Dissociation constants of acids and bases

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. 

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... 

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 ... 

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... 

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. 

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. 

In the calculation of protolytic equilibria, the ionic product of water, equations for dissociation constants of acids and bases, equations of analytical concentrations and equations of electroneutrality or proton balance are taken for the starting point. Due to the difficulty of the numerical calculation of pH these systems are generally solved by graphical methods. 

Over the years, spectrophotometry (UV-VIS) has been used to study such physicochemical phenomena as heats of formation of molecular addition compounds and complexes in solution, determination of empirical formulas, formation constants of complexes in solution, hydration equilibria of carbonyl compounds, association constants of weak acids and bases in organic solvents, tautomeric equilibria involving acid base systems, protein-dye interactions, chlorophyll-protein complexes, vitamin A aldehyde-protein complex, association of cyanine-dyes, determination of reaction rates, determination of labile intermediates, and dissociation constants of acids and bases. 

Kanakare JJ (1972) Determination of dissociation constants of acids and bases by potentio-static titiarion. Anal Chem 44 2376-2379... 

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