Base ionization constant, defined

A strong base is a substance that reacts completely with water, so that the base ionization constant defined in Eq. (14) or (15) is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. A weak base will be characterized by a base ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Eq. (12) favors the existence of unreacted free base. 

The higher the ionization constant, the stronger the acid, and consequently the weaker the base. Thus the value of Ka is at the same time a measure of the strength of the base there is no need to define a base ionization constant separately. 

Base ionization constants You won t be surprised to leam that like weak acids, weak bases also form equihbrium mixtures of molecules and ions in aqueous soluhon. Therefore, the equilibrium constant provides a measure of the extent of the base s ionization. The equilibrium constant for the ionization of methylamine in water is defined by this equilibrium constant expression. 

The extent of ionization of a weak base is quantified with the base ionization constant, K ). For the general reaction in which a weak base ionizes water, we define as follows ... 

The book is organized into eight chapters. Chapter 1 describes the physicochemical needs of pharmaceutical research and development. Chapter 2 defines the flux model, based on Fick s laws of diffusion, in terms of solubility, permeability, and charge state (pH), and lays the foundation for the rest of the book. Chapter 3 covers the topic of ionization constants—how to measure pKa values accurately and quickly, and which methods to use. Bjerrum analysis is revealed as the secret weapon behind the most effective approaches. Chapter 4 discusses experimental... 

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. 

More important and more widely applicable criteria of protonation sites are available in the effects of more distant substituents, in particular meta and para in benzene derivatives, which are as a rule purely electronic in origin. Exceptionally, they may be complicated by steric effects (e.g., in polysubstituted derivatives). The most widely useful approach is that of Hammett (1940), who suggested that the effects of substituents on the ionization constants of benzoic acid may be taken as a measure of their effectiveness in other systems involving other reaction centres and in reactions other than acid-base equilibria. He thus defined substituent constants, a, by the equation... 

Relative quantitative strengths of acids and bases are given either by their ionization constants, and K, or by their pK and pK values as defined by ... 

It is usual to discuss the ionization of a base B in terms of its conjugate acid BH+ in order to use the same set of equations for both acids and bases. The ionization constant Ka is defined by... 

All spectroscopic lines have a natural line width, and this can be of great use to the kineticist. This natural line width is determined by the lifetime of the excited state of the molecule. If this is short the line width is broad, while longer lifetimes give more sharply defined lines. If reaction occurs, this can alter the lifetime of the excited state and so change the natural line width of the transition. A detailed spectroscopic analysis gives relations between the width of the line, the lifetime of the reacting species and the rate constant for the reaction. This has proved a very important tool, especially for reactions in solution such as proton transfers and acid/base ionization processes. 

In Chapter 4, the ionization constant (i.e., the reaction constant of dissolution) for weak acids and acid phosphates was defined. The concept of the ionization constant is very general and useful while discussing dissolution of sparsely soluble oxides in acid-base reactions. We assign the symbol K for this constant. 

This table lists the dissociation (ionization) constants of over 1070 organic acids, bases, and amphoteric compounds. All data apply to dilute aqueous solutions and are presented as values of p7C, which is defined as the negative of the logarithm of the equilibrium constant K for the reaction... 

In practice one prefers to use the pKa value instead of ionization constant. Kg (la). In 1923 Bronsted extended the use of acidic ionization constants to bases defining Kb as well as pKb (lb). Consequently, we may infact be able to use pKg values to measure basicities of various nitrogen atoms in complex heterocyclic molecules. 

For acid-base complexes, the lone difference between a salt and a co-crystal is the location of the acidic H atom(s) in the crystal structure. While it is generally accepted that proton transfer will occur between an acid and base to form a salt in solution when the difference between their acid ionization constants (pXa of base - pXa of acid or ApXa) is greater than two or three units, crystallization may yield salts, co-crystals, or disordered solid forms that exhibit partial proton transfer when the ApXa is less, with the exact location of the acidic proton being strongly dependent on the specific crystal packing environment. Here, it must be understood that the value is a solution property that is not specifically defined in crystals and as such, cannot be transferred to the solid state in a general way. " ... 

The importance of the equilibrium law is widespread throughout chemistry. We have seen in Chapter 7 that defining the optimum conditions for certain key industrial gas phase reactions is dependent on a thorough understanding of the factors that determine the proportions of reactants and products in an equilibrium mixture, and we will return to a consideration of the Haber process later in this chapter. However, chemists use the equilibrium law to represent the extent to which a weak acid or base ionizes or dissociates, defining terms such as the dissociation constants and in relation to these effects. The behaviour of acid-base indicators is also explained in terms of the equilibria involved and the application of Le Chatelier s principle. 

In Chapter 13, conjugate acid-base pairs were defined HA is the conjugate acid of A, and A" is the conjugate base of HA. A simple relation between the acidic and basic ionization constants in a conjugate pair was established (page 251) KJi.ty = Ky,. Thus, if we know the ionization constant for an acid or base, we can find the ionization constant for its conjugate base or acid. 

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