled, then the ISE behaviour can be obtained from the equation for the distribution equihbrium... 

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]. 

In acid media the sensitivity for divalent metals is decreased because of formation of HjS consequently electrodes with selenides are preferable in acid media [155], In the absence of complexes the electrodes have a response for divalent ions with Nernstian slope in the concentration range 10 to 10" moldm- ... 

A good deal of attention has been devoted to the effect of chlorides on the behaviour of Cu ISEs [221, 325,441] (for a review see [129]). While in the absence of chloride the ISE potential depends linearly on the logarithm of the Cu activity, with Nernstian slope of 0.0296 V (25 °C), at chloride concentrations greater than the Cu concentration the slope changes to 0.0591 V and a marked response to the chloride ion activity appears. Westall et al. [441] state that contact of Cu with the membrane leads to electrode oxidation,... 

The electromotive force (emf) of liquid membrane electrodes depends on the activity of the ions in solution and their performance is similar in principle to that of the glass electrode. To characterize the behavior of liquid membrane electrodes, the linearity of the emf measurements vs. concentration of a certain ion in solution is checked. Additional performance data are the Nernstian slope of the linear range and the pH range over which the potential of the electrode is constant. 

Five liquid membrane electrodes (Table 13.3) are now commercially available and have found wide application in the testing of electrolytes in biological and technological systems. All five electrodes perform well in the concentration range over which the Nernstian slope is maintained, i.e., from 10 -10 moldm . These electrodes to a certain extent have replaced in both chemical and clinical laboratories the more traditional instrumental methods of analysis, such as flame photometry and atomic absorption spectrometry. There are, of course, many more liquid membrane electrodes, but the availability of satisfactory solid electrodes has greatly restricted their development and practical application. 

Fig. 2 Plot illustrating the relatively linear relationship between the Cu"/ L redox potential and logarithmic value of the Cu" L stability constant, Cu"L- The line drawn has the Nernstian slope (0.059) and represents a Cu L stability constant value of 10 All complexes represented involve uncharged ligands (see Table 4).

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. 

A screen-printed Pt electrode (Fig. 27.1) served as the sensor. Comparison of this sensor with Pt electrodes ( Phoenix , USA Metrohm , Switzerland) showed that their electrochemical behavior (speed of potential establishment and Nernstian slope) differs insignificantly. The advantage of the former is its cost effectiveness, small size and possibility to be used as disposable if the need arises. 

The pH response was initially tested at 90°C and one atmosphere pressure by immersing the sensor in standard pH buffer solutions. The EMF of the sensor was then compared with the EMF generated by a commercial Ross-type glass electrode. Figure 3 shows the EMF versus pH response of a representative tube, PSU-T1-18. The response over a pH range of 2.5 to 9.5 is linear and displays a slope which is 98% of the theoretical Nernstian slope for this temperature. 

E° is the standard potential in V of the silver/silver chloride electrode and ya the activity coefficient of the chloride ion. The Nernstian slope k in V is given by Eq. (4) ... 

In the microchip ISE, the calibration plot of the measured potential versus log [Ba2+] gives a slope of 36 mV. What is the theoretical value of the Nernstian slope Explain the difference between the experimental and Nernstian slopes. [766] (2 marks)... 

At room temperature -> Nernstian slope (59 mV/decade of concentration change) is usually observed. Partial pressure can be derived by applying Henrys law. A catheter configuration suitable for measurements inside blood vessels has been described [ii]. Using other electrolyte constituents and membranes semipermeable for other types of gas sensors for other analytes were developed (including N02, S02, H2S, HF [iii]). Various CO2 sensors with galvanic solid electrolyte cells have been designed [iv]. 

Sub-nernstian slope — An -> ion-selective electrode is said to have a sub-nernstian slope when the slope of the linear part of the calibration plot of the potential vs. the... 

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