Inner Potential Standards

The standard hydrogen electrode is the universally accepted reference electrode in aqueous solutions. Unfortunately, such a universal reference electrode does not exist for non-aqueous solutions. By using reference electrodes such as the calomel electrode or the silver/silver chloride electrode with aqueous reference electrolyte 

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It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

All quantities in Eq. (12.6) are measurable The concentrations can be determined by titration, and the combination of chemical potentials in the exponent is the standard Gibbs energy of transfer of the salt, which is measurable, just like the mean ionic activity coefficients, because they refer to an uncharged species. In contrast, the difference in the inner potential is not measurable, and neither are the individual ionic chemical potentials and activity coefficients that appear on the right-hand side of Eq. (12.3). 

Although the inner potential difference is not measurable in principle, it would be useful to have at least good estimates. We can see from Eq. (12.3) that this problem is equivalent to determining the difference in the chemical potential of individual ions. If we knew the standard Gibbs energies of transfer of the ions ... 

Calculate the inner potential difference across (a) anNa 7Na+ standard reversible... 

Fig. 5.1 The standard chemical potentials (jif), the activities (qi), and the inner potentials ((p) in the case of a neutral, hydrophobic polymer and an aqueous solution containing (KA K+ + A ) electrolyte...

If the inner reference of the electrode is such that the iso-pH value (isotherm intersection point) lies at pH = 7 0.5 (according to a DIN standard), which may agree with the Ege vs. pH plot in Fig. 2.11, then the overall potential of this... 

Similar to the standard SBC, GSBP partitions the system into inner and outer regions and the effects of the outer region on the inner, reaction region are represented implicitly within the total effective potential (potential of mean force) [36],... 

The potential developed is determined by the chloride concentration of the inner solution, as defined by the Nemst equation. As can been seen from the above reaction, the potential of the electrode remains constant as long as the chloride concentration remains constant. Potassium chloride is widely used for the inner solution because it does not generally interfere with pH measurements, and the mobility of the potassium and chloride ions is nearly equal. Thus, it minimizes liquid-junction potentials. The saturated potassium chloride is mainly used, but lower concentrations such as 1M potassium chloride can also be used. When the electrode is placed in a saturated potassium chloride solution, it develops a potential of 199 mV vs the standard hydrogen electrode. 

The relativistic effects (Rl) and (R2) can be simulated by adjusting the sizes of basis functions used in a standard variational treatment. This adjustment is usually combined with an effective-core-potential [ECP] approximation in which inner-shell electrons are replaced by an effective [pseudo] potential of chosen radius. The calculations of this chapter were carried out with such ECP basis sets in order to achieve approximate incorporation of the leading relativistic effects.)... 

It is thus reasonable to anticipate that HOC1 could behave as an outer-sphere one-electron oxidant. Indeed, the standard potential for the HOC1/HOC1 - couple is estimated at 0.25 V (9). In prior reports where such a pathway might have been uncovered, alternative pathways generally have been found, such as inner-sphere mechanisms and reactions via Cl2. Reaction via Cl2 is often a viable pathway because of the presence of Cl- either as a contaminant or reaction product and its reaction with HOC1 as in Eq. (4). 

In the treatment of the kinetics of the electron transfer illustrated in Section 4.1, it has been assumed that the propulsive force for the electron transfer was the electrochemical potential E i.e. a quantity directly related to 4>M — < >s). However, since the solvated ions cannot enter the inner layer of the double layer (IHP), the true propulsive force should be < )M — standard rate constant, k°, and the exchange current, i0, should become respectively ... 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

Secondly, instead of a pure and simple electron transfer, the redox reaction can be coupled to a chemical reaction in such a way that the electron transfer takes place either after incorporation of the substrate or an intermediate into the inner coordination sphere of a metal ion ( inner-sphere electron transfer), by formation of a charge transfer complex, or in form of a hydrogen or hydride atom abstraction, respectively. In these cases the reaction between redox catalyst and substrate does not directly depend on the difference of the two standard potentials (see Sect. 2.3). 

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