Transfer Activity Coefficients Research Articles

Validation of pH Standards and Estimation of the Activity Coefficients of Hydrogen and Chloride Ions in an Ionic Liquid, Ethylammonium Nitrate.

We selected and validated the pH values of three standard materials that function in the protic ionic liquid, ethylammonium nitrate (EAN). The pH values of 0.05 mol kg-1 phthalate, oxalate, and phosphate buffers were 4.93 (0.04), 2.12 (0.04), and 7.13 (0.06), respectively (the values in the parentheses denote the standard deviation). Because the pH of EAN ranges from 0 to 10, with a neutral pH of 5, these materials are usable as acidic, basic, or neutral standards. The standard electrode potential of silver-silver chloride in EAN was 127.2 (1.7) mV. The activity coefficients of hydrogen and chloride ions remain equal to unity in EAN of a wide concentration range, which indicates that the effective ionic strength is independent of the solute ion concentration. In addition, the estimated value of the transfer activity coefficient of chloride ion suggests a weaker solvation in EAN compared with water in spite of a ubiquitous cation (C2H5NH3+). These behaviors of ions in EAN can be explained by the unique solvation in the ionic liquid through direct ion-ion electrostatic interactions.

Electrochemical behaviours of Eu(III)/E(II) and Ce(IV)/Ce(III) in H3PO4-H2O media : solvation and complexation reactions

Many kinds of rare earth elements (REE) such as europium and cerium have been make them essential elements in many high-tech components. The electrochemical studies can be presented as an interesting indication for europium and cerium extraction from phosphoric solutions, including solvation and complexation reactions. The normal redox potentials of Eu3+/Eu2+and Ce4+/Ce3+systems have been determined in H3PO4-H2O media with various phosphoric acid concentration. The solvation of these elements in phosphoric media is characterized by their transfer activity coefficients "f" calculated from the corresponding normal redox potentials. The corresponding solvation increases with increasing the H3PO4concentration. For each REE, the electrochemical properties depend on its number of charges and on its basic properties. Results suggest that solvation and complexation of REE phosphates are important in controlling REE concentration.

Electrochemical behaviours of Eu(III)/E(II) and Ce(IV)/Ce(III) in H3PO4-H2O media : solvation and complexation reactions

Many kinds of rare earth elements (REE) such as europium and cerium have been make them essential elements in many high-tech components. The electrochemical studies can be presented as an interesting indication for europium and cerium extraction from phosphoric solutions, including solvation and complexation reactions. The normal redox potentials of Eu3+/Eu2+ and Ce4+/Ce3+ systems have been determined in H3PO4-H2O media with various phosphoric acid concentration. The solvation of these elements in phosphoric media is characterized by their transfer activity coefficients "f" calculated from the corresponding normal redox potentials. The corresponding solvation increases with increasing the H3PO4 concentration. For each REE, the electrochemical properties depend on its number of charges and on its basic properties. Results suggest that solvation and complexation of REE phosphates are important in controlling REE concentration.

Distribution of 1-Alkyl-3-methylimidazolium Ions and Their Ion Pairs between Dichloromethane and Water

The distribution behavior of the salts of a series of 1-alkyl-3-methylimidazolium cations (RMeIm(+); R = butyl, hexyl, and octyl) with tetrafluoroborate (BF(4)(-)), hexafluorophosphate (PF(6)(-)), bis(trifluoromethanesulfonyl)amide (NTf(2)(-)), and 2,4,6-trinitrophenolate (Pic(-)) anions has been investigated in a dichloromethane-water system at 25 degrees C. The distribution constants (K(D)) of the ion pairs and the transfer activity coefficients ((o)gamma(w)) of the single ions were determined. For the ion pairs with a given anion, the log K(D) value increases linearly with the number of methylene groups (N(CH2)) in the cation, which can be explained by using the regular solution theory. A similar relationship was observed between log (o)gamma(w) and N(CH2) for the free RMeIm(+) ions, and the result was discussed by decomposing the transfer activity coefficient into the Born-type electrostatic contribution and the non-electrostatic one. For the free anions and their ion pairs with a given cation, the (o)gamma(w) and K(D) values increase with increasing molar volume of the anion: i.e., BF(4)(-) < PF(6)(-) < Pic(-) < NTf(2)(-). The features of the RMeIm(+) salts in the liquid-liquid distribution and the ion-pair formation in water are also discussed by comparing the present results with those of tetraalkylammonium salts previously reported.

Nikolai Izmailov (1907-1961): An appreciation

Nikolai Izmailov (1907-1961): An appreciation

Kinetics of Proton Transfer from Cationic Carbon Acids in Water and Aqueous DMSO. Effect of Activating Groups and Solvent on Intrinsic Rate Constants

[reaction: see text] Acidity constants and rates of reversible deprotonation of acetonyltriphenylphosphonium ion (1H+), phenacyltriphenylphosphonium ion (2H+), N-methyl-4-phenacylpyridinium ion (3H+), and N-methyl-4-(phenylsulfonylmethyl)pyridinium ion (4H+) by amines in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v) have been determined. From the respective Brønsted plots, log k(o) values for the intrinsic rate constants of the various proton transfers were obtained. Solvent transfer activity coefficients of the carbon acids and their respective conjugate bases were also determined which helped in understanding how the pKa values and intrinsic rate constants depend on the solvent. Some of the main conclusions are as follows: (1) The pK(a) values of 1H+, 2H+, and 3H+ are significantly higher than that of 4H+ because of a stronger resonance stabilization of the corresponding conjugate bases 1, 2 and 3, respectively. (2) The electronic effects of the PPh3+ and the N-methyl-4-pyridylium group are similar but the mix between inductive and resonance effect is different. (3) All four acids become more acidic upon addition of DMSO to the solvent. In all cases, the main factor is the stronger solvation of H3O+ in DMSO; for 1H+, 2H+, and 3H+ but not 4H+ this factor is significantly attenuated by stronger solvation of the carbon acid in DMSO. (4) The intrinsic rate constants for proton transfer are relatively high for all four carbon acids and show little solvent dependence; this contrasts with nitroalkanes which have much lower intrinsic rate constants and show a strong solvent dependence. These results can be understood by a detailed analysis of the interplay between inductive, resonance, and solvation effects.

Stability and Solvation of 1:1 Complexes of Dibenzo-21-crown-7 with Alkali Metal Ions in Polar Solvents

Stability constants of 1:1 complexes of dibenzo-21-crown-7 (DB21C7) with alkali metal ions have been determined at 25 °C in nitromethane, acetonitrile, propylene carbonate, and water by conductometry or capillary electrophoresis. The stability constants in acetonitrile, propylene carbonate, and water vary as Na + « K + Cs + ; this selectivity pattern is the same as that previously reported in methanol. In these solvents, the K + /Na + selectivity of DB21C7 is higher than that of dibenzo-18-crown-6 (DB18C6) and dibenzo-24-crown-8 (DB24C8). The stability constant in nitromethane, which has the weakest solvation power for the metal ions, decreases with increasing ionic size (Na + > K + > Rb + > Cs + ), reflecting the gas-phase selectivity governed by the electrostatic interaction between the ether oxygen atoms and the metal ion. The transfer activity coefficients of DB21C7 and its alkali metal ion complexes from nitromethane to the other solvents have been determined to evaluate the solvation of these solutes. It is shown that DB21C7 shields the alkali metal ions effectively from the solvents; the shielding by DB21C7 is generally more effective than by DB18C6 but less effective than by DB24C8.

Stabilities and transfer activity coefficients from water to polar nonaqueous solvents of 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene- and benzo-18-crown-6-cation complexes

Formation constants (K ML) for 1:1 complexes of 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (AC·B18C6), a linear counterpart of benzo-18-crown-6 (B18C6), and B18C6 with various mono- and divalent cations were determined in water at 25°C by conductometry; the K ML value for the B18C6-Li+ complex was also determined in acetonitrile. By using the K ML and literature values, transfer activity coefficients of the AC·B18C6- and B18C6-alkali metal ion complexes from water to polar nonaqueous solvents were calculated. The selectivity order in water of AC·B18C6 for univalent cations is different from that of B18C6, but for bivalent cations they are almost the same. For the same cation, the aqueous log K ML value is lower for AC·B18C6 than for B18C6 except for size-misfitting smaller cations (H+, Li+, Cd2+). In general, cyclization of the two binding-arms of AC·B18C6 increases the selectivity and stability for the cations in water. Although AC·B18C6 is always less hydrophilic than B18C6, the AC·B18C6-alkali metal ion complex is more hydrophilic than the corresponding B18C6 complex. It follows that the alkali metal ion in the AC·B18C6 complex is less effectively shielded and dehydration of AC·B18C6 upon complexation in water is less efficient, compared with B18C6. Hydrogen bonding between AC·B18C6 or B18C6 and water causes the unexpectedly lowest aqueous stability of the AC·B18C6- or B18C6-alkali metal ion complex among all the solvents. The same holds for the stabilities of the other metal ion complexes with AC·B18C6 or B18C6.

Stability and solvation of alkali metal ion complexes with dibenzo-30-crown-10

Stability constants for the 1:1 complexes of dibenzo-30-crown-10 (DB30C10) with alkali metal ions have been determined at 25 °C in nitromethane and water by conductometry and capillary electrophoresis, respectively. Transfer activity coefficients of DB30C10 and its complexes from nitromethane to S (S = water, acetonitrile, propylene carbonate, methanol, and N, N-dimethylformamide) have been determined at 25 °C to evaluate the solvation properties. The stability constant in the poorly solvating solvent, nitromethane, decreases with increasing metal ion size, Na + > K + > Rb + > Cs +, reflecting the intrinsic selectivity governed by electrostatic interaction between the metal ion and the ether oxygen atoms. It is also suggested that a part of the ether oxygen atoms does not bind to the metal ion in the Na(DB30C10) + complex. The aqueous stability constant varies as Na + ≪ K + ≈ Rb + ≈ Cs +; this selectivity pattern is similar to that in acetonitrile, propylene carbonate, and methanol. The complex stability in water is very low compared to that in the nonaqueous solvents, owing to hydrogen bonding of water to the oxygen atoms of the free crown ether. The transfer activity coefficient values show that DB30C10 shields all the metal ions effectively from the solvents and lead to the conclusion that the complexation selectivity in S receives a significant contribution from the solvation of the free metal ions. The Na(DB30C10) + complex has specific interaction with water, causing much lower K +/Na + selectivity in H 2O than in MeOH.

Stabilities in nitromethane of alkali metal ion complexes with dibenzo-18-crown-6 and dibenzo-24-crown-8 and their transfer from nitromethane to other polar solvents

Stability constants for the 1:1 complexes of Na +, K +, Rb +, and Cs + with dibenzo-18-crown-6 (DB18C6) and dibenzo-24-crown-8 (DB24C8) have been determined by conductometry at 25 °C in a poorly solvating solvent, nitromethane. For both the crown ethers, the stability constant decreases with increasing metal ion size, Na + > K + > Rb + > Cs +, regardless of the size compatibility between the metal ions and the ligand cavities. A comparison of the results with those in several other solvents (S: acetonitrile, propylene carbonate, water, methanol, and N, N-dimethylformamide) leads to the conclusion that the selectivity sequence of these crown ethers in nitromethane agrees with the intrinsic one in the absence of a solvent. Transfer activity coefficients of the crown ethers and their complexes from nitromethane to S have been determined to evaluate the solute–solvent interactions. It is shown that DB24C8 shields the alkali metal ions more effectively from the solvents than DB18C6 because of the larger number of oxygen atoms and the more flexible structure of DB24C8. Regarding the complexation in nitromethane as a reference, the complex stability and selectivity in S are discussed. The selectivities of these crown ethers in water, methanol, and N, N-dimethylformamide, which apparently obey the size-fit concept, are largely due to the solvation of the free alkali metal ions.

Electrochemistry in Concentrated Hydrochloric Acid (C ≤ 5 M) Progressively Neutralized by Sodium Hydroxide Pellets: Acid−Base, Oxidation−Reduction, and Solvation Properties of the Medium

The acid−base and oxidation−reduction properties of aqueous HCl + NaCl mixtures (HCl + NaCl ≤ 5 M) are characterized by means of the Ro(H) acidity function and the Ro(Cl-) function, which respectively represent their ability to give up the proton and the chloride ion. The electrochemical windows of these mixtures at various working electrodes including mercury drop, carbon, platinum, and gold are determined. The solvation properties of these media are characterized by means of the solvation transfer activity coefficients of the solutes, which are calculated from the normal potential values of the corresponding redox system.

The principles of migration and dispersion in capillary zone electrophoresis in nonaqueous solvents.

Nonaqueous solvents are interesting media for capillary zone electrophoresis as they can affect all relevant parameters governing the separation of sample zones. However, for a rational planning of the working conditions and an appropriate interpretation of the results obtained, the basic principles of ion migration and zone dispersion must be understood. Many solvent induced effects need to be carefully considered and recognized before full exploitation of nonaqueous solvents can take place. It is the goal of this overview to present the fundamental physicochemical aspects of capillary zone electrophoresis in nonaqueous solvent systems. Therefore, the detailed discussion is related to the effect of organic solvents on electrophoretic mobilities (based on the theory of conductance), acid-base dissociation behavior (based on the transfer activity coefficient and medium effect), pH, separation efficiency (with regard to mobility and diffusion coefficient in dilute solutions), resolution, and electroosmotic flow.

Medium effect (transfer activity coefficient) of methanol and acetonitrile on beta-cyclodextrin/benzoate complexation in capillary zone electrophoresis.

Association constants, Kc, were derived from the electrophoretic mobilities of the anionic solutes (seven benzoates with hydroxy or chloro substituents) by capillary zone electrophoresis in different solvent systems, consisting of binary mixtures of water with up to 20% (v/v) methanol or acetonitrile, respectively. The association constants expectedly are found to decrease with increasing organic solvent concentration. The effect of organic solvents on the Kc of the benzoates with beta-cyclodextrin was analyzed applying the concept of the transfer activity coefficient (or the medium effect). This concept enables the evaluation of the significance of the contributions of the individual species involved in the complexation equilibrium in the different solvents: the benzoate ion, beta-cyclodextrin, and the anionic benzoate-beta-cyclodextrin complex. The medium effect on benzoate was calculated from the change in acidity constant of benzoic acid in the different mixed solvents and the corresponding transfer activity coefficients of the proton and the molecular acid. The transfer activity coefficients for beta-cyclodextrin results from its solubility at saturation in the different solvents. In this way, an estimation of the standard free energy of transfer, deltaG(t)0, of each species involved in the complexation equilibrium was possible for the transfer from water into the respective mixed solvent. It was found that the organic solvents do not significantly affect deltaG(t)0 for the benzoate anion. However, the organic solvents play a different role concerning the stabilization of beta-cyclodextrin and the complex anion: whereas the addition of acetonitrile has nearly no influence on deltaG(t)0 of the anionic complex, the reduction in Kc is caused by the enhanced stabilization of beta-cyclodextrin (reflected by its better solubility). Addition of methanol, on the other hand, lowers the solubility of beta-cyclodextrin, thus giving positive values for deltaG(t)0. Thus, the overall effect on Kc in methanolic solutions must be related to the pronounced destabilization of the benzoate-beta-cyclodextrin complex.

クラウンエーテル及びそのアルカリ金属イオン錯体の水-極性非水溶媒間移行活量係数

クラウンエーテル及びそのアルカリ金属イオン錯体の水-極性非水溶媒間移行活量係数

Stabilities and Transfer Activity Coefficients from Water to Polar Nonaqueous Solvents of Benzo-15-Crown-5- and 15-Crown-5-Alkali Metal Ion Complexes

Stability constants (KML) of 1 : 1 benzo-15-crown-5 (B15C5) complexes with alkali metal ions were conductometrically measured in water at 25°C. Transfer activity coefficients of B15C5 and 15-crown-5 (15C5) from water to polar nonaqueous solvents were determined at 25°C. By using these data and the literature values, transfer activity coefficients of the B15C5 and 15C5 complexes with alkali metal ions from water to the polar nonaqueous solvents were calculated to study the solute-solvent interaction of the crown ether complexes. The stability of the B15C5 complex is lower in water than in any other nonaqueous solvent. The KML value for B15C5 is always smaller than the corresponding K ML value for 15C5. The interaction of the B15C5 or the 15C5 complex with the solvents depends on the alkali metal ion in the crown cavity. All the B15C5 and 15C5 complexes undergo hydrophobic hydration, which is particularly stronger for the B15C5 complexes with Na+ and K+. The unexpectedly lowest stability of the B15C5- or the 15C5-alkali metal ion complex in water among all the solvents is caused by the hydrogen bonding between ether oxygen atoms of uncomplexed B15C5 or 15C5 and water.

Stabilities in Water of Alkali Metal Ion Complexes with Dibenzo-24-crown-8 and Dibenzo-18-crown-6 and Their Transfer Activity Coefficients from Water to Nonaqueous Solvents

Stability constants KML for the 1:1 complexes of Na+, K+, Rb+, and Cs+ with dibenzo-24-crown-8 (DB24C8) and dibenzo-18-crown-6 (DB18C6) in water have been determined by a capillary electrophoretic technique at 25°C. The KML sequence is Na+ Rb+ > Cs+ for DB18C6. Compared with DB18C6, DB24C8 exhibits higher selectivity for K+ over Na+, but lower selectivity for K+, Rb+, and Cs+. To evaluate the solvation of the complexes in water, their transfer activity coefficients sγH2O between polar nonaqueous solvents and water have been calculated. The sγH2O values provide the following information: interactions with water of the metal ions and of the crown-ether oxygens are greatly reduced upon complexation and the complexes undergo hydrophobic hydration in water; the character of each alkali metal ion in solvation is more effectively masked by DB24C8 than by DB18C6, because of the larger and more flexible ring structure of DB24C8. Solvent effects on the complex stabilities are discussed on the basis of the sγH2O values.

Propylene carbonate as a nonaqueous solvent for capillary electrophoresis: mobility and ionization constant of aliphatic amines.

The two properties of aliphatic amines were investigated in propylene carbonate as solvent that are decisive for capillary electrophoretic migration: the actual mobilities and the pKa* values. Solutes were eight primary, secondary, and tertiary amines. Roughly, the actual ionic mobilities of the ammonium ions are inversely proportional to the solvent viscosity, fairly obeying Walden's rule. The pKa* values of the cation acids, HB+ (the corresponding acids of the amines, B), were related to the conventional pH* scale of the buffers. Determined from the effective mobilities as a function of the pH*, they are increased by approximately 7 units compared to water. This increase was interpreted based on the concept of the standard free energy of transfer of the individual species in the acid-base equilibrium. The corresponding medium effect on the proton, log mgammaH+ (the logarithm of the transfer activity coefficient mgammaH+) is approximately +8. The medium effect on the free base, B, was obtained from solubility data; it is about -1 and smaller. Plausible values for the medium effect on the cation HB+ (-1 to -2) lead to a sum of the increments, which corresponds with the overall effect, expressed by the change in pKa*. Examination of the individual contributions shows that the drastically lower basicity of propylene carbonate compared to water is mainly responsible for the increase in pKa upon transfer of the acid-base equilibrium of aliphatic ammonium/amine from the aqueous to the organic solvent.

Stabilities in Water and Transfer Activity Coefficients between Nonaqueous Solvents and Water of 1 : 1 Complexes of 15,15-Dimethyl-16-crown-5 with Metal Ions

Abstract The stability constants of 1 : 1 complexes of 15,15-dimethyl-16-crown-5 (DM16C5) with Li+, Na+, K+, Ag+, Tl+, Sr2+, Ba2+, and Pb2+ and of 16-crown-5 (16C5) with Li+ have been determined by conductometry in water at 25 °C. The transfer activity coefficients (sγ H2O) of the alkali metal ion complexes between nonaqueous solvents (s: acetonitrile, propylene carbonate, and methanol) and water have been calculated. These values have been compared with the literature values for 16C5 and 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (L16C5). In water, 16C5 and DM16C5 selectively complex Li+ over Na+ and K+. The complexing ability of DM16C5 for a given metal ion is generally lower than that of 16C5 and L16C5. The alkali metal ion complexes of all the crown ethers are much less stable in water than in s, and the selectivity and the substituent effects also depend upon the solvent. The sγ H2O values provide the following information: the alkali metal ion and the ether oxygen atoms are greatly dehydrated upon complexation in water; the dehydration is most drastic for Na+; the dehydration for Na+ is hindered by the methyl groups of DM16C5 and facilitated by the electron-donating side arm of L16C5. The solvent effects upon complexation are explained in terms of solute-solvent interactions.

Stabilities in Polar Nonaqueous Solvents and Transfer Activity Coefficients from Water to Nonaqueous Solvents of 19-Crown-6-Alkali Metal Ion Complexes

Formation constants of 1 : 1 19-crown-6(19C6) complexes with alkali metal ions weredetermined conductometrically at 25 °Cin acetonitrile(AN), propylene carbonate (PC), methanol, DMF, andDMSO. 19C6 always forms the most stable complex withK+. The selectivity order of 19C6 forheavy alkali metal ions isK+ > Rb+ > Cs+.The selectivity for Na+ varies withthe solvent; that for Li+ is the second lowest(AN, DMSO) or the lowest (PC). Transfer activity coefficients(SγH2 O) of19C6 from water to the nonaqueous solvents (S) weremeasured at 25 °C. The contributions of a methylenegroup and an ether oxygen atom to thelog SγH2 Ovalue of a crown ether wereobtained. The SγH2 Ovalues of the 19C6–alkali metal ion complexes(SγH2 O (ML+)) werecalculated, M+ and L denoting an alkali metal ionand a crown ether, respectively. For AN, PC, andCH3OH, although the M+ ion is more stronglysolvated by water than by AN, PC, or CH3OH, thelog SγH2 O (ML+) islarger than the correspondinglog SγH2 O (L)expect for the case of M+ = Li+.The higher lipophilicity of the19C6 complex ion is attributed to an enforcement ofthe hydrogen-bonded structure of water for the complexion caused by the greatly decreased hydrogen bondingbetween ether oxygen atoms and water uponcomplexation. For DMF and DMSO, thelog SγH2 O (ML+) is also greater thanthe correspondinglog SγH2 O (L).It was concluded from thisfinding that the unexpectedly lowest stability of the19C6 complex ion in water is due to the hydrogenbonding between 19C6 and water. The stabilities and thelog SγH2 Oof 19C6–alkali metal ion complexes were compared with those of 18C6complexes.

ChemInform Abstract: Stabilities in Water and Transfer Activity Coefficients Between Nonaqueous Solvents and Water of Metal Ions Complexed with Macrocyclic Compounds

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.