Dissociation Constant

The product of ionic concentrations, K_W = [H⁺] [OH], is called the Ion Product Constant for water, or more frequently, the Ionization Constant or Dissociation Constant. At 25C, K_W equals 1 x 10^-14. K_W varies with temperature and, at 37C (body temperature), the value is about 3.4 x 10^-14. Figure 17 illustrates the relationship of the Dissociation Constant with water temperature. As indicated earlier, a change in temperature causes a change in equilibrium in a chemical reaction. Because the ionization of water is an equilibrium process, it is also affected by temperature. The Dissociation (or Ionization) Constant, K_w, for water increases with increasing temperature up to about 500F and then decreases. Experimental values of K_W at various temperatures are listed in Table 5 and graphed in Figure 16. Notice in Table 5 that the pH of pure water changes with temperature. For pure water at any temperature, however, [H⁺] = [OH^-]. It should be noted that the equation pH + pOH = 14 is true only at or near 25C (77F).

Example: The hydroxyl concentration [OH^-] of a water solution at 25 C is 7.2 x 10^-9 moles/liter. Calculate the pH of the solution.

Solution:

At 25C, an equilibrium exists between pure molecular water and its ions. The [H+] equals the [OH^-] and both have values of 1 x 10^-7 moles/liter. Using the pH definition, it follows that the pH of pure water at 25C is 7. pH values less than 7 indicate an acidic solution and values greater than 7 indicate a basic or alkaline solution.

Figure 17 Ion Product Constant for Water

Summary

The important information in this chapter is summarized below.

Acids, Bases, Salts, and pH Summary

The following terms are defined in this chapter:

Acid - substances that produce hydrogen (H+) in water solutions

Base - substances that produce hydroxide ions (OH^-) in water solutions

Salt - a compound composed of positive and negative ions held together with an ionic bond

Alkalies - certain soluble salts, principally sodium, potassium, magnesium and calcium, that have the property of combining with acids to form neutral salts

pH - the negative logarithm of the hydrogen concentration [H+] in moles/liter

pOH - the negative logarithm of the hydroxyl concentration [OH^-] in moles/liter

Dissociation constant of water (K_w) - the product of ionic concentrations, ([H+] [OH^-]); at 25C K_w = 1 x 10^-14

The formula for pH is: pH = -log [H+] Therefore, [H+] = 10 ^-pOH

The formula for pOH is: pOH = -log [OH^-] Therefore, [OH] = 10 ^-pOH

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PLANT CHEMISTRY - BASIC SEPARATION THEORY

Introduction

The fundamental principles upon which a gaseous diffusion plant is based are as follows.

1.All the particles (atoms, molecules, or ions) which make up the gas are continuously moving in straight lines in all directions. The particles collide with anything in their path (e.g., other particles or the walls of the container), thereby altering the course of moving particles but not their average velocities.

2.All of the particles have the same average kinetic energy. Accordingly, if the masses of the particles are different, so must their velocities be different with the lighter particles having the greater average velocities.

On the basis of the two principles stated above, if a quantity of gas is confined by a porous membrane or barrier, some of the gas will escape through the pores in the barrier. If the confined gas is isotopic (a mixture of particles of different molecular weights), the lighter particles of the gas will have a greater tendency to diffuse through the barrier because of their greater average velocities. Consequently, the gas which has passed through the barrier will be enriched in the light constituent while that gas which has not diffused through the barrier will be depleted in the light constituent.