Concentrated salt solution, proton dissociation

The most trivial explanation for the effect of electrolytes on rate of proton dissociation is to consider the effect of salts on the dielectric constant of the solution (see also Equation 1). In concentrated salt solutions, a considerable fraction of the water molecules are oriented in an hydration shell around the ions thus, their dielectric constant is smaller than in pure water (Hasted et al., 1948). A decreased dielectric constant will accelerate ion-pair recombination and slow down ion-pair separation. 

In concentrated salt solutions, the vapor pressure is lower than that of pure water, and hence it exhibits reduced water activity. This phenomenon is explained by the fact that a considerable fraction of the water molecules are associated with the hydration of the salt ions. The binding energy of these water molecules (which forms the first and the second hydration shells) to the center ion is larger than 10 kcal/mol therefore, they are less likely to participate in the hydration of the newly formed proton. To observe successful proton dissociation, the thermodynamic stable complex must be formed within the ion-pair lifetime. The depletion of the solution from water molecules available for this reaction will lower the probability of the successful dissociation. As demonstrated in Figure 9, this function decreases with the activity of the water in the solution. 

STRATEGY Because NH4+ is a weak acid and Cl- is neutral, we expect pH < 7. We treat the solution as that of a weak acid, using an equilibrium table as in Toolbox 10.1 to calculate the composition and hence the pH. First, write the chemical equation for proton transfer to water and the expression for Ca. Obtain the value of Ka from Kh for the conjugate base by using K, = KxJKh (Eq. 11a). The initial concentration of the acidic cation is equal to the concentration of the cation that the salt would produce if the salt were fully dissociated and the cation retained all its acidic protons. The initial concentrations of its conjugate base and H30+ are assumed to be zero. 

The virial methods differ conceptually from other techniques in that they take little or no explicit account of the distribution of species in solution. In their simplest form, the equations recognize only free ions, as though each salt has fully dissociated in solution. The molality m/ of the Na+ ion, then, is taken to be the analytical concentration of sodium. All of the calcium in solution is represented by Ca++, the chlorine by Cl-, the sulfate by SO4-, and so on. In many chemical systems, however, it is desirable to include some complex species in the virial formulation. Species that protonate and deprotonate with pH, such as those in the series COg -HCOJ-C02(aq) and A1+++-A10H++-A1(0H), typically need to be included, and incorporating strong ion pairs such as CaSO aq) may improve the model s accuracy at high temperatures. Weare (1987, pp. 148-153) discusses the criteria for selecting complex species to include in a virial formulation. 

The correlation between the availability of water and the rate constant of proton dissociation was measured in two systems. In one system, the ratio water methanol of a mixed solution modulated the availability of water [38]. In the other system, made of concentrated electrolyte solutions, the activity of the water was modulated by the salt [39]. The dependence of the measured rate of dissociation [60, 67, 68], either from photoacid or ground state acids, on the activity of the solvent yielded a straight log-log correlation function with respect to the activity of the water... 

We attribute the effect of the amine at C-6 as deriving from the extent of dissociation of the ammonium salt at C-6, and it is an equilibrium effect which depends on the relative concentrations of the protonated phenolate group and the dissociated salt form. Strongly basic amines remove the proton completely and form the ammonium salt. Weakly basic amines do not cause complete ionization of the C-6 phenol. This same effect has been observed for monomeric models in solution. (We can ignore free amine quenching because the concentration of the amine is very low.)... 

Dissociation constant of silicic acid calculated according to the a + = [(Kacx Kw)/c]1/2fbrmula for dissociation of salts formed from weak acid and strong base a+ is the activity of protons (from pH), K w is the ionization constant of water, and c is the concentration of silicate solution. 

Strong bases are those compounds that totally dissociate in water, yielding some cation and the hydroxide ion. It is the hydroxide ion that we normally refer to as the base, because it is what accepts the proton. Calculating the hydroxide ion concentration is really stredghtforward. Suppose that you have a 1.5 M (1.5 mol/L) NaOH solution. The sodium hydroxide, a salt, completely dissociates (breaks apart) into ions ... 

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