Acid-dissociation constant binary acids

Tam, K. Y., Multiwavelength spectrophotometric determination of acid dissociation constants. Part VI. Deconvolution of binary mixtures of ionizable compounds, Anal. Lett. 33, 145-161 (2000). 

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 ... 

Factors affecting the acid dissociation constants of binary acids. 

Recall that, because of the strong H-F bond, hydrofluoric acid is a weak acid with a small acid dissociation constant Ka of 6.8 x 10-4. In contrast, the other binary acids of the halogen family—HC1, HBr, and HI—are strong acids that completely dissociate in water.) The fluorosilicic acid produced, H2SiF6, is a water-soluble substance with a structure as in Fig. 6.3.1. 

The applicability of the equation to predict pK values was confirmed by the good fits obtained for experimental and predicted values. A graphical example is given in Figure 1 for the dissociation constants of picric acid in binary mixtures62. 

FIGURE 1. Dissociation constants of picric acid in binary mixtures62. Reprinted with permission from Reference 62. Copyright (1994) American Chemical Society... 

The micro-constants of enkephalin and tyrosine peptides... were determined according to tiie modified metiiod of complementary tri-stimulus colorimetry (CTS method) (Flaschka H, Applications of complementary tri-stimulus colorimetr). -1. Analysis of binary and ternary colorant systems, Talanta, 7, 90-106 (1960). The acid dissociation and micro-constants thus determined are summarized in Table 1 and Table 2. 

I should like to further point out that a similar reaction mechanism is also applicable to eukaryotic systems, naturally with several modifications. As can be seen in Table 1, the dissociation constant of EF-2-GDP was smaller than that of the prokaryotic EF-G-GDP by two orders of magnitude. Thus, the EF-2-GDP binary complex could be detected on the nitrocellulose membrane. Furthermore, EF-2-GDP had a higher affinity toward ribosomes than EF-G-GDP and could form a ternary complex in the absence of fusidic acid. However, the stabifity of the complex was lower than that of the corresponding Gpp(CHa)p complex. On the other hand, the eukaryotic EF-la-GDP has a dissociation constant two orders of magnitude greater than that of EF-Tu GDP, and hence, it is not likely that it requires EF-1 for the displacement of bound GDP. A preliminary experiment indicated that, in the absence of EF-1, EF-la was rather slowly released from ribosomes after the hydrolysis of GTP (Nagata et al., unpublished). If this is the case, the function of the eukaryotic EF-l/8 is more similar to the prokaryotic IF-1 which promotes the release of IF-2 from ribosomes. 

CT is a (complementary against PT) technique, applied for determination of stabihty constants of complexes and proto-complexes, in particular. The knowledge of physico-chemical parameters related to dissociation of mono- and polyprotic acids H L in electrolytic systems is important both from cognitive and analytical viewpoints. Dissociation of the acids in such systems is characterized by pK = -logKj values for acidity parameters. In binary-solvent systems, the acidity parameters at the ends of x-scale (i.e., x = 0 and 1) are called as acidity constants," referred to solutions in pure (mono-component) solvent SI (pKjsi), S2 (pK s2), and W (pK ), where SI, S2 - organic solvents, W - water. We refer later to the solvents characterized by mutual miscibility, in all proportions, within the pairs W+Sl, W+S2, and S1+S2. 

CT - conductometric titration D - tittand (solution titrated), s - relative permittivity (dielectric constant) k - conductivity - dissociation constant pKj = -logKj - acidity constant (in pure solvent) pKj = pKj(x) - acidity parameter PT - potentiometric (pH) titration SI = CH3OH S2 = ((I H3)20 T - titiant (titrating solution), x - mole fraction of a co-solvent in binary-solvent system W = H2O. 

M. Sadiq Khan Niazi, Conductivities and thermodynamic dissociation constants for chloroacetic acid in binary mixed solvent systems at 298.15 K, 7. Chem. Eng. Data, 38 (1993) 527-530. http //puhs.acs.Org/doi/abs/10.1021/je00012a011... 

Thus the large separation in successive pK s for the oxo acids is attributable to the electrostatic effects of the negative charge left by the dissociation of one proton on the remaining ones. In bifunctional binary adds, where the negative charge due to the removal of one proton is concentrated on the very atom to which the second proton is bound, the separation of the constants is extraordinarily great Kt and K2 for H2S are 10 7 and 1014, respectively, whereas for water we have... 

Kinetic parameters have been established for solvolysis of the pentacyanofer-rate(III) derivative [Fe(CN)5(N02)] . For aquation, which is acid-catalyzed at pH <5, A//= = 43kJmol-, =-80 J K" mol", and A =+2 cm" mol". Intrinsic and solvational contributions are presumably closely balanced in the case of A Rate constants for solvolysis of [Fe(CN)5(N02)] in water, methanol, dimethyl sulfoxide, and dimethylformamide correspond with the electron-donating abilities of the respective solvents. Activation volumes for the nonaqueous solvents, between +20 and +27 cm" mor reflect the dissociative nature of these solvoly-ses. " Rate constants for dissociation of the [Fe(CN)5(2,6-Me2pyrazine)]" anion in binary aqueous solvents containing methanol, acetone, or acetonitrile correlate well with acceptor numbers for the respective media, though with a very different... 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the... 

All the investigations cited so far have been in aqueous solution medium dependence of dissociation rate constants in binary aqueous solvents (cosolvents methanol, r-butyl alcohol, 1,2-ethanediol, 1,2,3-propanetriol, and sucrose) has been reported for the 4-cyanopyridine and 4,4 -bipyridyl penta-cyanoferrates(II). For water-rich media (IQi lD < 16.5), plots of logarithms of rate constants against mole fraction organic component are linear, against 1/D almost linear. Slopes differ greatly between cosolvents, but results can be correlated quite satisfactorily with a three-parameter equation incorporating acidity and basicity parameters and the excess function... 

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