Dissociation constant of water

First Carbonation. The process stream OH is raised to 3.0 with carbon dioxide. Juice is recycled either internally or in a separate vessel to provide seed for calcium carbonate growth. Retention time is 15—20 min at 80—85°C. OH of the juice purification process streams is more descriptive than pH for two reasons first, all of the important solution chemistry depends on reactions of the hydroxyl ion rather than of the hydrogen ion and second, the nature of the C0 2 U20-Ca " equiUbria results in a OH which is independent of the temperature of the solution. AH of the temperature effects on the dissociation constant of water are reflected by the pH. 

Figure 8,4 Ion dissociation constant of water as a function of P and T conditions. From Eugster and Baumgartner (1987). Reprinted with permission of The Mineralogical Society of America.

Table 8,3 Ionic dissociation constant of water for T between 0 and 60 °C at P = 1 bar.

It is customary to describe the dissociation constants of both acids and bases in terms of pK values. This is possible in aqueous biological systems because a simple mathematical relationship exists between pK , pK, and the dissociation constant of water pK . 

Explain why the equality of the hydrogen ion and hydroxyl ion concentrations is violated when certain salts are dissolved in water. Compare the values of the dissociation constants of water, acetic acid, carbonic acid, the bicarbonate ion, and aluminium hydroxide. How can the hydrolysis process be explained from the viewpoint of the law of mass action In what cases is hydrolysis reversible and in what cases does it proceed virtually to the end ... 

As discussed in Section 3.10.3, in the gas phase the basicity of simple amines follows the order NMe3 > NHMe2 > NH2Me > NH3 because of the electron donating effect of the methyl (Me) groups. In solution, however, we can define a basicity constant as the equilibrium constant for the reaction shown in Equation 3.4. Note it is important to specify temperature, solvent (usually water) and solution ionic strength, 1 Basicity constants are related to the acid dissociation constants (/Q of the base s conjugate acid via the dissociation constant of water, K = 10 14 at 25 °C. Thus Kbx K = Kw. 

The Na+/H+ exchanger is usually considered to catalyze the exchange of Na+ for H+. It should be noted, however, that because the kinetics of H+ binding are identical to the kinetics of OH de-binding (by virtue of the small dissociation constant of water), it cannot be determined whether the system acts as an Na+/H+ exchanger or as an Na+/OH cotransporter. 

The basic properties of water such as viscosity, dissociation constant, dielectric constant, compressibility, and the coefficient of thermal expansion play a major role in determining optimal reaction conditions for obtaining maximum benefits in both SCWO and WAO processes. The properties of water change dramatically with temperature, particularly near the critical point [24-26]. A well-known example, the variation of pAw with temperature at the saturation pressure, is shown in Fig. 3. The dissociation constant of water goes through a maximum around 250°C (pAw minimum), and then undergoes a sharp decline as the temperature approaches the critical point. The density and the dielectric constant of water also show sharp changes close to the critical point, as shown in Fig. 4. 

In this case, indeed, the integration constant can be derived from data which we have already calculated. The equihbrium constant of this reaction can be calculated from the dissociation constant of water and of carbon dioxide K. Thus we have... 

The change in the dissociation of water with the temperature is also in agreement with the theory. According to the latest experiments of Noyes, Kato, and Sosman on the hydrolysis of ammonium acetate at high temperatures, the dissociation constant of water increases rapidly with the temperature. This is shown by the following table ... 

The coefficients in equations (26) and (27) for the dissociation of a number of acids and the solubility of calcium carbonate are given in Table A6.5 (Millero, 1979, 1995). The results for carbonic and boric acid are taken from the measurements of Culberson and Pytkowicz (1968). The effect of pressure on the solubility of calcite and aragonite has been determined from the measurements of Ingle (1975). The effect of pressure on the dissociation constants of water, hydrogen sulfate, hydrogen sulfide, ammonia, and hydroffuoric and phosphoric acids have been estimated from molal volume and compressibility data. 

This calculation relies on the prediction of the dissociation constant of water, K, at elevated temperatures. The prediction involves the dielectric constant of water and a power series consistent with non-electrostatic interactions in the absence of a dielectric medium (12). The dissociation constant of water was calculated based upon the assumption that the entropy of the dissociation of water is the sum of the contributions of the electrostatic and nonelectrostatic interactions. The deviation between the predicted and the experimental (12) values of pK is smaller than 0.01. 

One of the most accurate methods for determining the dissociation constant of water is based on e.m.f. measurements of cells of the type... 

Inspection of the van t Hoff equation shows that the effect of a temperature change on is proportional to the magnitude of the enthalpy of the reaction. The larger the AH°, the stronger the temperature dependence of For example, Fig. 1.5 shows the steep temperature dependence of the dissociation constant of water (H2O = + OH ) for which AH° = +13.362 kcal/mol. This... 

Equation (5) is the basis of analytical neutralizations. Kv, is the dissociation constant, or rather, the ionization constant or ion product of water. Hydroxyl and hydrogen ions are simultaneously formed as a result of the slight dissociation of water. The ion product of water, though very small, has been determined by various investigators, with excellent agreement. The constant varies considerably with temperature. The following table contains the values given by Kohlrausch and Heydweiller for the dissociation constant of water at various temperatures. 

It seems surprising at first to refer to water as a base. This is probably due to the fact that we have always considered the hydrogen ion concentration in water identical with the hydroxonium concentration, and because the dissociation constant of water as well as of other solvents is exceedingly small. Alcohol is about five hundred times weaker than water as a base. Were we to add water to an alcoholic solution of an acid, the water would act qualitatively as does ammonia when it is added to an aqueous acid solution ... 

ScHOOEL has already pointed out the influence of heat on the color of an indicator. He found that boiling a solution of an alkali sensitive indicator shifted the color to the basic side, while the color of acid sensitive indicators was displaced towards the acid side. He explained this on the basis of an increase in the dissociation constant of water. This interpretation is illustrated by the following discussion. 

At room temperature the corresponding hydroxyl ion concentration is about 10 . The increased dissociation of water at higher temperatures will not affect the [OH ]] of a solution which is already 1/10,000 normal with respect to this ion (at least if we work with not too dilute alkali solution). The [OH ] will thus remain approximately 10". The dissociation constant of water at 100°, however, is about 100 times as large as at 18°, so that the hydrogen ion concentration at 100° will be 100 times as great... 

Table 1. Ion Product (Dissociation Constant) of Water at Various Temperatures...

The above technique was applied by Harned and his colleagues to determine acidity constants for a variety of weak acids in both water and in water-non-aqueous solvent mixtures [3]. It may also be used to determine the self-dissociation constant of water. In the case of moderately weak acids the extrapolation procedure requires a more careful consideration of the contribution of H to the ionic strength. More details can be found in the monograph by Harned and Owen [3]. 

The stability line for gibbsite is calculated from the known solubility product of gibbsite, defined by equation 6.21. The dissociation constant of water ... 

Remember that [OH ] is fixed by pH because the dissociation constant of water requires that [OH-] = 10 V[H+]. 

The estimation of temperature variations depends rather strongly on the temperature dependence of, the dissociation constant of water, and agreement with measured values is not uniform, hi the case of HCl solutions, the discrepancy is minimal at concentrations below, for example, 0.02 mol dm-3 at = 450 mT, but is not negligible at B= 640 mT. In the case of NaOH solutions, the discrepancy is minimal even at 0.025 mol dm" 2 when B = 450 mT. In NaBr solutions, the discrepancy is large even in a 0.005 mol dm- 3 solution. 

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