Base-Ionization Equilibria

Many over-the-counter drugs are salts of bases 

These salts are often chlorides (frequently called hydrochlorides) and are analogous to ammonium chloride.You might find it instructive to read the ingredients from the labels for these products. 

Equilibria involving weak bases are treated similarly to those for weak acids. Ammonia, for example, ionizes in water as follows  

In terms of the thermodynamic equilibrium constant, the activity of H2O is nearly constant and essentially 1, so it does not appear explicitly in the equilibrium constant. 

Because the concentration of H2O is nearly constant, you can rearrange this equation as you did for acid ionization.  

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Weak acid and weak base ionization equilibria have extremely small ionization constants. For example, the ionization constant of acetic acid is 1.8 X 10 . The ionization constant of ammonium hydroxide is 1.7 x 10 This means that there are very few ions present at equilibrium most of the acid or base is present as the un-ionized species. This is the reason they are described as weak acids or bases. The position of ionization equilibrium in each case would be as illustrated in Figure 11.4. 

CH3COO (ary) + HjOCO 5= CHjCOOHCa ) + OH (ary) and the base ionization equilibrium expression is written as... 

Membrane uptake of nonionized solute is favored over that of ionized solute by the membrane/water partition coefficient (Kp). If Kp = 1 for a nonionized solute, membrane permeability should mirror the solute ionization curve (i.e., membrane permeability should be half the maximum value when mucosal pH equals solute pKa). When the Kp is high, membrane uptake of nonionized solute shifts the ionization equilibrium in the mucosal microclimate to replace nonionized solute removed by the membrane. As a result, solute membrane permeability (absorption rate) versus pH curves are shifted toward the right for weak acids and toward the left for weak bases (Fig. 7). 

The method proposed by Blumstein at is based on the conductivity measurements. It is suitable for the systems in which shift of ionization equilibrium during polymerization takes place. This method was successfully applied to follow template polymerization of p-styrene sulfonic acid in the presence of polycationic ionenes used as template. The results confirm data obtained for the same system by another methods. 

Although the mechanism of eqns (7) and (8) may be a reasonable one for the breakdown of [109], for some of the more reactive species, especially those derived from chloroacetic or dichloracetic acid, it may not be, as shown by the following considerations. For this mechanism to be valid the value of fc2 calculated from the relationship /cHO- = A fc2must be (i) less than the time constant for a molecular vibration (1012-1014s-1) fii) less than the value of based on the estimated Ka and the assumption that the ionization equilibrium (7) is diffusion controlled in the thermodynamically favourable direction, i.e. k1 = 10nM-1s-1. These two conditions are easily fulfilled with the hemiacetals and the less reactive hemiorthoesters derived from pinacol monoesters (see Table 15), but with the more reactive hemiorthoesters the calculated values of k2 lie close to or are greater than the cal-... 

We fitted the combined data with model spectra based on the atomic data compiled by Raymond and Smith (1977). The model spectra employed here are both for collisional ionization equilibrium (CIE) and non-equilibrium ionization (NEI) models with cosmic abundances (Allen, 1977). Single Te spectrum for both models can not fit the data. Two components of different Te models can reproduce the data well for both models. The physical parameters obtained with CIE models are self inconsistent because the ionization parameter r ( the electron density n X the elapsed time t the after shock heating ) is about 1011cm 3sec which is too short by an order of magnitude for the CIE condition to be reached. 

In performing a calculation based on an acid or base ionization constant expression such as Eqs. (13-7) or (13-8), there are often many unknowns. Remember that in an algebraic problem involving multiple unknowns, one needs as many equations as there are unknowns. The equilibrium constant expression itself is one equation, and the Kw expression is always available. Two other types of equation are often useful equations expressing... 

EXAMPLE 1 Aparticularly simple and common case of the buffer solution is one in which the weak acid and weak base are conjugates of each other. We could choose acetic acid as the weak acid and the acetate ion is, then, the weak base. Since relatively large amounts of each are needed, it would not be possible to use just a solution of the acid because the ionization equilibrium favors the acid to a large extent. However, we can still make an acid-acetate buffer by these methods ... 

Well, in case you hadn t noticed, sodium ions don t seem to do much in chemistry. They are almost always spectator ions, because they don t participate in any of the chemical reactions. Their job is to provide a charge balance to the anions in solution. So, in calculating the pH of sodium acetate, we ignore sodium. The acetate ion, however, is the conjugate base of the weak acid, acetic acid. Therefore, the acetate ion is a base, and we can write this ionization equilibrium equation. 

Just like sodium ions, chloride ions are spectator ions in acid-base chemistry. Their job is to provide a charge balance to the cations in solution. So, in calculating the pH of lidocaine hydrochloride we ignore the chloride ion. Now we could draw out the structure or write the molecular formula of lidocaine and its conjugate acid, but it is tedious to do so. Let s do what most chemists do, and postulate a temporary abbreviation for these species. How about using L for lidocaine, and HL+ for its conjugate acid Now, we can write an equation for the acid ionization equilibrium reaction. 

This same process can be seen in bases. Consider the equilibrium of a weak base ionization, as shown below ... 

At this point, the calculation becomes a little different. Because the benzoate ion is a base, the equilibrium constant expression must be the base ionization constant, Kb. You haven t been given the value of Kb, however, so you will have to generate it using Equation 13.8 ... 

Strong acids and bases ionize completely in solution, while weak acids and bases partially ionize. The partial ionization of weak acids and bases creates solutions in which an equilibrium is established. 

On the other hand, if HA is an uncharged acid z = — V, e.g. CH3—CO2H), the right-hand side of Eq. (4-10) involves the sum of two reciprocal radii (zha = 0) and a strong influence of the relative permittivity on the ionization equilibrium is expected. Because in acid/base reactions of this charge type, neutral molecules are converted into anions and cations, which attract each other, reaction (4-5) will shift to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Ionization increases when increases. This rule is qualitatively verifiable for water and alcohols as... 

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. 

Because HX is a strong acid, its conjugate base is weak. That is, HX is nearly 100% ionized because H2O is a stronger base (in the forward reaction) than is the conjugate base X (in the reverse reaction). In other words, the ionization equilibrium lies almost completely to the right because the base H2O has a much greater attraction for the H+ ion than does the base X . You can think of this as the battle of the bases Which of the two (H2O or X ) has a... 

In contrast to strong bases, a weak base ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion. The weak base methylamine (CH3NH2) reacts with water to produce an equilibrium mixture of CH3NH2 molecules, CH3NH3+ ions, and OH ions. 

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 constant is called the base ionizahon constant. The base ionization constant is the value of the equilibrium constant expression for the ionizahon of a base. The smaller the value of K, the weaker the base. values and ionization equahons for several weak bases are listed in Table 19-4. 

For weak acids, the magnitude of is very small, and as a result the resulting H3O+ and A ions will be produced in small amounts. Under those conditions, both Yjj+ and y will be approximately equal to one, and then one can approximate the thermodynamic equilibrium constant, K, by the concentration-based ionization constant, Ka. 

Since water is amphiprotic, that is, it can act both as an acid and as a base, every aqueous solution is characterized by the auto-ionization process, in which one H2O molecule transfers a proton to another H2O molecule. The water auto-ionization equilibrium must always be satisfied, whether other acids or bases are present in the solution or not. 

Thus the change of the concentration of free base of one order of magnitude will produce an increase of potential of 59 millivolts at 25°, i.e., the increase of base concentration from 1CH to 10 mol per liter will produce the same effect on the potential as the change from 10-3 to lO2 mol per liter. A plot of E against the volume of added base will be very closely of the form given in Fig. 2 (o). A more adequate discussion of the theory of the acid-base titration, including the ionization equilibrium of water, is given later in this chapter. 

Compared with the total concentration of water, very few water molecules are consumed by this reaction, so we can treat [H2O] as a constant. Thus we can write the base ionization constant (Ky), which is the equilibrium constant for the ionization reaction, as... 

Because this reaction produces OH ions, the sodium acetate solution will be basic. The equilibrium constant for this hydrolysis reaction is the base ionization constant expression for CHaCOO, so we write (see p. 615)... 

Step 3 From the above discussion we write the equilibrium constant of hydrolysis, or the base ionization constant, as... 

Consequently, when a strong acid or base is added to water, the water ionization equilibrium shifts, and the pH of the solution changes due to the added H" ions (for an acid) or hydroxyl ions OH" (for a base). 

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