Constant base-dissociation

Weak bases only partially accept protons from the solvent and are characterized by a base dissociation constant, kj,. For example, the base dissociation reaction and base dissociation constant for the acetate ion are... 

Polyprotic bases, like polyprotic acids, also have more than one base dissociation reaction and base dissociation constant. 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

Two useful characterization applications involving acid-base titrimetry are the determination of equivalent weight, and the determination of acid-base dissociation constants. 

The initial goal of the kinetic analysis is to express k as a function of [H ], pH-independent rate constants, and appropriate acid-base dissociation constants. Then numerical estimates of these constants are obtained. The theoretical pH-rate profile can now be calculated and compared with the experimental curve. A quantitative agreement indicates that the proposed rate equation is consistent with experiment. It is advisable to use other information (such as independently measured dissociation constants) to support the kinetic analysis. 

The equilibrium constant of the proton transfer (125), omitting the activity of the H20, is known as the base dissociation constant and is denoted by K , to distinguish it from the KA of (124). 

The reaction generates hydroxide anions, so the solution is basic. Fluoride acts as a base, so the equilibrium constant is a base dissociation constant, Zj,. 

Chemists have calculated the extent to which most acids and bases will dissociate in water. This mathematical value is called the acid dissociation constant (Ka) for acids and the base dissociation constant (Kb) for bases. The higher the value for Ka or Kb, the more the acid or base dissociates in water and the stronger it is. 

Identify each of the following terms (a) hydronium ion, (h) Bronsted theory, (c) proton (Bronsted sense), (d) acid (Bronsted sense), (e) base (Bronstcd sense), (/) conjugate, (g) strong, (h) acid dissociation constant, (/) ionization constant, (/) base dissociation constant, (k) autoionization, (/) pH, and f/w) K .. 

Base dissociation constant in a micelle based on concentration as a mole ratio... 

Step 2 Write the expression for the base-dissociation constant. 

Br0nsted-Lowery acids are H+ donors and bases are H+ acceptors. Strong acids dissociate completely in water. Weak acids only partially dissociate, establishing an equilibrium system. Weak acid and base dissociation constants (Ka and Kb) describe these equilibrium systems. Water is amphoteric, acting as both an acid or a base. We describe water s equilibrium by the Kw expression. A pH value is a way of representing a solution s acidity. Some salts and oxides have acid-base properties. A Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. 

The equilibrium constant expression is called the weak base dissociation constant, K[, and has the form ... 

The p/<, of a base is actually that of its conjugate acid. As the numeric value of the dissociation constant increases (i.e., pKa decreases), the acid strength increases. Conversely, as the acid dissociation constant of a base (that of its conjugate acid) increases, the strength of the base decreases. For a more accurate definition of dissociation constants, each concentration term must be replaced by thermodynamic activity. In dilute solutions, concentration of each species is taken to be equal to activity. Activity-based dissociation constants are true equilibrium constants and depend only on temperature. Dissociation constants measured by spectroscopy are concentration dissociation constants." Most piCa values in the pharmaceutical literature are measured by ignoring activity effects and therefore are actually concentration dissociation constants or apparent dissociation constants. It is customary to report dissociation constant values at 25°C. 

The concentration of water is almost constant in dilute solutions. Multiplying both sides of the equilibrium expression by [H2O] gives the product of two constants on the left side. The new constant is called the base dissociation constant, Kb. 

Table 8.3 lists the base dissociation constants for several weak bases at 25°C. Nitrogen-containing compounds are Bronsted-Lowry bases, because the lone pair of electrons on a nitrogen atom can bond with H+ from water. The steps for solving problems that involve weak bases are similar to the steps you learned for solving problems that involve weak acids. 

The characteristic taste of tonic water is due to the addition of quinine. Quinine is a naturally occurring compound that is also used to treat malaria. The base dissociation constant, Kb, for quinine is 3.3 x 10 . Calculate [OH ] and the pH of a 1.7 x 10 mol/L solution of quinine. 

Pyridine, C5H5N, is used to manufacture medications and vitamins. Calculate the base dissociation constant for pyridine if a 0.125 mol/L aqueous solution has a pH of 9.10. 

The use of capillary electrophoresis (CE) during the synthetic drug development is described from the preclinical development phase to the final marketed stage. The chapter comprises the determination of physicochemical properties, such as acid—base dissociation constants (pKJ, octanol—water distribution coefficients (logP), and analysis of pharmaceutical counterions and functional excipients. 

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