Acid strength definition

The p/<, of a base is actually that of its conjugate acid. As the numeric value of the dissociation constant increases (i.e., pKa decreases), the acid strength increases. Conversely, as the acid dissociation constant of a base (that of its conjugate acid) increases, the strength of the base decreases. For a more accurate definition of dissociation constants, each concentration term must be replaced by thermodynamic activity. In dilute solutions, concentration of each species is taken to be equal to activity. Activity-based dissociation constants are true equilibrium constants and depend only on temperature. Dissociation constants measured by spectroscopy are concentration dissociation constants." Most piCa values in the pharmaceutical literature are measured by ignoring activity effects and therefore are actually concentration dissociation constants or apparent dissociation constants. It is customary to report dissociation constant values at 25°C. 

When carefully prepared and dried, the salt has a perfectly definite composition. In acid strength, it may be regarded as a tribasic acid and is often used to standardize acid and alkali solutions by direct weighings as well as for preparing a standard reducing solution. 

No rigorous practical definition of acid strength exists outside of very dilute aqueous solutions where activity coefficients can be assumed to be unity. Each method of measuring the acid strength of surface sites is liable to give somewhat different answers, the results being qualitative at best. The dielectric constant of the medium, in which the acid or base probe is used influences the measured acid-base strength. 

When equations of the type (8) and (9) are applied to reactions in nonaqueous solution, qualitative rather than quantitative relationships are usually found. This can be attributed to the difficulties in unequivocally assigning catalytic constants on the basis of the experimental data. Bell has reviewed this problem and the problem of the acid-strength scale to be used (Bell, 43). In most cases the dissociation constants in water are used as the basis of acid strength, and a general discussion of this problem will follow the presentation of the other definitions of acids. 

The concept of acid site is based on the idea that protons are fixed at definite position. Thus, the measures of the acid strength, which are described so far, are basically based on the static properties of OH groups. However, the solid acid catalysed reactions are often carried out at higher temperatures than room temperature. In general, the catalysts undergo structural and chemical change under reaction conditions. Therefore, the characterization of properties of zeolites at high temperatures is more desirable. 

Some of the most important processes in chemical and biological SYSTEMS ARE ACID-BASE REACTIONS IN AQUEOUS SOLUTIONS. In THIS FIRST OF TWO CHAPTERS ON THE PROPERTIES OF ACIDS AND BASES, WE WILL STUDY THE DEFINITIONS OF ACIDS AND BASES, THE pH SCALE, THE IONIZATION OF WEAK ACIDS AND WEAK BASES, AND THE RELATIONSHIP BETWEEN ACID STRENGTH AND MOLECULAR STRUCTURE. WE WILL ALSO LOOK AT OXIDES THAT CAN ACT AS ACIDS OR BASES. 

The particular virtue of these definitions is that the Lewis acid and the Lewis base strengths are both estimates of the valence of the bond that links the cation with the anion. The most effective bonds will therefore occur between a cation whose Lewis acid strength (Sg) is close to the Lewis base strength (Sb) of the anion. This is known as the Valence Matching Principle. Compounds between badly matched ions, i.e. Be (Sa = 0.5 v.u.) and ClO j" (Sb = 0.08 v.u.), are difficult if not impossible to form as both the cation and the anion will be forced into unusual coordination. If they... 

The Arrhenius Acid-Base Definition The Acid-Dissociation Constant (KJ Relative Strengths of Acids and Bases... 

Acids and bases are essential substances in home, industry, and the environment. In aqueous solution, water combines with the proton released from an acid to form the hydrated species represented by HgO laq). In the Arrhenius definition, acids contain H and yield HaO in water, bases contain OH and yield OH in water, and an acid-base reaction (neutralization) is the reaction of and OH to form HgO. Acid strength depends on [HaO" ] relative to [HA] in aqueous solution. Strong acids dissociate completely and weak acids slightly. The extent of dissociation is expressed by the acid-dissociation constant, K. Weak acids have values ranging from about 10 to 10 . Many acids and bases can be classified qualitatively as strong or weak based on their formulas. 

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