Acid—base properties

Different approaches are used to explain acid-base properties of substances. According to Bronsted and Lowry [16,17], an acid is a substance able to act as proton donor, while a base is a substance able to act as proton acceptor. According to Lewis [18], acids are the compounds that are able to accept electron pairs forming covalent bond, while bases are the compounds that act as donors of electron pair. 

The strength of bases and acids in solution is characterized by their dissociation constants 

For solid acids and bases, the acidic and basic properties can also be expressed by a similar equation, but it incorporates the concentrations of acidic or basic sites present on the surface instead of concentrations of hydrogen or hydroxyl ions  

The acidic and basic properties of aqueous solutions are changed within the pH range from 0 to 14, while for solid acids and bases, they can be changed within the range -30 Ho, H. +40. A boundary point between Ho and H. is Ho = 7. One can see that these limits are much broader than in the case of aqueous solutions. Acids with Hq value within 7 + -12 correspond to usual acids, while above -12, they are superacids. Similarly, the bases with H. from 7 to 26 correspond to usual bases while above 26 they are superbases. The concept of superacids and superbases is widely used to explain the processes that take place in acid-base catalysis [21]. 

The strength of solid acids and bases is characterized by proton affinity (PA). For a base B, PA is equal to the enthalpy of reaction B + H - BH in gas phase, where B is electrically neutral base and BH is its protonated form. The methods have been developed to determine PA for various compounds by the combined application of different indicators, sorbents and IR spectroscopy methods [22]. 

Potentiometric titration curves of humic compounds against strong base are usually broad and ill-defined thus reflecting the diversity in acidic groups. The apparent acidity of these groups can be expressed as  

The potentiometric titration of simple acids characterised by an acid dissociation constant Ka is described by the Henderson—Hasselbalch equation (Stumm and Morgan, 1970)  

Huizenga (1977) investigated the acid—base properties of twelve marine DOM samples and one combined river water sample and used the Katchalsky— Spitnik model (eq. 2) to numerically estimate pKa, m and total number of Type 1 sites (—COOH groups), in the range pH 3—8. Although the overall 

The a coefficients 3.30 and 1.27 were determined graphically from Wilson and Kinney s (1977) data. /3, co and n values [see Wilson and Kinney (1977) for explanation of symbols] for the Gulf of Alaska sea-water sample were not provided, and these were assumed to be equal to the lake-water data provided. 

Summary of the acid—base properties of marine, fresh-water and soil humic compounds 

The interpretation of titration curves of peptides and proteins can be quite tricky. In addition to the number of groups that may be involved, their pAa values can be perturbed by several factors. For example, when charged groups are in close proximity and when salts are present, pA, values are influenced by electrostatic effects. Titration thus gives apparent pAa values and the intrinsic values have to be computed by applying a correction factor based on the Debye-Hiickel theory  

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. 

The interaction between selective metal oxides and molecules to be oxidized is, of course, based on electron-accepting and electron-donating properties, respectively. In this way, Mo6+, Vs+, etc. act as electron acceptors and molecules with 7r-bonds as donors. Ai et al. [5—12] have drawn attention to the fact that this can also be described by acid—base properties. An electron donor molecule like butene is a basic entity interacting with acidic sites on the catalyst. Hence it follows that activity and selectivity depend on the relative acidity and basicity. Mo03, for example, is an acidic oxide, while Bi203 is a basic oxide. Different compositions Bi Mo have different acidities. The rate of oxidation depends on the number of acid sites (=acidity) and the acid strength, viz. 

Ai and Suzuki [5,9] investigated the combination V2Os—P2Os. The acidity was measured indirectly by the activity for dehydration of isopropanol and was shown to decrease with increasing P2Os content. The activity for the oxidation of butene-1 and butadiene to maleic acid anhydride decreased accordingly. It was shown that the adsorption equilibrium constant of the olefin on the catalyst also decreased in the same way. 

With tin vanadates, the selectivity for the formation of butadiene goes through a maximum at an atomic ratio Sn/V = 9. Below this ratio, the acidity is greater, leading to more maleic acid anhydride in the reaction products. Butadiene will adsorb more with increasing acidity and will have a greater opportunity to be oxidized. The resulting acid anhydride will desorb relatively easily from an acid catalyst. A basic catalyst will result in more combustion products. 

Combinations of Bi203 and Mo03, promoted by P2Os at a constant P/Mo ratio (0.2) were studied over a full composition range by Ai and Ikawa [6], Acidity (and basicity) were measured directly by adsorption of compounds like ammonia, pyridine and acetic acid. The effect of the Bi/Mo ratio on the acidity (Fig. 14) parallels the effect on the overall butene oxidation activity [presented in Fig. 5, Sect. 2.3.2(a)(i)]. 

With respect to the reaction products, the catalysts can be classified into three groups. The first group is very acidic in nature (Bi/Mo = 0—0.3) and converts olefins to acidic products (e.g. butene to maleic anhydride), the second group has medium acidity (Bi/Mo = 0.5—3) and provides the optimal conditions for the dehydrogenation of butene to butadiene, while the third group (Bi/Mo 3), which has a basic character, only forms combustion products. 

The concept of acid-base interaction is often used in the physics of adhesion to account for the formation of interfacial bonds, in heterogeneous catalysis to analyze the reactivity and selectivity of solid catalysts, and in colloid physics to describe the double-layer interactions between small particles in suspension in a liquid. This chapter will present a brief historical sketch of the acid-base concept in molecular physics and will specify its relevance in adhesion and catalysis. Then, the acid-base characteristics of oxide surfaces will be described, with special attention paid to the properties of hydroxyl groups on these surfaces. 

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Ireland J F and Wyatt PAH 1976 Acid-base properties of electronically excited states of organic molecules Adi/. Rhys. Org. Chem. 12 131-221... 

Acid-Base Properties of Amino Acids with Neutral Side Chains... 

Individual ammo acids differ m their acid-base properties This is important m peptides and proteins where the properties of the substance depend on its ammo acid constituents especially on the nature of the side chains It is also important m analyses m which a complex mixture of ammo acids is separated into its components by taking advantage of the differences m their proton donating and accepting power... 

Electrophoresis is used primarily to analyze mix tures of peptides and proteins rather than individual ammo acids but analogous principles apply Because they incorporate different numbers of ammo acids and because their side chains are different two pep tides will have slightly different acid-base properties and slightly different net charges at a particular pH Thus their mobilities m an electric field will be differ ent and electrophoresis can be used to separate them The medium used to separate peptides and proteins is typically a polyacrylamide gel leading to the term gel electrophoresis for this technique... 

Acid-Base Properties of Ammo Acids (Tables 27 2 and 27 3, p 1059)... 

Now the overall effects due to hydrogen bonding, dipole moment, acid-base properties, and molecular configuration can be expressed as... 

Conditional Metal—Ligand Formation Constants Recognizing EDTA s acid-base properties is important. The formation constant for CdY in equation 9.11 assumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, then equation 9.11 provides an adequate description of the formation of CdY . for pH levels less than 12, however, K overestimates the stability of the CdY complex. 

To correct the formation constant for EDTA s acid-base properties, we must account for the fraction, ayi-, of EDTA present as Y . 

As with EDTA, which we encountered in Chapter 9, o-phenanthroline is a ligand possessing acid-base properties. The formation of the Fe(o-phen)3 + complex, therefore, is less favorable at lower pH levels, where o-phenanthroline is protonated. The result is a decrease in absorbance. When the pH is greater than 9, competition for Fe + between OH and o-phenanthroline also leads to a decrease in absorbance. In addition, if the pH is sufficiently basic there is a risk that the iron will precipitate as Fe(OH)2. 

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). 

Usually the acid-base properties of poly electrolyte are studied by potentiometric titrations. However it is well known, that understanding of polyelectrolyte properties in solution is based on the knowledge of the thermodynamic properties. Up to now, there is only a small number of microcalorimetry titrations of polyelectrolyte solutions published. Therefore we carried out potentiometric and microcalorimetric titrations of hydrochloric form of the linear and branched polyamines at 25°C and 65°C, to study the influence of the stmcture on the acid-base properties. 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

Classification according to Br nsted acid-base properties is useful. 

Proteins are the indispensable agents of biological function, and amino acids are the building blocks of proteins. The stunning diversity of the thousands of proteins found in nature arises from the intrinsic properties of only 20 commonly occurring amino acids. These features include (1) the capacity to polymerize, (2) novel acid-base properties, (3) varied structure and chemical functionality in the amino acid side chains, and (4) chirality. This chapter describes each of these properties, laying a foundation for discussions of protein structure (Chapters 5 and 6), enzyme function (Chapters 14-16), and many other subjects in later chapters. 

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