Base-dissociation constant and

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). 

In order to determine to what extent these speculations have validity, it is necessary to be able to evaluate more quantitatively the relative contributions of these interactions to the free energies of protein and nucleic acid molecules in water and nonaqueous solvents. For this purpose, a substantial body of quantitative data is required concerning the properties of suitable model compounds in a variety of solvents, including their solubilities, acid-base dissociation constants, and thermodynamics of hydrogen bond formation. The dearth of pertinent data on hydrogen bonds in solvents of interest is particularly frustrating to even a semiquantitative evaluation of the scheme presented in Fig. 7. 

Alpha values for polyfunctional bases are generated in an analogous way, with the equations being written in terms of base dissociation constants and [OH ],... 

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. 

Delphonine shows an unusually high basic dissociation constant, and that of aconine is only a little less. In the case of tertiary vinyl cyclic amines, which prove to be unexpectedly strong bases, Adams and Mahan suggested that they exist in solution as equilibrium mixtures consisting of the tertiary unsaturated base and a quaternary arrangement in which the double bond has moved to the nitrogen, which may be represented thus —. . . +. . . +... 

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 Henderson-Hasselbalch equation provides a general solution to the quantitative treatment of acid-base equilibria in biological systems. Table 2.4 gives the acid dissociation constants and values for some weak electrolytes of biochemical interest. 

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. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

Fig. 10. The formulae of some of the bases in DNA and RNA. The acid dissociation constants and the methylmercury constants and binding centres axe shown (after R. B. Simpson)...

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 .. 

One of the characteristics of acids and bases is that they catalyze certain reactions. Many years ago, J. N. Bronsted studied the relationship between acid strength as measured by the dissociation constant and the rate of a reaction that is catalyzed by the acid. The relationship that Bronsted recognized can be written as... 

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 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. 

The earliest LFER, advanced by Bronsted, correlates the acid dissociation constant and base strength (1/A h) of species with its effectiveness as a catalyst in general acid (At h) and base (Atgl-catalyzed reactions respectively. The relationships take the form... 

D Waldron-Edward. The microdetermination of acid and base dissociation constants by paper electrophoresis. J. Chromatogr. 20 556-562 (1965). 

A similar situation exists for bases. Strong bases such as KOH, NaOH, and Ca(OH)2 dissociate completely in water. Weak bases don t dissociate completely in water, and their strength is measured by the base dissociation constant,... 

X10 . The molarity in this problem tells you that the concentration of benzoic acid is 5.0 X10 . You also know that the concentration of benzoic acid s conjugate base is the Scime as the given H+ concentration. All that remains is to write an equation for the acid dissociation constant and plug in these concentrations. 

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