On the effect of the porosities of microporous membranes on their ionic selectivities

In order to help clarify the correlation between the structure and the transport parameters in the microporous membranes mentioned in this paper, the effect of porosity upon ionic selectivity is examined for Nucleopore®, Millipore® and Pall® membranes. These membranes separate two NaCl solutions of concentrations c₁ (constant for all the experiment,) and c₂ (fixed for each experiment, c₂ >c₁). The concentrations are small enough to take the place of the activities in the transport equations. The temperature is 298.0 ± 0.1 K and the stirring speed used to homogenize the solutions is 50 rpm.The ionic selectivity is characterized, in the steady state, by an experimental parameter K, which is obtained from measurements of the membrane potential as a function of the relative saline concentration n_c = c₂/c₁ (with c₁ being constant). This parameter K makes it possible to obtain the relative ionic permeability n_p = P_-/P₊ and the transport numbers. The relation between K, n_p and t₊ or t_- is obtained by integrating the Nernst-Planck equations and making use of the Goldman hypothesis of constant electric field. The porosity θ, expressed in percents of the empty volume over the total volume of the membrane, is measured by means of a pycnometric method which is explained in this paper. From the values of K and θ, obtained for the membranes and experimental conditions of these experiments, it can be concluded that ionic selectivity increases linearly with porosity.     

Complexation study and spectrofluorometric determination of the binding constant for diquat and p-sulfonatocalix[4]arene

The interactions between diquat (DQ) and p-sulfonatocalix[4]arene (C4S) were studied in an aqueous solution as a function of the ionic strength. Evidence for the formation of a complex between DQ and C4S was obtained using fluorescence measurements, while the stoichiometry of the complex was confirmed as 1:1 for DQ/C4S using UV–vis spectroscopy. The ionic strength had no influence on the stoichiometry of the complex, but exerted a significant influence on the complexation constant, K_c, decreasing with an increase in the ionic strength. The thermodynamic complexation constant, K_c′, was computed as 5.25±1.11×10⁷ using the extended Debye–Hückel law. The rate constants for the heterogeneous electron transfer for the reduction of DQ at an electrode surface were evaluated as 0.150±0.010 cm s^?1 in the absence of C4S and 0.065±0.010 cm s^?1 when C4S was added to the solution in a 1:1 ratio.     

Complex Formation and Ionic Association in Methanol Solutions of Poly(Ethylene Oxide) and Alkaline Earth Salts

Complex formation of poly(ethylene oxide) (PEO) with divalent barium and strontium salts was investigated in methanol. In these systems the complexation was accompanied by a considerable degree of ionic association. An analytical model for the polymer-ion complexation based on a one-dimensional lattice model was proposed. According to this model, the electrostatic effects between the bound ions were separated from the total free energy change of the binding. Three binding constants, i.e., the ionic association constant K _A, the cation binding constant, K _c, and the anion binding constant, K _a, could be estimated. K _A for barium and strontium salts was comparable, and the effect of counteranions on K _A was not large. K _c for barium salts was almost independent of the kind of counteranion and larger than that for corresponding strontium salts, indicating stronger polymer-ion interaction for barium salts. The anion binding constant, K _a, was strongly dependent upon the kind of anion, and the order was CI^? ? ? 4 ^?. The pronounced ion binding for larger anions may be explained by the more favorable free energy change of desolvation. Finally, the concentration of free and bound ionic species was determined as a function of PEO concentration.     

Spectrophotometric investigation of monobromo and dibromo complexes of copper(II) in methanolic medium

Complex formation between copper(II) and bromide in anhydrous methanol was investigated spectrophotometrically. At a constant copper(II) concentration of 3.0 x 10^?4M, Cu²⁺ and CuBr⁺ are at equilibrium for [Br^?] < 1.0 x 10^?3M while CuBr⁺ and CuBr₂ exist at equilibrium in the range of [Br^?] 2.0 x 10^?3 ?40 x 10^?3M. An isosbestic point at 235 nm indicated the equilibrium of Cu²⁺ and CuBr⁺ while a second isosbestic point at 290 nm showed the equilibrium of CuBr⁺ and CuBr₂. Stability constants for the formation of CuBr⁺ and CuBr₂ (K₁, and K₂, respectively) were determined as a function of ionic strength in the range 0.01–0.10. Log K₁ and log K₂ values at zero ionic strength were obtained by extrapolation of the plot of log K vs ionic strength, the values being 3.97 ± 0.01 and 2.31 ± 0.01.     

13C NMR measurements of acid-base equilibrium constants in alkaline aqueous solutions of D-glucose anomers

The concentration dissociation constants pK _c of the α and β D-glucose anomers were measured at pH 12–14 by the ¹³C NMR method. The pK _c values for L ? H_?1L + H equilibria at 25°C and an ionic strength of 1.0 mol/l (NaCl) were pK _c ^α = 12.31 ± 0.03 and pK _c ^β = 12.03 ± 0.01. Dissociation was found to shift all the spectrum lines downfield. The most probable hydroxyl group responsible for the dissociation of β-D-glucose was determined.     

Carbamate Formation and Amine Protonation Constants in 2-Amino-1-Butanol–CO<Subscript>2</Subscript>–H<Subscript>2</Subscript>O System and Their Temperature Dependences

The apparent protonation constant of an amino butanol (AB) and its carbamate formation constant in the CO₂–H₂O–AB system were determined at T = 298.15–328.15 K, and at various ionic strengths up to 0.2 mol·L^?1 by potentiometric titration. The Debye–Hückel equation was used to extrapolate the protonation constants to zero ionic strength. The variation of the thermodynamic equilibrium constant for carbamate formation with temperature was modeled according to the relationship of log₁₀ K₁ = 280.91/T ? 0.1105, while the temperature dependency of the amine protonation constant was correlated by log₁₀ K₂ = 1926.53/T + 2.9482. Van’t Hoff type plots of the pK values showed linear relationships indicating that the standard enthalpy changes of reaction are constant over this range of temperatures. Hence, our current findings are crucial for designing efficient unit operations involving separation of CO₂ from natural or flue gases.     

A critical study of the application of ultraviolet spectroscopy to the self-association of adenine,adenosine and 5′-AMP in aqueous solution

UV spectra of adenine, adenosine and 5′-AMP in aqueous solution have been measured over the concentration range 1 × 10⁻⁶−5 × 10⁻² M. The apparent molar absorptivity of these compounds changes upon concentration, showing two hypochromic effects at c < 5 × 10⁻³ M and c &>; 5 × 10⁻³ M, respectively, which may be explained in terms of self-association.A method for calculating self-association constants from these experimental data is developed, based on an association model in which the first hypochromic effect is interpreted in terms of formation of dimers, with an equilibrium constant K₂, and the second effect is interpreted in terms of formation of polymers, with an equilibrium constant K_n. The value of K₂ is of the order of magnitude of 10⁴ for the three compounds. The value of K_n is dependent on the model chosen for the analysis of the second effect, having an order of magnitude of 10².The features of the self-association model are discussed, as well as the method for calculating self-association parameters from experimental data.     

Spectkophotometric determination of the dissociation constants of n,n'-bis(3-hydroxypropyl)dithiooxamide and n,n'-bis(2-hydroxypropyl)dithiooxamide

The dissociation constants of N,N'-bis(3-hydroxypropyl)dithiooxamide and N,N'-bis(2-hydroxypropyl)dithiooxamide were determined by a spectrophotometric method using a weighted least squares technique for the calculations. For N,N'-bis(3-hydroxypropyl) dithiooxamide a thermodynamic constant pK₁^T of 11.37 was found. At ionic strength μ = 1, pK₁ = 11.27 and pK₂ = 14.29. For N,N'-bis-(2-hydroxypropyl) dithiooxamide, these values were respectively: pK₁^T = 11.11; pK₁ = 10.99 and pK₂ = 13.75.     

Electronic spectral studies and solvent effects on the reaction of iodine with 1,4,8,11-tetraazacyclotetradecane

The reaction between iodine and 1,4,8,11-tetraazacyclotetradecane (TACTD) is studied photometrically in various solvents like CCl₄, CHCl₃, CH₂Cl₂ and 1,2-dichloroethane. The results reveal that in each solvent the (TACTD):I₂ ratio is 1:2 and the iodine complex is formulated as (TACTD)I⁺·I₃⁻. The obtained values of the formation constant (K), extinction coefficient (ε) and oscillator strength (ƒ) for the iodine complex are shown to be strongly dependent on the polarity of the solvent. A linear correlation is obtained between either (ƒ) or (ε) and the dielectric constant (D) of the solvent. The important role of the solvent is mainly suggested to be due to the interaction of the ionic iodine complex with the solvent.     

Thermodynamics of the formation of Sm3+-ethylenediamine-N,N′-disuccinic acid complexes in aqueous solutions at 298.15 K

The thermodynamic parameters of the formation of the complexes of ethylenediamine-N,N′-disuccinic acid (H₄Y) with Sm³⁺ ion are determined by means of calorimetry and measuring pH at 298.15 K and ionic strengths of 0.5, 1.0, and 1.5 (KNO₃). The values of logK, Δ _r G, Δ _r H, and Δ _r S are calculated for the formation of SmY^? and SmHY complexes at fixed and zero values of ionic strength. The resulting values are interpreted.     

Electrochemically catalyzed cleavage of carbon-carbon bond The intermediate formation of metal—Carbene complexes

This paper descirbes a polarographic method for the determination of the formation constant for the binding of molecular oxygen with several cobalt(II) Schiff-base complexes. The method consists of determining, from the polarographic diffusion current for the reduction of the oxygenated complex, the oxygen partial pressure at which half of the complex is oxygenated. At this point, the partial pressure determined with a Clark-type membrane electrode is equal to the reciprocal of the formation constant, K_oxy, for the oxygenated species. K_oxy values of 0.128, 0.162 and 0.178 Torr^?1 were obtained for the Co(II) SALEN, SALOPH and 3-methoxySALEN complexes, respectively, at 0° C in pyridine. This method can be used to determine K_oxy values as large as 2.0 Torr^?1 with an estimated uncertainty of 5–10%, based on the uncertainty of the oxygen electrode measurement.     

The dependence of the magnitude of the spontaneous polarization on the cell thickness in ferroelectric liquid crystals

The magnitude of the spontaneous polarization (P) was measured for S-3,7-dimethyloctyl 4-n-hexadecyloxybenzoyloxybenzoate, a material with an isotropic to smectic C phase transition (T_c), using a triangular wave method. The temperature dependence of the measured values was well represented by the expression P = P₀(T_c-T)^α· P₀ was found to depend on the cell thickness for thicknesses below 18 μm, but saturated to a constant value above this limit. The value of the exponent α, on the other hand, was found to be insensitive to the cell thickness. The decreased value of the polarization in the thin cells is thought to be a result of polarization screening due to ionic impurities.     

IONIC STRENGTH EFFECTS ON THE GROUND STATE COMPLEXATION and TRIPLET STATE ELECTRON TRANSFER REACTION BETWEEN ROSE BENGAL and METHYL VIOLOGEN

Abstract— The equilibrium constants, K_c, for complexation between methyl viologen dication (MV²+) and Rose Bengal, or Eosin Y, decrease with increasing ionic strength. At zero ionic strength K_c is 6500 (± 500) mol^?1 dm³ for Rose Bengal and 3200 (± 200) mol^?1 dm³ for Eosin Y, and these values decrease to 1500 (± 100) and 680 (± 40) mol^?1 dm³, respectively, at an ionic strength of 0.1 mol dm^?3. K_c is independent of pH between 4.5 and 10. ΔH is -25 (± 1) kJ mol^?1 for complexation with either dye, whereas ΔS is -15 (± 3) J K^?1 mol^?1 for Rose Bengal, and - 23 (± 3) J K^?1 mol^?1 for Eosin Y. The complexation constant for Rose Bengal and the neutral viologen, 4,4'-bipyridinium-N, N'-di(propylsulphonate), (4,4'-BPS), is 420 (± 35) mol^?1 dm³, and independent of ionic strength. No complexation could be observed for either Rose Bengal or Eosin with another neutral viologen, 2,2'-bipyridinium-N,N'-di(propylsulphonate), (2,2'-BPS). MV²+ quenches the triplet state of Rose Bengal with a rate constant of 7 × 10⁹ mol^?1 dm³ s^?1, and this rate constant decreases slightly as ionic strength increases. The cage escape yield following quenching, Φ_cc is very low (Φ_cc= 0.02 (± 0.005), and independent of ionic strength. 4,4'-BPS quenches the triplet state of Rose Bengal with a rate constant of 2.2 (± 0.1) × 10⁹ mol^?1 dm³ s^?1, and gives a cage escape yield of 0.033 (± 0.006). 2,2'-BPS quenches the Rose Bengal triplet with a rate constant of 6 (± 1) × 10⁸ mol^?1 dm³ s^?1 and gives a cage escape yield of 0.07 (± 0.01). Conductivity measurements indicate that MV²+(Cl^?)₂ is completely dissociated at concentrations below 2 × 10^?2 mol dm^?3.     

Quantitative parameters for the sequestering capacity of polyacrylates towards alkaline earth metal ions

The complex formation constants of polyacrylic (PAA) ligands (1.4≤log N≤2.4, N=number of monomer units) with calcium and magnesium ions were determined in different ionic media at different ionic strengths, 0≤I≤1 mol l⁻¹, at t=25 °C. Experimental pH-metric data in the presence of Ca²⁺ or Mg²⁺ were firstly analysed in terms of apparent protonation constants, log K^H*, using the “three parameter model” proposed by Högfeldt; differences in log K^H*, determined in different ionic media, were interpreted in terms of complex species formation. The only species present in the system M-PAA (M=Ca²⁺ or Mg²⁺) is ML₂: attempts to find species of different stoichiometry were unsuccessful. The stability dependence of this species on ionic strength, on the degree of neutralisation (α) and on PAA molecular weight is discussed using empirical equations. The formation constant, log β₂, is significantly higher for Ca²⁺ than for Mg²⁺: at I=0.1 mol l⁻¹ (NaCl), log N=1.8 and α=0.5, log β₂^Ca=4.43 and log β₂^Mg=4.24. The formation of polyacrylate-alkaline earth metal complexes is discussed in the light of sequestering effects in natural waters.     

Amine oxides—XIII: Iodine complexes with non-aromatic amine oxides

EDA-iodine complex with trialkylamine oxides in dichloromethane have been investigated spectrophotometrically. Thermodynamic (K_c^AD, —ΔG°) and spectral (ε_λ^AD) characteristics of these complexes were determined. The absorption band of iodine in the visible region was blue-shifted with a variable maximum, depending on the structure of the amine oxide. The K_c^AD and ε_λ^AD values were calculated using a modified method. A non-linear relationship K_c^AD = f(R), where R is the number of carbon atoms in the alkyl chain, was found.     

Ruthenium(III) catalysed oxidation of l-leucine by a new oxidant,diperiodatoargentate(III) in aqueous alkaline medium

The kinetics of Ru(III) catalysed oxidation of l-leucine by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and a constant ionic strength of 0.60 mol dm⁻³ was studied spectrophotometrically. The oxidation products are pentanoic acid and Ag(I). The stoichiometry is [l-leucine]:[DPA] = 1:2. The reaction is of first order in Ru(III) and [DPA] and has less than unit order in both [l-leu] and [alkali]. The oxidation reaction in alkaline medium has been shown to proceed via a Ru(III)–l-leucine complex, which further reacts with one molecule of monoperiodatoargentate(III) (MPA) in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test and spectral studies. The reaction constants involved in the different steps of the mechanism are calculated. The catalytic constant (K_c) was also calculated for the Ru(III) catalysed reaction at different temperatures. From the plots of log K_c versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The active species of catalyst and oxidant have been identified.     

Thermodynamic properties of alkali halides. IV. Apparent molal volumes,expansibilities, compressibilities,and heat capacities in urea-water mixtures

The apparent molal volume φ_v, expansibility φ_E, compressibility φ_K, and heat capacity φ_c of NaCl were measured in urea-water mixtures, as a function of salt (<1.5m) and urea (<13m) concentrations at 25°C. At a fixed urea concentration, the transfer functions from H₂O to 3m urea are linear functions of the NaCl aquamolality. At a fixed salt aquamolality, (0.1m), the sign of the transfer functions is in the direction of a decrease in the structure-breaking effect, and the absolute values of the transfer functions tend to level off at high urea concentrations (13m). The functions φ_v, φ_E, φ_K, φ_c, and (?φ_v/?T)_p were measured for the sodium halides and alkali, bromides (chlorides in the case of φ_K) at a fixed salt aquamolality of 0.1m and fixed urea molality of 3m. The corresponding transfer functions from H₂O to 3m urea are opposite those from H₂O to D₂O and similarly are relatively independent of ionic size. This suggests that urea, shows no specific interaction affinity for ions and that the overall number of water molecules influenced by the ions is relatively constant for all alkali halides. The lithium halides are an exception in that Li⁺ seems to have hardly any structure-breaking effect.     

A new sensitive method of dissociation constants determination based on the isohydric solutions principle

The paper provides a new formulation and analytical proposals based on the isohydric solutions concept. It is particularly stated that a mixture formed, according to titrimetric mode, from a weak acid (HX, C₀ mol/L) and a strong acid (HB, C mol/L) solutions, assumes constant pH, independently on the volumes of the solutions mixed, provided that the relation C₀ = C + C² · 10^pK₁ is valid, where pK₁ = −log K₁, K₁ the dissociation constant for HX. The generalized formulation, referred to the isohydric solutions thus obtained, was extended also to more complex acid-base systems. Particularly in the (HX, HB) system, the titration occurs at constant ionic strength (I) value, not resulting from presence of a basal electrolyte. This very advantageous conjunction of the properties provides, among others, a new, very sensitive method for verification of pK₁ value. The new method is particularly useful for weak acids HX characterized by low pK₁ values. The method was tested experimentally on four acid-base systems (HX, HB), in aqueous and mixed-solvent media and compared with the literature data. Some useful (linear and hyperbolic) correlations were stated and applied for validation of pK₁ values. Finally, some practical applications of analytical interest of the isohydricity (pH constancy) principle as one formulated in this paper were enumerated, proving the usefulness of such a property which has its remote roots in the Arrhenius concept.     

The association constant of the first chloro-copper(II) complex in water to 100°C

The equilibrium constant, K₁, for the reaction Cu²⁺ Cl^? ? [CuCl]⁺ has been determined in the range 15–100°C from spectrophotometric data. The method of analysis, which is widely used, requires both slope and intercept from a reciprocal plot of absorbance against total metal ion concentration. The calculated values of K₁, particularly at low temperature, are very sensitive to the chosen method of data treatment such that the application of weighting factors to the absorbance data for a given photometric accuracy can halve the values. The large variations in K₁ found in the literature are likely to be caused by underestimation of the instability inherent in the method as much as by the neglect of chemical effects such as the formation of higher complexes. By appropriate weighting of the absorbance data an increase in K₁ is apparent above 30°C and the method may have limited usefulness at high temperatures.     

Thermodynamic characteristics of complexation between Ho<Superscript>3+</Superscript> and ethylenediamine-<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>'-disuccinic acid in aqueous solutions at 298.15 K

The thermodynamic characteristics of complexation between ethylenediamine-N,N'-disuccinic acid (H₄Y; EDDA) and Ho³⁺ ion were determined calorimetrically and potentiometrically at 298.15 K and ionic strengths of 0.1, 0.5, 1.0, and 1.5 (KNO₃). The logK, Δ_rG, Δ_rH, and Δ_rS values for the formation of HoY^– and HOHY complexes were calculated at the studied and zero ionic strength values. The changes in thermodynamic parameters of the reactions are discussed.