Acid base system conjugate

The most effective buffering system contains equal concentrations of the acid, HA, and its conjugate base, A-. According to the Henderson-Hasselbalch equation (2.6), when [A-] is equal to [HA], pH equals pKa. Therefore, the pKa of a weak acid-base system represents the center of the buffering region. The effective range of a buffer system is generally two pH units, centered at the pKa value (Equation 2.9). 

Such an equilibrium system is termed a conjugate (or corresponding) acid-base system. A and B are termed a conjugate acid-base pair. It is important to realize that the symbol H+ in this definition represents the bare proton (unsolvated hydrogen ion), and hence the new definition is in no way connected to any solvent. The equation expresses a hypothetical scheme for defining the acid and base - it can be regarded as a half reaction which takes place only if the proton, released by the acid, is taken up by another base. 

The scavenging reactions of OH radicals by carbonate or bicarbonate ions (Eqs. 6-25 and 6-26) are usually referred to as electron transfer reactions. The carbonate radical is the conjugated base of the bicarbonate radical the acid/base system HCO /CO exhibits a plvv of between 7 and 9.6 (Hoigne, 1998). 

The acid and base which differ by a proton according to this relationship are said to be conjugate to one another every acid must, in fact, have its conjugate base, and every base its conjugate acid. It is unlikely that free protons exist to any extent in solution, and so the acidic or basic properties of any species cannot become manifest unless the solvent molecules are themselves able to act as proton acceptors or donors, respectively that is to say, the medium must itself have basic or acidic properties. The interaction between an acid or base and the solvent, and in fact almost all types of acid-base reactions, may be represented as an equilibrium between two acid-base systems, viz.,... 

Note that nothing has been stated about the charge of the acid HA, which can be a neutral molecule HA or a cation BH such as the ammonium ion. The add cannot, however, be an anion HA , because in general such an anion is also a weak base and so two acid-base systems in addition to water are involved. For a cationic add BH the ionization constants of the acid and of the conjugate base are related. Thus the ionization constant of BH as an acid is... 

Control of pH is vital in synthetic and analytical chemistry, just as it is in living organisms. Procedures that work well at a pH of 5 may fail when the concentration of hydronium ion in the solution is raised tenfold to make the pH 4. Fortunately, it is possible to prepare buffer solutions that maintain the pH close to any desired value by the proper choice of a weak acid and the ratio of its concentration to that of its conjugate base. Let s see how to choose the best conjugate acid-base system and how to calculate the required acid-base ratio. 

When hydrogen-bonding involves titratable groups, the pmay be increased or decreased according to circumstances. If the acidic form of an acid is acting as a donor, removal of the proton will be inhibited and the pwill be increased. Conversely, if the basic form of a conjugate acid-base system is acting as an acceptor, addition of a proton to the base will be inhibited and the pwill be lowered. 

One point that is often overlooked when considering co-solvents is their influence on buffers or salts. Since these are conjugate acid-base systems, it is not surprising that by introducing solvents into the solution, a shift in the pKa of the buffer or salt can result. These effects are important in formulation terms, since many injectable formulations that contain co-solvents also contain a buffer to control the pH (Rubino 1987). 

This sequence of reactions illustrates the buffering of the blood by the carbonic acid-bicarbonate conjugate acid-base system. The protons generated at the tissue level by the ionization of carbonic acid are removed at the lungs when bicarbonate ion reenters the red blood cell and reacts with them to produce carbonic acid. H2CO3 then rapidly dissociates, under the influence of carbonic anhydrase, and the resultant CO2 diffuses into the alveolar space because its concentration in the plasma is higher than in the alveoli. 

Many acids, including HCN, do not react completely in water. We learned in Section 3.3 that these compounds are weak acids. A weak acid and water react to produce a conjugate acid-base system in which the acid and base on the right are... 

The acid-base properties of isoxazole and methylisoxazoles were studied in proton donor solvents, basic solvents or DMSO by IR procedures and the weakly basic properties examined (78CR(Q(268)613). The basicity and conjugation properties of arylisoxazoles were also studied by UV and basicity measurements, and it was found that 3-substituted isoxazoles were always less basic than the 5-derivatives. Protonation increased the conjugation in these systems (78KGS327). 

Most proton transfer reactions are fast they have been carefully studied by relaxation methods. A system consisting of a conjugate acid-base pair in water is a three-state cyclic equilibrium as shown in Scheme IV. [The symbolism is that used by Bemasconi. ... 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

Now suppose that the substrate of a chemical reaction is a weak acid, with both the conjugate acid HS and conjugate base S being capable of undergoing reaction. Usually these two species will react at different rates because of the considerable difference in their electronic configurations. The rate equation for this system is... 

Buffers are solutions that tend to resist changes in their pH as acid or base is added. Typically, a buffer system is composed of a weak acid and its conjugate base. A solution of a weak acid that has a pH nearly equal to its by definition contains an amount of the conjugate base nearly equivalent to the weak acid. Note that in this region, the titration curve is relatively flat (Figure 2.15). Addition of H then has little effect because it is absorbed by the following reaction ... 

STRATEGY The OH ions added to the buffer solution react with some of the acid of the buffer system, decreasing the amount of acid and increasing the conjugate base by the same amount. We solve this problem in two steps. First, we find the new molar concentrations of the acid and its conjugate base. Then we can rearrange the expression for Ka to obtain the pH of the solution, just as in Example 11.1. 

With a given weak acid, a buffer soiution can be prepared at any pH within about one unit of its p vaiue. Suppose, for exampie, that a biochemist needs a buffer system to maintain the pH of a soiution ciose to 5.0. What reagents shouid be used According to the previous anaiysis, the weak acid can have a p Z a between 4.0 and 6.0. As the p deviates from the desired pH, however, the soiution has a reduced buffer capacity. Thus, a buffer has maximum capacity when its acid has its p as ciose as possibie to the target pH. Tabie 18-1 iists some acid-base pairs often used as buffer soiutions. For a pH - 5.0 buffer, acetic acid (p Za — 4.75) and its conjugate base, acetate, wouid be a good choice. 

A practical problem in solution preparation usually requires a different strategy than our standard seven-step procedure. The technician must first identify a suitable conjugate acid-base pair and decide what reagents to use. Then the concentrations must be calculated, using pH and total concentration. Finally, the technician must determine the amounts of starting materials. The technician needs a buffer at pH = 9.00. Of the buffer systems listed in Table 18-1. the combination of NH3 and NH4 has the proper pH range for the required buffer solution. 

In order to arrive at values of the virtually intrinsic acidity, i.e., an acidity expression independent of the solvent used (Tremillon12 called it the absolute acidity), Schwarzenbach13 used the normal acidity potential as an expression for the potential of a standard Pt hydrogen electrode (1 atm H2), immersed in a solution of the acid and its conjugate base in equal activities analogously to eqn. 2.39 for a redox system and assuming n = 1 for the transfer of one proton, he wrote for the acidity potential... 

As free proton cannot exist alone in solution, reactions in which a proton is split off from an acid to form a conjugate base cannot occur in an isolated system (in a homogeneous solution although this is possible in electrolysis (Section 5.7.1)). The homogeneous solution must contain another base Bn that accepts a proton from the acid HAr (acid HA is, of course, not conjugate with base Bn). It will be seen that this second base can even be the solvent molecule, provided it has protophilic properties. Acid-base reactions thus depend on the exchange of a proton between an acid and a base that are not mutually conjugate ... 

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