Activities of species

Conversely Figure 6.17 shows that when XjTI<0, then for any fixed 0j one has an increase in pj, or surface activity aj, in relation to the pj or aj value corresponding to 11=0. In fact, denoting by pjor a the activity of species j in absence of the double layer (FI=0), then one obtains from equations (6.41) or (6.42) for any fixed j ... 

Focusing on the deposition potential of species A, the activity of species A in the alloy, aA, can be expressed as... 

The case of a real solution, shown by equation 2.62, is handled by introducing a dimensionless correction, called the activity coefficient (y ). In the high dilution limit (mi -> 0), Xi = 1, and equation 2.62 reduces to 2.61. The activity of species i is simply a, = ypnjma. 

In this article, a brief discussion will be given on the relevance of continuum theory in explaining the rate of electron transfer and the activation of species in solution we will concentrate in particular on molecular and quantum mechanical models of ET reactions at the electrode/electrolyte interface that are needed to replace those based on the continuum approach. ... 

E q = a. constant emf, including that of the reference electrode and the standard potential of the ion being measured = activity of species A with charge Z ... 

The activity of species C is its concentration multiplied by its activity coefficient. The activ-ity coefficient measures the deviation of behavior from ideality. If the activity coefficient were I, then the behavior would be ideal and the form of the equilibrium constant in Equation 8-1 would be correct. 

To measure an equilibrium constant, we measure concentrations (actually activities) of species at equilibrium. This section shows how spectrophotometry can be used for this purpose. 

The above discussion has shown that the effects of amine basicity and concentration on catalytic activity are consistent with Equilibrium 3. No explanation has been presented for the increased hydrogenation activity of species II as compared with the amine-free hydrido carbonyl (species III). The data in Table IV and the first-order dependence of aldehyde provide the basis for such an explanation. 

According to Nernst s equation, there should be linear relationship between the equilibrium potential of the metal/metal-ion electrode (M/M2+) and the logarithm of the concentration of 1VF+ ions [Eq. (5.13)]. This linear relationship was experimentally observed for low concentration of the solute MA, for instance, 0.01 mol/L and lower. For higher concentrations a deviation from linearity was observed, see, for example, Figure 5.12. The deviation from linearity is due to ion-ion interactions. In the example in Figure 5.12, the ion-ion interactions include interaction of the hydrated Ag+ ions with one another and with NO 3 ions. The linear relationship between the equilibrium potential E and the log of concentration is obtained if the square brackets in Eq. (5.13) signify the activity of species within those brackets. The activity of the species i is defined by the equation... 

Thus, the Nernst equation relates the potential difference k = (pP — (ps at the interface to the activities of species i in phases m and 5. The standard state of the metal cation in the metal is unity. In other cases, it is possible to assume that the activity inside the nonmetallic electrode phase is constant. Therefore, the Nernst equation simplifies to... 

Typical for ion-selective electrodes is the selective exchange of one or a limited number of species dissolved in solution at the surface of the membrane. This can be obtained through different mechanisms, as will be discussed in the next section of this chapter. The intensity of this exchange determines the values of Ex and E2 and is therefore the value of AE. As a consequence, the higher the activity of species i in the solution, the higher the value of AE will be. This principle can be used for electroanalytical purposes - hence the value of potentiometric sensors. 

A comparison of Eqs. (1.28) and (1.34) indicates that for a redox reaction at the interface electrode-solution under equilibrium the Galvani potential difference at the interface is a function of the activities of species O and R in the solution and it is not possible to fix this potential difference and these activities in an independent way. In contrast, in a chemical equilibrium the concentrations of all reactant species are fixed. 

In the simplest case in which the activities of species O and R are equal to the unity and the SHE reference electrode is considered, the measured potential... 

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