Ionic strength dependence of the

If the rate equation contains the concentration of a species involved in a preequilibrium step (often an acid-base species), then this concentration may be a function of ionic strength via the ionic strength dependence of the equilibrium constant controlling the concentration. Therefore, the rate constant may vary with ionic strength through this dependence this is called a secondary salt effect. This effect is an artifact in a sense, because its source is independent of the rate process, and it can be completely accounted for by evaluating the rate constant on the basis of the actual species concentration, calculated by means of the equilibrium constant appropriate to the ionic strength in the rate study. 

A more recent examination confirmed the existence of an unusual ionic-strength dependence of the reaction rate , which features a minimum at /r 0.83, but it was noted that the initial kinetics differ from those occurring later in the reaction. Consideration of the equilibria prevailing in aqueous solutions of Cr(VI) produced a simplified rate law... 

Austin et al. [132] measured the ionic strength dependence of the liposome-water distribution of several acidic and basic drugs and modelled the data with a combination of electrostatic and ion pair models. They concluded that the increased apparent Dmw values at higher ionic strength were due primarily to the reduction in surface potential and not to ion pairing. Ion pairing was also excluded because the apparent Dmw varied at fixed ionic strength with the... 

Recognizing the ionic strength dependence of the effect of short range forces in binary interactions, Pitzer was able to develop an empirical relation for B,-a(I). The expression for systems containing strong electrolytes with one or both ions univalent is... 

Baes and Mesmer [3] use the function F I ) proposed by Pitzer to express the ionic strength dependence of the ion interaction coefficient fimx in Guggenheim s equations. For a single electrolyte... 

X is an anion, whose participation in the rate-determining step is suggested by the marked ionic-strength dependence of the rate. 

The interphase between an electrolyte solution and an electrode has become known as the electrical double layer. It was recognized early that the interphase behaves like a capacitor in its ability to store charge. Helmholtz therefore proposed a simple electrostatic model of the interphase based on charge separation across a constant distance as illustrated in Figure 2.12. This parallel-plate capacitor model survives principally in the use of the term double layer to describe a situation that is quite obviously far more complex. Helmholtz was unable to account for the experimentally observed potential dependence and ionic strength dependence of the capacitance. For an ideal capacitor, Q = CV, and the capacitance C is not a function of V. 

Fig. 2 Ionic strength dependence of the radius of gyration for sample Et-PVPLwBr in aqueous NaBr solution Lw=400 nm ( ) Lw=1000 nm (A) Lw=1800 nm (v)- The solid lines are calculated with fa=0.17 and w0=-0.03...

The above discussion has focused attention on electron transfer through the exposed edge of the heme group. This model is suggested from experience with small metal complexes and is substantiated by the rate constants and ionic strength dependence of the cytochrome c crossreactions. 

The zeta potential is the electric potential of a double layer at the slipping point (Fig. 9.8). The slipping point is known to be in front of the colloid s surface at a distance of at least a single hydrated cationic layer (Stern layer) (the exact location of the slipping point is not clear). Based on what was said above with respect to pH dependence and/or ionic strength dependence of the electric double layer, it is also known that as the ionic strength increases, or pH approaches the PZC, the zeta potential decreases (Fig. 9.9). 

Finally, this section has focused primarily on ions in aqueous solution but it should be remembered that aqueous solutions also contain important dissolved neutral species, e.g. O2, CO2, H2CO3, Si(OH)4. For non-ideal solutions, the Setschenow Equation (1899) has traditionally been used to describe the ionic strength dependence of the activity coefficients for dissolved neutral species and can be written as... 

Harris, T. K., Davidson, V. L., Chen, L. Y., Mathews, F. S., and Xia, Z. X., 1994, Ionic strength dependence of the reaction between methanol dehydrogenase and cytochrome 05511 evidence of conformationally coupled electron transfer. Biochemistry 33 12600912608. 

In the radiation chemistry of water, many steps of the process were clarified. The radical mechanism for the radiation chemistry of water was confirmed and the existence of multiple additional species other than H, OH and HjOj, such as HO2, O and other similar radicals were deduced. One additional complication became clear there appeared to be two types of H atoms, with different reactivities. Dainton writes of having suggested that one of them might be an electron in solution however, he was assured by James Franck that that species could not live for chemically significant times.After several years, Dainton shook off this pronouncement and both he and Czapski and Schwarz measured the ionic-strength dependence of the reaction of the H-atom and showed that it had a negative charge. 

In the second line of the program, Length[zlist] is used to calculate the number of p/fs the reactant has. The Mathematica operation Table makes a list of values. The ionic strength dependencies of the two p fs of atp can be calculated as follows ... 

The ionic strength dependence of the rate constant is completely contained within the X, Y and Z terms. Eq. 8 allows a non-linear least-squares fitting of experimental data to be carried out, yielding values for /tx and V. In practice, the monopole and dipole terms cannot be uniquely characterized by this procedure, and using only the Fii term in the fitting procedure yields a satisfactory value for V [. 

Figure 4. Ionic strength dependence of the observed rate constant for electron transfer from reduced Fd to oxidized FNR. The dashed line corresponds to a theoretical fit to the higher ionic strength data using the parallel-plate model. The solid line is a smooth curve drawn through the data points.

S Derived from the ionic strength dependence of the osmotic and mean activity coefficient of Ni(C104)2 solution in [2005GAM/BUG] (cf. [2005GAM/BUG], Section V.4.3). 

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