Ionization constant solvent

It can be seen from Table 2 that the intrinsic values of the pK s are close to the model compound value that we use for Cys(8.3), and that interactions with surrounding titratable residues are responsible for the final apparent values of the ionization constants. It can also be seen that the best agreement with the experimental value is obtained for the YPT structure suplemented with the 27 N-terminal amino acids, although both the original YPT structure and the one with the crystal water molecule give values close to the experimentally determined one. Minimization, however, makes the agreement worse, probably because it w s done without the presence of any solvent molecules, which are important for the residues on the surface of the protein. For the YTS structure, which refers to the protein crystallized with an SO4 ion, the results with and without the ion included in the calculations, arc far from the experimental value. This may indicate that con-... 

The ionization eonstant should be a function of the intrinsic heterolytic ability (e.g., intrinsic acidity if the solute is an acid HX) and the ionizing power of the solvents, whereas the dissoeiation constant should be primarily determined by the dissociating power of the solvent. Therefore, Ad is expeeted to be under the eontrol of e, the dieleetrie eonstant. As a consequenee, ion pairs are not deteetable in high-e solvents like water, which is why the terms ionization constant and dissociation constant are often used interchangeably. In low-e solvents, however, dissociation constants are very small and ion pairs (and higher aggregates) become important species. For example, in ethylene chloride (e = 10.23), the dissociation constants of substituted phenyltrimethylammonium perchlorate salts are of the order 10 . Overall dissociation constants, expressed as pArx = — log Arx, for some substanees in aeetie acid (e = 6.19) are perchloric acid, 4.87 sulfuric acid, 7.24 sodium acetate, 6.68 sodium perchlorate, 5.48. Aeid-base equilibria in aeetie acid have been earefully studied beeause of the analytical importance of this solvent in titrimetry. 

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

The authors studied, as they call it, "acid-base equilibria in glacial acetic acid however, as they worked at various ratios of indicator-base concentration to HX or B concentration, we are in fact concerned with titration data. In this connection one should realize also that in solvents with low e the apparent strength of a Bronsted acid varies with the reference base used, and vice versa. Nevertheless, in HOAc the ionization constant predominates to such an extent that overall the picture of ionization vs. dissociation remains similar irrespective of the choice of reference see the data for I and B (Py) already given, and also those for HX, which the authors obtained at 25° C with I = p-naphthol-benzein (PNB) and /f B < 0.0042, i.e., for hydrochloric acid K C1 = 1.3 102, jjrfflci 3 9. IQ-6 an jjHC1 2.8 10 9 and for p-toluenesulphonic acid Kfm° = 3 7.102( K ms 4 0.10-6) Kmt = 7 3.10-9... 

The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in... 

Long, F. A. Ballinger, P, Acid ionization constants of alcohols in the solvents water and deuterium oxide, in Pesce, B. (ed.), Electrolytes, Pergamon Press, New York, 1962, pp. 152-164. 

Woolley, E. M. Hepler, L. G., Apparent ionization constants of water in aqueous organic mixtures and acid dissociation constants of protonated co-solvents in aqueous solution, Anal. Chem. 44, 1520-1523 (1972). 

Table 1.5 Self-ionization constants of solvents. (According to B. Tremillon)... 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

Lines 21 -40. Physical data. The usual crystalline shape, density (note two values reported.), sublimation notation, boiling point data, and so on. K at 25° is the ionization constant of the acid the pH of the saturated solution (2.8 at 25°C) is given. The solubility data (Soly) is very complete, including water solutions at various temperatures, a bit about the phase diagram of the compound, and solubility in other solvents. Note that numerical data is given where possible. 

The dielectric properties of the solvent have also an influence on the ionization constant of an incompletely ionized substrate. By the process of ion dissociation the concentration of associated ions is decreased this results because the latter are in equilibrium with non-ionized species and the ionization equilibrium will be restored by the formation of additional associated ions. 

Sarmini, K., and Kenndler, E. (1999). Ionization constants of weak acids and bases in organic solvents. /. Biochem. Biophys. Methods 38, 123—137. 

The ionization constants of 2,2 -bipyridine in various organic solvent and water mixtures, - " at different ionic strengths " " and at... 

The papers in the second section deal primarily with the liquid phase itself rather than with its equilibrium vapor. They cover effects of electrolytes on mixed solvents with respect to solubilities, solvation and liquid structure, distribution coefficients, chemical potentials, activity coefficients, work functions, heat capacities, heats of solution, volumes of transfer, free energies of transfer, electrical potentials, conductances, ionization constants, electrostatic theory, osmotic coefficients, acidity functions, viscosities, and related properties and behavior. 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

Ionization reactions have been investigated (I) by a variety of methods that lead to reasonably accurate values of equilibrium constants over rather wide ranges of temperatures, pressures, and dissolved salt concentrations. However, the status of measurements leading to ionization constants in aqueous organic mixed solvents has not been developed nearly so well, in spite of the excellent work of Harned, Grunwald, Bates, and others (1-10). Experimental methods have been difficult and those methods that utilize the hydrogen electrode can be applied only to systems in which there are no complicating reduction reactions. 

We have recently devised a rapid and convenient method for determination of the ionization constant for water in mixed aqueous organic solvents (11-16). The method utilizes glass electrodes and gives results in satisfactory agreement with earlier work. 

The assumptions made in the use of these methods to obtain the above ionization constants in mixed solvents have been summarized (16). The fact that the pK values calculated using the above assumptions are in good agreement with those values reported in the literature is an indication that any errors resulting from these assumptions are probably relatively small. 

Solvents. Reagent grade THF (nD25 = 0.888, bp = 64-66°C) containing 0.025 wt-vol % di-ferf-butyl-p-cresol which served as an antioxidant was used for the preparative GPC fractionation. The solvent TFE (nD20 = 1.2907, d25 = 1.3823, bp = 76°C, ionization constant Ka — 4.3 X 10"13) was obtained from Halocarbon Products Corp., Hackensack, N. J., and was used for both analytical GPC and viscometry. The recovery and... 

Water is solvent for all compounds except diphenylmethane, in which case ethyl ether is used. Ionization constants for carbonyl compounds and nitriles are gross acid constants, uncorrected for tautomerism. 

Moreover, many hydrogen compounds, when dissolved in such solvents, ionize more or less completely lo give solvated protons and anions, In the case uf polybasic acids, ionization constants are reported for each step in this dissociation. 

The rate data were analyzed by assuming Equation 6.17. In this equation kt is the titrametric rate constant for product formation, ks is the solvent-assisted ionization constant, and FkA is the fraction of the aryl-assisted rate constant that gives rise to product (as opposed to the fraction that gives starting material through internal return). 

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