Extraction constant

Catalyst Cation. The logarithms of extraction constants for symmetrical tetra- -alkylammonium salts (log rise by ca 0.54 per added C atom. Although absolute numerical values for extraction coefficients are vastly different in various solvents and for various anions, this relation holds as a first approximation for most solvent—water combinations tested and for many anions. It is important to note, however, that the lipophilicity of phenyl and benzyl groups carrying ammonium salts is much lower than the number of C atoms might suggest. Benzyl is extracted between / -propyl and -butyl. The extraction constants of tetra- -butylammonium salts are about 140 times larger than the constants for tetra- -propylammonium salts of the same anion in the same solvent—water system. 

Table 1. Anion Extraction Constants Eq, of Tetra-n-Butylammonium Ion Pairs Between Water and Chloroform...

Calestani et al. [8] prepared dietylamide of the tetra-carboxymethyl ether of p-tert-h W.y calix[4]arene and reported alkali metal picrate extraction constants of 1.9... 

The rate of reaction not only depends on the actual chemical reaction with the halide (T, Br or Cl ), but also on the competitive extraction of Y versus X (see Fig. 3.56), which can be expressed as the extraction constant ( qx) of the salt QX ... 

The catalyst anion has also been shown to have a large influence on the reaction rate. The extraction constant of tetra-n-butylammonium salts between water and chloroform decreases with different anions as follows picrate CIO4 > T > toluene sulphonate > NO.-i > Br > benzoate > Cf > acetate > OH (Esikova, 1997 Dehmlow, 1993). 

Such ion-pair constants are conditional , in that they depend on the concentration of the counterion with which the charged drug molecule enters into the octanol phase as an ion-pair. This is due to the low dielectric property of octanol, inducing charge neutrality upon uptake of charged drug molecules. Extraction constants may be used to explicitly include the participahon of the counterion [18]. 

Equation (31) is true only when standard chemical potentials, i.e., chemical solvation energies, of cations and anions are identical in both phases. Indeed, this occurs when two solutions in the same solvent are separated by a membrane. Hence, the Donnan equilibrium expressed in the form of Eq. (32) can be considered as a particular case of the Nernst distribution equilibrium. The distribution coefficients or distribution constants of the ions, 5 (M+) and B X ), are related to the extraction constant the... 

Figure 4.6 Hypothetical lipophilicity profiles (a) fixed extraction constant with differing salt concentrations (b) fixed salt concentration with differing extraction constants. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission fromBentham Science Publishers, Ltd.]...

To cap off the topic of salt dependence, is the following example (also using acebutolol), which will indeed surprise most readers, at first. It is possible to have a peak in a logD/pH profile of a monoprotic molecule In Fig. 4.6b, we simulated the case by assuming that the level of salt was kept constant and equal to the concentration of the sample, and proceeded to explore what should happen if the log of the extraction constant Ke [162,225,275,277]... 

Johansson, P.-A. Gustavii, K., Potentiometric titration of ionizable compounds in two phase systems. 3. Determination of extraction constants, Acta Pharm. Suecica 14, 1-20 (1977). 

Table 10 Values of pAa s, extraction constants, separation factors, and calculated point charges in...

This equilibrium may be used to define an extraction constant (Kex) in the usual way. It should be noted that under some conditions, especially when larger ions are involved, a second equilibrium involving complexation of a further molecule of (non-deprotonated) ligand may also be present. 

Increasing the hydrophobicity of quaternary ammonium salts increases the apparent extraction constants for the ion pair and therefore leads to a higher catalytic activity (Brandstrom, 1977). The same phenomenon has been observed by Cinquini and Tundo (1976) for crown ether catalysis (Table 35). The catalytic activity of 18-crown-6 [3] and alkyl-substituted derivatives [117]—[ 119] in the reaction of n-CgH17Br with aqueous KI follows the order [117], [118] > [119] s> [3]. The alkyl-substituted [2.2]-cryptand derivatives are also much more efficient than the parent compound [86]. Increasing the hydrophobicity of [2.2.2]-cryptand (Cinquini et al., 1975) and even of polypode ligands (Fornasier et al., 1976) leads to higher catalytic activity. The tetradecyl-substituted compounds show the reactivity sequence [2.2.2]-cryptand at 18-crown-6 > [2.2]-cryptand on the reactivity scale that can be distilled from Table 35. 

The number of lanthanide ions that can be studied simultaneously in the same aqueous solution is dependent on several considerations. Firstly, the extraction constants Ke of the lanthanides change by approximately 104 on transition from La3+ to Lu3+. It is thus difficult to provide a constant TTA concentration across the lanthanide series because of the inaccuracy of the D values. Secondly, the gamma ray spectra emitted by the tracers from each phase must contain one or more characteristic peaks which can be easily counted for a desired element. We must choose several lanthanide mixtures. 

Several typical extraction constants, Kgr for the transfer of permanganate from an aqueous solution into methylene chloride are listed in Table 1. 

Table I. Extraction Constants for the Transfer of Permanganate from Water into Methylene Chloride (32)...

For a two phase system (water and a water nonmiscible organic solvent) containing a hydrophobic salt QX dissolved in water (Q + X QXs), the conditional extraction constant e is defined Cqxs = QX[Q ]w [X 1w where Cqxs is the total concentration of Q (quat) and X (corresponding anion) present in the organic phase in the molar ratio 1 to 1 and [Q ]w [X ]w te the respective... 

The extraction constant, can be expressed as the product of several equilibrium constants for other assumed equilibria in the net reaction ... 

If the solute A does not undergo any reaction in the two solvents, except for the solubility caused by the solvation due to the nonspecific cohesive forces in the liquids, the distribution of the solute follows the Nernst distribution law, and the equilibrium reaction can be described either by a distribution constant or an (equilibrium) extraction constant... 

Since [HAJa, and [BJotj are easily measurable qnantities, it is common to define the extraction constant K.. for this model ... 

For simplicity, we write the adduct HLB, instead of HB L. The extraction constant is... 

In order to calculate Du from Eq. (4.36), several equilibrium constants as well as the concentration of free A" are needed. Though many reference works report stability constants [19, 20] and distribution constants [4, 21], for practical purposes it is simpler to use the extraction constant for the reaction... 

Fig. 4.15 The system La(III) acetylacetone (HA) - IM NaC104/benzene at 25°C as a function of lanthanide atomic number Z. (a) The distribution ratio Hl (stars, right axis) at [A ] = 10 and [HA] rg = 0.1 M, and extraction constants (crosses, left axis) for the reaction Ln + 4HA(org) LnA3HA(org) + 3FE. (b) The formation constants, K , for formation of LnA " lanthanide acetylacetonate complexes (a break at 64Gd is indicated) circles n = 1 crosses n = 2 triangles w = 3 squares w = 4. (c) The self-adduct formation constants, for the reaction of LnA3(org) + HA(org) LnA3HA(org) for org = benzene. (A second adduct, LnA3(HA)2, also seems to form for the lightest Ln ions.) (d) The distribution constant Ajc for hydrated lanthanum triacetylacetonates, LnAs (H20)2 3, between benzene and IM NaC104. (From Ref. 28.)...

Table 4.17 Comparison of the Extraction Constants Aei for the Ilh-Suhgroup Divalent Ions, and inh Lanthanide Ions, with Sulfur or Oxygen Dialkyl Phosphoric Acids ...

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