Measure of acidity strength

The parameters and Ca are associated with the Lewis acid, and Eg and Cb with the base. a and b are interpreted as measures of electrostatic interaction, and Ca and Cb as measures of covalent interaction. Drago has criticized the DN approach as being based upon a single model process, and this objection applies also to the — A/y fBFs) model. Drago s criticism is correct, yet we should be careful not to reject a simple concept provided its limits are appreciated. Indeed, many very useful chemical quantities are subject to this criticism for example, p o values are measures of acid strength with reference to the base water. 

Acidity constant, Ka (Section 2.8) A measure of acid strength. For anv acid HA, the acidity constant is given bv... 

Tables of KA furnish a quantitative measure of acid strengths with which we can compare different acids and predict their properties. Several values of KA are given in Table 11-IV.

The pXa (- -log,oKJ is a measure of acid strength, with the lower the value the stronger the acid. It is the pH at which a weak acid is 50% ionized ... 

What then is the difference between an acid and an electrophile, or between a base and nucleophile No great difference until we try to use the terms in a quantitative sense. For example, if we refer to acid strength, or acidity, this means the position of equilibrium in an acid-base reaction. The equilibrium constant Ka for the dissociation of an acid FIA, or the pKa, is a quantitative measure of acid strength. The larger the value of Ka or the smaller the pKa, the stronger the acid. 

Nonaqueous methods include the use of amine titration and adsorption of indicators for visual measurement of acid strength. This procedure allows both the determination of the total amount of acid sites and also the acid strength distribution. A disadvantage is that bulky molecules (amines and indicators) arc used and these may be excluded from entering small pores. With zeolites, the slow rate of diffusion and equilibration has to be taken into account. Spectroscopic measurement of acid strength may also be performed using amine titration and indicator adsorption. Ultraviolet or fluorescent indicators may be used. 

Superficially, we would expect the high electronegativity of fluorine to stabilise a carbanion centre, but measurements of acid strengths and exchange rates for a variety... 

A. E. Hirschler (Sun Oil Co., Marcus Hook, Pa. 19061) I should like to make a comment relating to your statement that activity is not a simple function of acidity. This statement appears to contain the implicit assumption that all acid sites able to protonate pyridine would have the same catalytic activity. Actually, one might expect an intensity factor (acid strength) to be operative in acid catalysis as well as an extensive factor, and the apparent lack of correlation you observed may result from an influence of acid strength on catalytic activity. Pyridine adsorption data alone do not completely define Bronsted acidity. One may need also some measure of acid strength. Acidity measurement with Hr indicators, such as we and others have reported, may supply useful information relative to acid strength as well as number of acid sites. 

Since the electromotive force of a cell depends on the activity of the ions of the electrolyte it is possible to determine activities potentiometrically, in particular that of the hydrogen ion which is a measure of acid strength. Strictly, however, the activity of a single ionic species is not measurable, for positive ions always exist in the presence of negative ions. Nevertheless, the mean activity of HCl which is measurable in a cell does conveniently indicate the mean activity of the ions which carry the current, and —logjQ of this activity is the pH of the solution. The basic principle of potentiometric pH determination may be seen by considering a cell divided into two halves by a silver wall coated on both sides with silver chloride ... 

Although the Brpnsted-Lowry model helps explain acid strength, the model does not provide a quantitative way to express the strength of an acid or to compare the strengths of various acids. The equilibrium constant expression provides the quantitative measure of acid strength. 

It is useful to derive a numerical measure of acid strength, which is the amount of hydrogen ion released when a given amount of acid is dissolved in water. Such an expression, called the acid dissociation constant, or K, can be written for any acid, HA, that reacts according to the equation... 

The pA is a more convenient numerical measure of acid strength. The smaller its value, the stronger the acid. This is the reverse of the situation with A, where larger values imply stronger acids (Table 2.6). 

Acid dissociation constant (Ka) A mathematical measure of acid strength, determined from the equilib-... 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

In order to study the interactions of molecules at these sites without the complication of subsequent reaction, much use has been made of probe molecules with different basicities, and therefore with different strengths of interaction at acid sites. The interactions have been monitored by many different techniques, including microcalorimetry and TPD, particularly of amines, and infrared spectroscopy and solid state NMR of the adsorbate-sorbate complex. This work has primarily aimed to establish a measure of acid strength that can be linked to catalytic activity in hydrocarbon transformations. 

We have seen that the magnitude of indicates the strength of a weak acid. Another measure of acid strength is percent ionization, defined as... 

Another method for quantitative determination of acid sites on solid catalysts is calorimetric nitration with amines [129]. This method uses the very simple concept that the enthalpy of adsorption for simple bases is a direct measure of acid strength. If a variation of the heat of adsorption with surface coverage is... 

Apart from discernment of acid type, qualitative and quantitative measures of acidic strengths in solid acids are also crucial for the detailed understanding of the mechanism and performance of the catalyst during catalytic reactions. Assorted illustrations for characterization of acidic strength in various solid acid catalysts invoking the aforementioned P SSNMR approaches wiU be discussed in this section. 

Quantitative Measure of Acid Strength 2.17 (a) Pyruvic acid (b) Phosphoric acid... 

Isosteric heats of adsorption of strong bases (eg, pyridine) may be considered as measures of acid strength. However, discrimination between Lewis and Br0nsted sites is not possible. 

The acid dissociation constant is a quantitative measure of acid strength. 

The theory of indicators, pK values (as a measure of acid strength) and the choice of indicators for acid-base titrations is discussed in Chapter 18. 

The terms strong and weak acids are not entirely satisfactory since they are qualitative descriptions. Strong and weak are not absolute terms and some acids, such as phosphoric(v) acid, H3P04(aq), are described as moderately strong. In Chapter 18, a quantitative measure of acid strength that does not vary with dilution, known as the acid dissociation constant, will be introduced. 

As theories of acidity were developing, so was the practical measurement of acidity. This has a long history and is at the foundation of analytical chemistry. The concept of pH is perhaps the most important. It is not a measure of acid strength but the measurement of the pH values of acid solutions at the same concentration allows the relative strengths of acids to be compared. 

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