Constants of Acids and Bases

If the pH Of the solution and the concentrations of HB and B are measured, pKa can be readily calculated. Spectrophotometry is employed in determining the ratio (chb/cb ). Since the conjugate acid and base forms of a substance will be in equilibrium, the relative concentration of each form will depend on the pH of the solution. For moderately weak acids or bases, the pH of the solution may be adjusted so as to make one of the forms predominant. For example, the spectrum of an acid HB in alkaline solution will correspond to the absorption of the anion ( b ) and in acid solution, to that of the undissociated acid (Dhb). At intermediate pH values, however, both forms will be present and the observed absorption (D) will be a linear combination of those of the two forms (provided Beer s law holds). Since the optical densities (jDb- and Dhb) of the two forms (for a fixed concentration of the solute) are known, the relative concentrations of the two forms at any intermediate pH may be calculated. A similar spectrophotometric procedure may be applied to bases. The acidity function, H0, which is a measure of the proton-donating tendency of a medium towards a neutral base, has also been determined by spectrophotometry85. 

The pA 0 of acetic acid at 25°C was taken as 4-76 and pKa values of benzoic acid were calculated from the absorption data at different wavelengths (Table 7.5). 

Wavelength, ntyi Extinction coefficients (K) in 0-liV HjSOi OllVNaOH Buffer pK 

Flexer, Hammett and Dingwall have determined the dissociation constants of several weak acids and bases by this method. Brown and co-workers have used a similar procedure for the determination of base strengths of pyridine derivatives. 

An alternative method for determining dissociation constants involves plotting the observed optical density at a particular wavelength against the pH of the solution and obtaining the midpoint, where pH — pit (i.e. chb = cB ), graphically. This type of titration is tedious since it involves measurement of pH and optical density of a number of samples. 

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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. 

A. Albert and E. P. Serjeant, Ionization Constants of Acids and Bases. Methuen, London, 1962. 

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... 

Table 3.2. Acidity and Basicity Constants of Acids and Bases in Aqueous Solutions (25°C)... 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

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. 

Albert A, Serjeant EP. The Determination of Ionization Constants of Acids and Bases A Laboratory Manual. 3rd Ed. New York, Chapman and Hall, 1984. 

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