Charge transfer potentiometric

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

The equivalent circuit corresponding to this interface is shown in Fig. 6.1b. The charge-transfer resistances for the exchange of sodium and chloride ions are very low, but the charge-transfer resistance for the polyanion is infinitely high. There is no direct sensing application for this type of interface. However, it is relevant for the entire electrochemical cell and to many practical potentiometric measurements. Thus if we want to measure the activity of an ion with the ion-selective electrode it must be placed in the same compartment as the reference electrode. Otherwise, the Donnan potential across the membrane will appear in the cell voltage and will distort the overall result. 

Uses, aminothiazoles derivatives, 132-172 hydroxythiazole derivatives, 438-442 mercaptothiazole derivatives, 438-442 UV spectra, 21 of alkylaminothiazoles. 38 and amino-imino equilibrium, 19, 20 and charge transfer, 21 of iminothiazolines, 38 and PPP calculations, 21 and potentiometric measurements, 21 representative data, 22 of Schiff bases, 41 and solvent effects, 21 and substituent effects, 22 of sulfathiazoles, 20 -of sulfonamidothiazoles, 116 of A-2-thiazoline-4-one, 422 pKa determination, 423 of A-4-thiazoline-2-one, in relation with protomerism, 387 representative data, 389 of A-4-thiazoline-2-thiones, 380. 381 in relation with protomerism. 378 of thiazolylcarbamates, 97 of thiazolyl-2-oxides, 409 of thiazolyl-4-oxides, 426 of 2-thiazolylthioethers, 404 of thiazolylthioureas, 94, 96 of thiazolylureas, 93... 

The stoichiometries of the principal species in solution can be determined from both the (concentration-dependent) potentiometric as well as spectrophotometric data. If there are only two siderophore species in equilibrium, spectrophotometric titration data typically show a shift of the ligand-to-ferric ion charge transfer band to longer wavelength with decreasing pH, and an isosbestic point. Using the Schwarzenbach equation (5)125) the data should fit to a linear correlation of Aobs vs (A0 — Aobs)/[H+]n,... 

The formation of Hg(SeCN)4 is well established by the potentiometric work of Toropova [56TOR], while her experimental data pertaining to the formation and the formation constant of Hg(SeCN)3 only comprise a few points. In their polarographic work Murayama and Takayanagi [72MUR/TAK] studied the anodic mercury wave in the presence of 0.001 to 0.003 M SeCN . The electrode process was assumed to comprise the charge transfer Hg(l) Hg + 2e combined with the formation of Hg(SeCN)2(aq) and Hg(SeCN)3. No primary data are provided and the evaluation procedure is rather involved, which makes the assessment difficult. The results are mixed equilibrium constants, since an activity coefficient correction was applied to the Hg ion. The following complexes are thus proposed to prevail in the Hg -SeCN system ... 

Although potentiometry provide most valuable data that reflect charge transfer processes in the enzyme, getting molecular insights from the experiments proved to be difficult because of lack of theory for proper quantitative interpretation of the data. Until recently, it has not been possible to directly relate the potentiometric data to a specific molecular mechanism of charge translocation in CcO. 

In the group of second-kind electrodes, half-ceUs based on the charge transfer reactions of silver play a very important role. Silver(I) forms many ill-soluble compounds with a variety of anions. Among these compounds silver(I) halides, mainly silver chloride, were used quite intensely for the formation of electrodes with different applications including potentiometric measurements. There are advantages of these electrodes [23] such as simple preparation, possibility to make them very compact, and also possibility to use them in some cases directly in the solution, which avoids the uncertainties connected with liquid junction. 

Both potentiometric (equilibrium) and amperometric (dynamic) modes of operation, however, require facile electron transfer between the biomolecule and the surface of an electrode. For most large, electroactive biomolecules such direct heterogeneous electron transfer is extremely slow and even for the measurement of equilibrium potentials, which requires only very small currents, a suitable mediator must be added to the solution. Clearly, provision for charge transfer mediation is essential for the successful construction of sensors of both types. The selection of an appropriate mediator is usually based on the fulfillment of certain criteria. Its heterogeneous and homogeneous redox reactions must take place rapidly at a well-defined potential in the medium of interest and involve the transfer of a definite number of electrons to and from stable redox species. In addition the mediator should be adequately soluble, usually in aqueous media at pH 7, and should not inhibit the reaction of the biomolecule with its substrate. Very few substances meet all these demands. 

Metallicenium cations and ferrocene charge transfer complexes The ferricenimn cation may be isolated with anions such as Is [77,93] BF4 [93] and FeCU" [94,95]. The ferricenium ion has one unpaired electron but it shows no electron spin resonance since the ground state is orbitally degenerate [95a, 80]. It has been suggested that the ferricenium ion forms complexes in solution with the anions Is , FeCU [96a] and carboxylates [956]. However, the Mdssbauer spectrum of solid ferricenium tetrachloroferrate suggests it to be the salt [(7r-C5H5)2Fe]+FeCl4 [94]. Further, potentiometric studies show that there is no appreciable complex formation between the ferricenium and chloride ions in water at 25 [96a]. 

Note that all cations are initially in solution and will certainly be solvated to some extent. In addition, notice that the symbol for the electron is again subscripted to show that charge comes from an electrode rather than from a homogeneous electron-transfer reaction in solution (cf. the potentiometric titrations we discussed in the previous chapter). 

The acid/base behavior of aromatic nitro compds in DMF, Me2CO, MeCOEt, and a mixt of solvents were studied using high frequency titration (Ref 42), Pr2CO/EtOH, MeEtCO/ MeOH and MeEtCO/EtOH are reported as suitable solvents for the potentiometric titration of TNT (Ref 43). Low concns of TNT in air were detected in their negatively charged state via electron transfer from ionized SF6. The charged... 

It is important to note that the electrode potential is related to activity and not to concentration. This is because the partitioning equilibria are governed by the chemical (or electrochemical) potentials, which must be expressed in activities. The multiplier in front of the logarithmic term is known as the Nernst slope . At 25°C it has a value of 59.16mV/z/. Why did we switch from n to z when deriving the Nernst equation in thermodynamic terms Symbol n is typically used for the number of electrons, that is, for redox reactions, whereas symbol z describes the number of charges per ion. Symbol z is more appropriate when we talk about transfer of any charged species, especially ions across the interface, such as in ion-selective potentiometric sensors. For example, consider the redox reaction Fe3+ + e = Fe2+ at the Pt electrode. Here, the n = 1. However, if the ferric ion is transferred to the ion-selective membrane, z = 3 for the ferrous ion, z = 2. 

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