Acid Dissociation Constants of

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

Reference must be made to textbooks of physical chemistry (see Bibliography, Section 3.39) for details of the methods used to evaluate true dissociation constants of acids. 

The dissociation constants of acids and bases are determined either exactly, by means of a suitable cell without liquid junction and without measuring the pH directly, or approximately on the basis of a pH measurement in a cell with liquid junction, the potential of which is reduced to a minimum with the help of a salt bridge. In the former case we shall use, for example, the cell... 

Let us consider a reaction which is catalyzed by H+ produced by weak acid HA. The dissociation constant of acid is given as... 

The study of solute-solvent and solvent-solvent interactions in mixed solvents has been gaining significance in recent years61-64, because of the increasing application of these solvents. Casassas and collaborators67 have used the Kamlet-Taft multiparametric equation for the correlation of dissociation constants of acids in 1, 4-dioxane-water mixtures. They found that when the main solvent is retained the property does not involve significant changes in the cavity volumes and, in those cases, the pK in binary solvents can be described by equation 8 ... 

Electrical Conductance of Aqueous Solutions of Ammonia and Metal Hydroxides. Check the electrical conductance of 1 W solutions of sodium hydroxide, potassium hydroxide, and ammonia. Record the ammeter readings. Arrange the studied alkalies in a series according to their activity. Acquaint yourself with the degree of dissociation and the dissociation constants of acids and bases (see Appendix 1, Tables 9 and 10). Why is the term apparent degree of dissociation used to characterize the dissociation of strong electrolytes ... 

By measuring the absorbance A of a sample of known e in a cell of known path length l, the concentration c may be determined. Because changes in absorbance reflect changes in concentration, it is possible to use absorbance measurements to follow rates of chemical reactions, to determine equilibrium constants (such as the dissociation constants of acids and bases), and to follow conformational changes in bio-organic molecules such as proteins and nucleic acids. 

Table 3.2 Solution Dissociation Constants of Acids and Bases...

With regard to stabilization, perhaps the most thoroughly investigated aspect of metallo-organic chemistry is the evaluation of stability constants. For the present discussion, a stability constant can be considered as analogous to the reciprocal of the dissociation constants of acids and bases. Thus, if L represents the 8-hydroxyquinoline anion and ML2 the metal complex, then ... 

Also, the organic content is expected to influence the dissociation constant of acidic analytes, resulting in an increase in the acidic analyte pKa and this could be described as the acidic analyte pKa shift, which is discussed in Section 4.6. On the other hand, the organic eluent will affect the dissociation of basic analytes in the opposite direction, resulting in a decrease in the basic analyte pKa, and is discussed in the Section 4.6 as the basic analyte pKa shift. 

Comparison of dissociation constants of acids and bases derived from electrochemical or conductivity measurements in solvents of different dielectric constants are meaningless (25, 28) since equilibrium constants determined in this way always represent the overall equilibrium constant K = Xform- sep-... 

Dissociation Constants of Acids and Bases.—If the law of mass action is applied to the equilibrium between an acid HA and the basic solvent S, i.e., to the equilibrium... 

Approximate Methods for Bases.—The procedures described for determining the dissociation constants of acids can also be applied, in principle, to bases analogous equations are applicable except that hydroxyl ions replace hydrogen ions, and vice versa, in all the expressions. Since the value of the product of an and ooir is known to have a definite value at every temperature (cf. Table LXI), it is possible to derive ooh" from obtained experimentally. 

A quantitative approach to the problem of the influence of the medium on the dissociation constants of acids, which eliminates the proton accepting tendency of the solvent, involves a comparison of the dissociation constants of a series of acids with the value for a reference acid. Consider the acid HA in the solvent S the dissociation constant is given by... 

Table 11. Relation between the dissociation constants of acids contained in esters subjected to alcoholysis with cellulose and the DS of the resulting cellulose...

To determine the in situ properties of the carbonate system in the ocean, it is necessaiy to determine the effect of pressure on the thermodynamic constants. This correction can be made in two ways (1) using direct measurements of the constants and (2) using partial molal volume and compressibility data (Mil-lero, 1979). The two methods are in good agreement (Millero, 1979) when comparisons are made for the carbonate system. The effect of pressure on the dissociation constants of acids (A ,) can be made from (Millero, 1979) equations... 

Table A6.5. Coefficients for the Effect of Pressure on the Dissociation Constants of Acids in Sea ater...

A Throughout the remainder of this chapter, we use K.,a represent the first and second dissociation constants of acids and to represent the... 

If the effect of increasing temperature on the dissociation constants of acids and bases may be disregarded, then the degree of hydrolysis will change only because the ion product of water, Kw, increases with increasing temperature. Thus the ion product of water at 100° is about one hundred times its value at room temperature. 

Ion activity constants (thermod3mamic dissociation constants) and stoichiometric dissociation constants of acids. 

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. 

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