Potentiometric determination of acidity constants of some Schiff bases in tetrahydrofuran-water mixtures

Using potentiometric measurements, dissociation constants of two Schiff bases, N,N′-bis(1′-hydroxy-2′-acetonaphthone)propylenediimine (L₁) and bis(1′-hydroxy-2′-acetonaphthone)-3,3′-diiminodpropaneamine (L₂) were determined in binary mixtures of tetrahydrofuran-water at 25.0 ± 0.1°C and an ionic strength of 0.100 M tetrabutylammonium perchlorate (TBAP). The results show that the pK _a values of these acids increases as the percentages of the tetrahydrofuran increase in solvent mixtures. There is a linear relationship between acidity constants and the mole fraction of tetrahydrofuran in the solvent mixtures. Effect of solvent composition on acidity constants is also discussed.     

Dissociation constants of phenols in methanol--water mixtures

A preferential solvation model that relates solute properties with solvent composition in binary mixtures has been applied to the dissociation pKa values of a set of 28 substituted phenols in methanol-water mixtures. The parameters of the model allow estimation of the pKa value of each phenol for any methanol-water composition. Moreover, it is demonstrated that the pKa values of the whole set of phenols at any methanol-water composition are linearly related to the pKa values of the phenols in water. Equations that relate the correlations' slope and intercept values with the solvent composition have been derived and tested with the set of phenols. The general parameters obtained for these equations allow an accurate calculation of the pKa value of any phenol, even of those not included in the original set, at any methanol-water composition solely from the pKa value of the phenol in water. These calculated pKa values can be used for quantitative structure-HPLC retention relationships. The method is tested by comparison of the calculated pKa values with the HPLC determined pKa values of 26 phenols in a polymeric column with a 50% methanol as mobile phase.     

Ionic equilibria in neutral amphiprotic solvents: Structural effects on dissociation constants of several substituted phenols and mercaptopyrimidines in isopropyl alcohol

The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK_a. The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK_a values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.     

Ionic equilibria in neutral amphiprotic solvents of low dielectric constant: Buffer solutions

The ionic equilibria in neutral amphiprotic solvents (isopropyl and tert-butyl alcohols) have been established, and equations to calculate pH values in solutions of acids, bases, salts or their mixtures, developed. The effect, on the dissociation equilibria, of the presence of small quantities of water or other solvents in the bulk solvent used has been taken into account in the proposed equations. On the basis of these equations some buffer solutions have been studied and recommended for electrode standardization. The results, tested by experimental work, show the importance of the incompleteness of dissociation of salts in these solvents, which decreases the pH of acid buffers and increases the buffer capacity.     

水/乙醇混合溶剂中氨基酸离解的取代基效应

用LKB-2277BioActivityMonitor测定了298.15K时甘氨酸,丙氨酸和丝氨酸在水/乙醇混合溶剂中的离解焓,计算了相应的离解熵。根据三个氨基酸分别与某一参考酸之间的质子交换反应,重点讨论了溶剂结构变化对取代基效应焓,熵变化的影响。实验结果表明在水/乙醇混合溶剂的最大结构化区域,取代基效应最为显著。认为在混合溶剂的富水区大块的似冰山结构的水分子簇的寿命大大延长。     

Liquid Chromatographic Behavior of Nitrogen Compounds

Abstract

Chromatographic behavior of nitrogen compounds differed from others. The column efficiency was poor for the compounds and sometimes solutes were not eluted out from a column. Therefore, the elution volume of alkylamines, anilines, pyridines, pyrazines, quinolines and aminopolyaromatic hydrocarbons was measured on a methacrylate gel and octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures. The solvent effect on the dissociation constant differs from that obtained for aromatic acids. The values in acetonitrile/water mixtures are smaller than those obtained in 100% water. The linear relation between log P and log k′ values is obtained in eluents of pH 7 where the retention of these compounds is maximized. Some hydrophobic fragmental constants are proposed from this result. Prediction of retention time of these compounds from their log P values can be done in the individual groups on octadecyl bonded silica gels in pH controlled acetonitrile/water mixtures.     

Assignment of reference pH-values to primary standard buffer solutions for standardization of potentiometric sensors in acetonitrile-water mixtures

Standard pH-values pH(PS) for seven primary standard buffer solutions in 0, 10, 30, 40, 50, 70 and 100% (w/w) acetonitrile-water mixed solvents at 298.15 K were determined according to the criteria recently endorsed by IUPAC. The preferential solvation of the ions in acetonitrile-water mixtures was studied in order to clarify the acid-base behaviour of the solutes in such mixtures. The influence of variation in the solvent composition on pH(PS) values was considered with a view to obtaining correlations that can be used to determine the pH(PS) values in any acetonitrile-water mixture up to 70% (w/w) acetonitrile. The pH(PS) values were then correlated with the weight and volume percentages and molar fraction of acetonitrile and with the Kamlet-Taft, pi(*), alpha and beta solvatochromic parameters of the acetonitrile-water mixtures. The equations obtained permit the standardization of potentiometric sensors in these mixtures.     

Conductance in isodielectric mixtures. II.i-butyronitrile with benzene,carbon tetrachloride,dioxane, and tetrahydrofuran

The conductance of tetrabutylammonium nitrate, perchlorate, and picrate and of tetraethyl-, tetrapropyl-, and tetrabutylammonium tetraphenylborides has been measured at 25°C ini-butyronitrile and in mixtures of this solvent with benzene, carbon tetrachloride, dioxane, and tetrahydrofuran, covering the range of dielectric constants 10–23.81. The association constant for a given salt is the same in isodielectric mixtures ofi-butyronitrile with the nonpolar liquids; association is greater at a given dielectric constant for the mixtures with tetrahydrofuran. Single-ion mobilities are specific for any ion-solvent combination and therefore cannot be described in terms of the radius of an equivalent sphere and the bulk dielectric constant and viscosity of the solvent.     

Solvent effects on the dissociation of aliphatic carboxylic acids in water—N,N-dimethylformamide mixtures

The autoprotolysis constants of several water-_N,N-dimethylformamide mixtures were determined. The acid dissociation constants of formic, acetic, propionic, n-butyric, isobutyric, n-valeric, isovaleric, 2-methylbutyric and trimethylacetic acids were evaluated in the same media by potentiometric titration. Empirical equations for the calculation of autoprotolysis constants and acid dissociation constants at 25° C in the mixed solvents are proposed. Approximate solvation numbers were obtained by multiple linear regression analyses of pK_a values.     

Solvent effects on the dissociation of aliphatic carboxylic acids in water-N,N-dimethylformamide mixtures: Correlation between acidity constants and solvatochromic parameters

The acid dissociation constants of nine aliphatic carboxylic acids in several N,N-dimethylformamide-water mixtures were subjected to factor analysis (FA) and two solvent factors emerged. A further target factor analysis (TFA) indicated that the Kamlet and Taft general equation is reduced in these mixtures to two terms: the independent one and that related to the hydrogen bond acceptor (HBA) basicity solvatochromic parameter. Accordingly, an excellent correlation is found between the logarithmic values of the acidity constants of each acid in the binary mixtures and their corresponding values.     

Ionization constants for very weak organic acids in aqueous solution and apparent ionization constants for water in aqueous organic mixtures

A potentiometric method using a glass electrode has been applied to determination of apparent ionization constants for water in binary mixtures of water with tetrahydrofuran, methanol, 1,3-propanediol, glycerol, sucrose, and glucose at 25°C. Further calculations with these apparent ionization constants, which are now based explicitly on the previously implicit assumption that ionization of the organic component is negligible compared to ionization of water, permit evaluation of ionization constants for several very weak acids in purely aqueous solvent. Resulting pK values derived from this work and from our earlier work are as follows: glucose (12.38), sucrose (12.75 and 12.80), glycerol (14.05 and 14.07), ethylene glycol (14.44 and 14.52), methanol (15.2), 1,3-propanediol (14.8 and 15.0), 1-propanol (15.1), 2-propanol (15.7), and 2-methyl-2-propanol (15.0).     

Ion mobilities in DMF-water mixtures at 25°C

Densities and viscosities of mixtures of N,N-dimethylformamide (DMF) with water at 25°C have been determined. Limiting equivalent conductances of cesium chloride, potassium chloride, potassium bromide and potassium thiocyanate in these solvent mixtures at 25°C are presented together with corresponding values of ion association constants and distance of closest approach parameters. The transference number of the potassium ion has been determined in solvent mixtures ranging from 0 to 0.75 mol fraction in DMF in water at 25°C. The conductimetric Hittorf method has been used for both potassium bromide and potassium chloride in solutions of up to 0.496 mole fraction of DMF. For solutions of potassium thiocyanate in 0.5 and 0.75 mole fraction in DMF the cationic transference number has been determined using the moving boundary method. Stokes radii have been evaluated. Transport properties are examined in relation to-solvent properties such as composition, dielectric constant, excess volume of mixing and free volume.To whom correspondence should be addressed.     

强酸与多元酸混合物中各组分酸含量测定方法研究

各级离解常数均大于10~(-7),而且相邻两级的离解常数之比K_(a1)/K_(a2)只有几十倍的多元酸与强酸混合情况下,提出通过测量多元酸的半中和点,分别测定各组分酸含量的酸碱滴定法。     

Linear titration plots for the analysis of mixtures of three weak acids or bases

Linearly related functions have been derived which enable mixtures of three weak acids or bases in solution to be analysed by means of pH-titration data. This theory has been tested with data from the titration of a variety of mixtures of acids. The data required for the functions are pH, volume of titrant, the equilibrium constants relevant to the mixture, and an independently determined equivalence volume. This may be the equivalence volume for one of the components of the mixture or the sum of the equivalence volumes for any two or all three of the components. It is immaterial how this equivalence volume is obtained, but it is usually possible to obtain the total equivalence volume from data in another part of the same titration curve (when a large excess of titrant has been added).     

乙醇—水混合溶剂中次氮基三乙酸离解平衡的研究

次氮基三乙酸(NTA)在非水或混合溶剂中离解常数的测定尚未见报道,本文用电位法测定了乙醇-水混合溶剂中溶解度较小的NTA的离解常数,用标准盐酸溶液返滴定其钾盐可增加NTA浓度,提高测定准确度,测量系统防止CO_2装置和数据处理方法也比文献简单。     

Spectrophotometric determination of acidity constants of 4-(2′-thiazolylazo)-resorcinol (TAR) in water-organic mixtures

<正>The acidity constants of TAR in different water-organic solvent binary mixtures at 25℃have been determined spectrophotometrically. DATa ANalysis(DATAN) program was used for calculations of the acidity constants and pure spectra of all formed species and their concentration distribution diagram in applied pH interval.Results show that there are a fairly inverse relationship between acidity constants and the mole fraction of the organic solvent in the binary mixtures.Effects of solvent composition on the pK_as and pure spectrum of each component are also discussed.     

A new method for the pH calculation of solutions and mixtures of acids,bases and salts

The improved successive approximation method calculates the exact [H⁺] and pH value of solutions and mixtures of acids, bases and salts, and can be applied to titration data. The procedure consists in the calculation of the concentration of all species present in solution, selection of the appropriate rule of electroneutrality and introduction of suitable correction factors in order to obtain a final quadratic equation which is easily solved. Compared with existing methods of pH calculation, which are based on the Newton-Raphson iteration procedure, the proposed method requires a smaller number of iterations and a shorter calculation time. As there are no limitations on the number of compounds, nor on the number or values of acidity and basicity constants, nor on the concentration range, complex problems can be solved and titration curves plotted in a short time.     

Dielectric studies of binary liquid-crystalline mixtures

Six binary liquid-crystalline mixtures have been studied by means of dielectric, microscopic and refractometric measurements. Induced smectic phases appear for four of them. The effective values of the components p_⊥ and p_∥ of the dipole moment per one molecule of the mixtures have been calculated from values of e_∥ and e_⊥. It has been established that the heterocomplexes formed in the mixtures are weak so that they do not cause any changes in the distribution of molecular charges. The observed deviations from linearity of the dielectric constants of the mixtures are caused either by dissociation of the cyanomesogen dimers or by a non-additive behaviour of the mixture density.     

Process monitored spectrophotometric titration coupled with chemometrics for simultaneous determination of mixtures of weak acids

A new spectrophotometric titration method coupled with chemometrics for the simultaneous determination of mixtures of weak acids has been developed. In this method, the titrant is a mixture of sodium hydroxide and an acid-base indicator, and the indicator is used to monitor the titration process. In a process of titration, both the added volume of titrant and the solution acidity at each titration point can be obtained simultaneously from an absorption spectrum by least square algorithm, and then the concentration of each component in the mixture can be obtained from the titration curves by principal component regression. The method only needs the information of absorbance spectra to obtain the analytical results, and is free of volumetric measurements. The analyses are independent of titration end point and do not need the accurate values of dissociation constants of the indicator and the acids. The method has been applied to the simultaneous determination of the mixtures of benzoic acid and salicylic acid, and the mixtures of phenol, o-chlorophenol and p-chlorophenol with satisfactory results.     

Acid dissociation and spectrophotometric behaviour of 8-quinolineselenol in water-dioxan mixtures

The arid dissociation constants of 8-quinolineselenol, 8-quinolinol and 8-quinolinethiol in water-dioxan mixtures of varying solvent composition were determined potentiometrically or spectro-photometrically. The absorption spectra of 8-quinolineselenol in the same solvents were also obtained. The acid dissociation phenomena, stability towards oxidation and the absorption spectra of the reagents are discussed in connection with the existing forms. It is verified that 8-quinolineselenol exists as a zwitterion in water-dioxan mixtures of low or high dioxan content.