Acid-dissociation constant types

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... 

A frequently encountered pH-rate profile exhibits a bell-like shape or hump, with two inflection points. This graphical feature is essentially two sigmoid curves back-to-back. By analogy with the earlier analysis of the sigmoid pH-rate curve, where the shape was ascribed to an acid-base equilibrium of the substrate, we find that the bell-shaped curve can usually be accounted for in terms of two acid-base dissociations of the substrate. The substrate can be regarded, for this analysis, as a dibasic acid H2S, where the charge type is irrelevant we take the neutral molecule as an example. The acid dissociation constants are... 

The acid dissociation constants for tri- and tetranuclear aqua chromium(III) species are summarized in Table XXIII. The structures of these species are not known. If, however, it is assumed that they have linear structures, such as structures 4a, 7b, and 7c shown in Fig. 1, then the observed acid strengths can be rationalized in terms of a -and /1-type hydrogen bond interactions, as discussed recently (118). 

The E1/2 for an electrode reaction of this type is pH dependent. As the pH increases, E1/2 shifts negatively due to decreasing availability of protons. A typical E1/2-pH plot is shown in Figure 3.29. The magnitude of negative shift per pH unit is related to the acid dissociation constants, Ka and Kaj, for the... 

In addition to influencing the rate of a reaction, pH may also control the products where alternate or sequential pH-dependent reactions take place. An example of this type of reaction is the chlorination of phenol. Lee and Morris (37) have shown that the chlorination of phenol proceeds by the stepwise substitution at the 2, 4, and 6 positions of the aromatic ring. The rate of each of these reactions depends on the product of phenate or chlorophenate anion and the hypochlorous acid concentrations. Since each phenolic compound has a slightly different acid dissociation constant, the species of chlorophenols that are formed depend on the pH of the solution. 

The amphoteric character of the hydroxylated Ti02 surface has been demonstrated by Boehm and co-workers in a series of potentiometric titration experiments performed both with anatase and rutile samples. Approximately half of the total amount of hydroxyls present on the Ti02 surface, which underwent neutralization with a diluted (0.01 M) sodium hydroxide solution, has been described as relatively strongly acidic, with a pK value of 2.9, and the remainder as weakly acidic with a pK of 12.7. However, the use of a Langmuir-type adsorption equation as a basis for estimating these acid dissociation constants has raised questions about the real significance of the above values. ... 

Symbols and abbreviations placidic dissociation constant Exchange H , exchangeable H at typical environment conditions bold type, actual functional group. 

This expression shows that an indicator with an acid dissociation constant of 1 X 10 - (pA a = 5) typically shows a complete color change when the pH of the solution in which it is dissolved changes from 4 to 6 (Figure 14-2). With a little more algebra, we can derive a similar relationship for a basic-type indicator. 

A WApH titration method was used for the determination of acid dissociation constants (pK a values) of ionizable compounds. Microspeciation was investigated by three approaches ( ) selective monitoring of ionizable group by spectrophotometry, ( ) deductive method, and (3) K(z) method for determination of tautomeric ratio from co-solvent mixtures. It has been shown that the WApH technique, for such types of ampholytes, is able to deduce the microconstants and tautomeric ratios, which are in good agreement with the literature data. 

Effect of Partition Coefficient and Dissociation Constant In the base case, the fraction of petroleum acid converted to soap (A/HA) is only 0.246, and the soap molar fraction is 0.309 (see Table 12.4). These values are affected by the partition coefficient Kd between water and oil and the acid dissociation constant Ka. Now let us see how sensitive these two parameters are. The data in Table 12.5 show that Kd is insensitive, whereas Ka is very sensitive. As Ka is increased, more acid is converted to soap. Accordingly, the soap molar fraction in the total surfactant becomes higher. As Xsoap is increased from the base case, the type III salinity limits are closer to those for the soap, which are lower. Thus, the mixture surfactant system becomes type II. As Xsoap... 

Acid dissociation constant, acidity constant, or the acid ionization constant, (Kg), is a specific type of equilibrium constant that indicates the extent of dissociation of ions from an acid. The acidity of a substance and its Ka value are directly proportional. A larger Kg (smaller pKg) means a stronger acid. Using the values of Kg, the strength of acids can be determined easily. 

In a recent study the acid dissociation constants of substituted l-amino-4-aryl-2(li/)-pyrimidinethiones of the type 1 and stability constants of their silver complexes have been measured pH-potentiometrically in a 75% (v/v) mixture of dioxane/water11. The influence of the substituents R1 and R2 on pKa and lgf n values has been discussed11. 

If the value of Kai is compared with the dissociation constant of the imidazolinium group ( NH N—) in the unchelated molecule (K2 = Iq-8.92, aHo i3 7 kcal mole-1), then the effect of chelating iron(II) is to decrease the pK of the imidazolinium group by 2.8 and the enthalpy by 6.8 kcal mole-1. Despite the difference in charge type between the ligand deprotonation and the chelate deprotonation reactions, the effect of chelation on the acid dissociation constant is quite large (101). 

Ka and Kb are particular types of equilibrium constants that give us an idea of the relative strengths of acids and bases, respectively. The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base. Likewise, the base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an OH ion. 

Based on a log acid dissociation constant (pKa) of 7.0 for hydrogen sulfide, as the sample pH decreases, H2S concentrations increase and toxicity increases [63,64]. Because of these imique speciation trends, the graduated pH manipulation can be used to characterize ammonia and hydrogen sulfide toxicity as well as other toxicants showing this type of behavior (e.g., some metals). 

The acid dissociation constants of type 1 and 2 acidic groups differ by about two orders of magnitude. Therefore, the complexation of divalent metal ions with HA can be written as... 

If we are dealing with an equilibrium of the type RH -f SH R " -h SH2 or R + SH RH" -hS , the right-hand side in each case contains more ions than the left, and the position of equilibrium should depend considerably on the dielectric constant of the medium, quite apart from other factors. In order to minimize this electrostatic effect we must consider equilibria such as RH -t-SH R-hSH or R -hSH RH-hS . The first of these is expressed by the conventional acid dissociation constant of a cation acid (e.g., the ammonium ion), while the second is often described as the solvolysis of the anion of a weak acid, and its equilibrium constant is obtained from the conventional dissociation constant of RH and the ionic product of the solvent. 

---