Ionization constant of water

Woolley, E. M. Hepler, L. G., Apparent ionization constants of water in aqueous organic mixtures and acid dissociation constants of protonated co-solvents in aqueous solution, Anal. Chem. 44, 1520-1523 (1972). 

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. 

Equilibrium constant on the molar concentration scale Michaelis constant Ionization constant of water Equilibrium constant on the mole fraction scale General rate constant Boltzmann s constant Rate constant of step i... 

The first equation defines the ionization constant of water at 25 °C (we omit the sign of the charges to simplify notation). The second is the same as Eq. (2.6.3), while the third is the conservation of the total initial concentration of the (weak) acid Nj- (we assume that there is no change in volume during the titration, hence this is the same as the conservation of the total number of acid molecules). The fourth equation is the electroneutrality condition, where [iV ] is the concentration of the added (strong) base. 

The second-order rate constant for the reaction of a hydrogen atom with a hydroxide ion to give an electron and water (hydrated electron) is 2.0 x 10 M s . The rate constant for the decay of a hydrated electron to give a hydrogen atom and hydroxide ion is 16M s. Both rate constants can be determined by pulse radiolytic methods. Estimate, using these values, the pA of the hydrogen atom. Assume the concentration of water is 55.5M and that the ionization constant of water is 10 M. 

Taking into account the ionization constant of water = aH+<5 oH ) 298.15 K, the equilibrium potentials can be calculated with the help of the Nernst equation at different pH values. Since ATw = 1.008 X lO-i at PH2 = 1 bar. 

CD reactions sometimes proceed via a metal hydroxide intermediate the concentration of OH ions in the solution is particularly important in such cases. Since almost all CD reactions are carried out in aqueous solutions, the pH of the deposition solution will give this concentration. In translating pH into OH concentration, the very temperature-dependent ioiuzation constant of water should be kept in mind, as mentioned previously. The reason for this can be seen from Table 1.2, which gives the OH concentration in water at a pH of 10 (a typical pH value for many CD reactions), calculated from the ionization constant of water, from the relation... 

Table 7. IONIZATION CONSTANTS OF WATER AT VARIOUS TEMPERATURES...

Kfo and Kia are quantitatively interrelated by the ionization constant of water (ion product of water), Kw ... 

The three independent rate constants /cqK, and kfc = kf + k, Kf fully determine the kinetic properties of Scheme 2, because the rate constants kf for enolization are related to those of the reverse reactions, Equation (9), where Kw is the ionization constant of water. We use primed symbols for the enolization of the neutral ketone K. In the rate equation for enolization, the terms k(f and k e ATW/AT are kinetically indistinguishable (see Equation (10) below). 

Values of the hydrolysis constant need not be measured and tabulated separately, because they are correlated to the dissociation constants of the weak acid (cf. equation (i)) and the ionization constant of water (cf. equation (ii)) ... 

Thus, the hydrolysis constant is equal to the ratio of the ionization constants of water and of the acid. 

Harned, H. S. and Hamer, W. J. The ionization constant of water and the dissociation of water in potassium chloride... 

In an aqueous solution, most of the water molecules are not ionized. In fact, the extent of ionization of pure water is constant at any given temperature and is usually expressed in terms of the ion product (or ionization constant) of water, K, ... 

Recall that an equilibrium-constant expression relates the concentrations of species involved in an equilibrium. The relationship for the water equilibrium is simply [H30" ][0H ] = K q. This equilibrium constant, called the self-ionization constant of water, is so important that it has a special symbol, Its value can be found from the known concentrations of the hydronium and hydroxide ions in pure water, as follows ... 

---