Algorithm for Calculating the Dissociation Constants of Ampholytes in Nonbuffer Systems

An algorithm for calculating the dissociation constants of ampholytes in aqueous solutions is developed on the basis of spectrophotometric data in the UV and visible ranges without pH measurements of a medium and without buffer solutions. The proposed algorithm has been experimentally tested for five ampholytes of different strengths. The relative error of measuring dissociation constants is less than 5%.     

Ionic equilibria in aqueous organic solvent mixtures the dissociation constants of acids and salts in tetrahydrofuran/water mixtures

The dissociation constants of several acids (perchloric, hydrochloric, phosphoric, acetic and benzoic acids) and of some sodium salts (chloride, acetate and benzoate) have been conductometrically determined in tetrahydrofuran/water mixtures up to a 90% of tetrahydrofuran in volume. The results demonstrate that conductometry can be successfully applied to determine the dissociation constants of salts and moderately weak and strong acids in the studied mixtures. The dissociation constants of the acids and some bases taken from the literature have been fitted to solvent composition through a previously derived equation, which is based on a preferential solvation model. The fitting parameters obtained allow calculation of the dissociation constant for any solvent composition inside the applicability solvent composition range. From the pK value, the pH of any buffered solution, such as those used in liquid chromatography, can be calculated for the particular tetrahydrofuran/ water composition of interest. Appreciable ion-pairing for sodium salts and strong acids has been observed for tetrahydrofuran contents higher than 60% in volume. Therefore, the accurate calculation of the pH values of buffers in tetrahydrofuran-rich solutions must take into account the pK values of the acid and salt.     

Dissociation constants of protolytic dissociation of glutamyl-glutamic and glycyl-glutamic acids in aqueous solution at 298 K

Protolytic dissociation constants of stepwise dissociation of glutamyl-glutamic and glycyl-glutamic acids in aqueous solutions at 298.15 K at ionic strengths of 0.1, 0.5, and 1.0 M on the NaCl background were measured by potentiometric titration. The thermodynamic dissociation constants were calculated. Using the obtained and published data, we discuss the effect of additional carboxy groups in the side chains of the molecule on the protolytic dissociation constants of peptides.     

General equation for determining the dissociation constants of polyprotic acids and bases from additive properties. Part I. Theory

Measured additive properties of polyprotic acids and bases as functions of the pH of the solution can be used for the determination of the dissociation constants. A rigorous general equation is derived in this paper for this purpose. The equation relates the additive property of the acid or base to the dissociation constants, the limiting values and activity coefficients of the solute species, and to the pH of the solution. The equation is used to obtain the dissociation constants by fitting it to the experimental data in a nonlinear least-square fitting procedure. Overlapping dissociation constants pose no problem in this method. Applications to spectrophotometry and reversed-phase high performance liquid chromatography are presented in a following paper (Part II of this series).     

Determination of dissociation constants of weak acids by deconvolution of proton binding isotherms derived from potentiometric data

Potentiometric titration of six carboxylic acid analytes were measured in aqueous and semiaqueous solvents to determine the effect of the solvent composition on the dissociation constants of the acids. The analytes studied were monoprotic (formic acid, acetic acid), diprotic (maleic and succinic acid) and triprotic (1, 2, ranging in composition from 0 to 80% by volume dioxane. The methodology used to assess the acidity constants was deconvolution of the proton binding isotherm of each analyte in each of the solvents. The dissociation constants agreed with those reported in the literature for titration in water. In the presence of the organic component, correlations relating the acidity constants with properties of the media were also found. In particular, affinity distributions derived from potentiometric titration data were used to assess the solvent media for resolving dissociation constants of weak acids and bases.     

Re-evaluated data of dissociation constants of alendronic, pamidronic and olpadronic acids

The dissociation constants of (4-amino-1-hydroxybutylidene)bisphosphonic (alendronic) acid, (3-(dimethylamino)-1-hydroxypropylidene)bisphosphonic (olpadronic) acid and (3-amino-1-hydroxypropylidene)bisphosphonic (pamidronic) acid were obtained in aqueous solutions (0.10 M KCl) and micellar solutions of cetylpyridinium chloride (0.10 M CPC) at 25.0°C. With the exception of the third dissociation constant of alendronic acid, the dissociation constants of alendronic, olpadronic and pamidronic acids in aqueous solutions matched literature data. The possibility of sodium alendronate determination by acid-base titration by NaOH solution was theoretically grounded on the basis of re-evaluated data of alendronic acid dissociation constants.      

General equation for determining the dissociation constants of polyprotic acids and bases from additive properties. Part II. Applications

Dissociation constants of some polyprotic acids and bases are determined from spectrophotometric and reversed-phase high performance liquid chromatographic data. A general equation relating additive properties of acids and bases to the pH of the solution is used for this purpose. The method is tested by applying it to the calculation of overlapping pK_as without the prerequisite of measuring the limiting values of the property for the individual species that result from the dissociation of the solute. The possibility of applying the same method to hyperpolarizability measurements is pointed out, and a procedure, based on the general equation, for obtaining the activity coefficients of the ionic species as a function of the ionic strength is also suggested.     

Simultaneous determination of equivalence volumes and acid dissociation constants from potentiometric titration data

New iterative methods for analysis of potentiometric titration data of (a) mixtures of weak monoprotic acids with their conjugate bases, (b) solutions of polyprotic (di- and triprotic) acids, and (c) mixtures of two diprotic acids are presented. These methods, using data exclusively resulting from the acidic region of the titration curve permits the accurate determination of the analytical concentration of one or more acids even if the titration is stopped well before the end point of the titration. For the titration of a solution containing a conjugate acid/base pair, the proposed procedure enables the extraction of the initial composition of the mixture, as well as the dissociation constant of the concerned acid. Thus, it is possible by this type of analysis to distinguish whether a weak acid has been contaminated by a strong base and define the extent of the contamination. On the other hand, for the titration of polyprotic acids, the proposed approach enables the extraction of the accurate values of the equivalence volume and the dissociation constants K(i) even when the ionization stages overlap. Finally, for the titration of a mixture of two diprotic acids the proposed procedure enables the determination of the composition of the mixture even if the sum of the concentrations of the acids is not known. This method can be used in the analysis of solutions containing two diastereoisomeric forms of a weak diprotic acid. The test of the proposed procedures by means of ideal and Monte Carlo simulated data revealed that these methods are fairly applicable even when the titration data are considerably obscured by 'noise' or contain an important systematic error. The proposed procedures were also successfully applied to experimental titration data.     

Aqueous Equilibria: Acids, Bases, and Solubility. A General Chemistry Laboratory Experiment

We refer to two major types of general chemistry freshman laboratory experiments: discovery and cookbook. The typical Freshman enrolled in general chemistry has the skills to perform an experiment that is somewhere between cookbook and discovery, and we have developed a laboratory experiment that is midway between these extremes. This experiment aids the students understanding of the concept of aqueous equilibria. In this experiment, students are asked to prepare and measure the pH of a variety of acidic and basic solutions. Students are also asked to calculate pH values from literature dissociation constants and dissociation constants from their experimental pH values. This experiment demonstrates the relationships between dissociation constants, percent ionization, pH and strength of acids and bases, as well as molar solubility and the solubility product constant, K_sp. A Mathcad-based prelaboratory activity provides a review of the concepts and engages students in what-if explorations.     

Determination of pK(a) values of N-heterocyclic bases by fluorescence spectrophotometry

Fluorimetry is a relatively fast and accurate means of determining the dissociation constants of sparingly soluble heterocyclic bases. Complications can arise, however, from the dependence of fluorescence on excited-state as well as ground-state acid-base chemistry. Several approaches to circumventing or compensating for this difficulty are discussed. To demonstrate the utility of the methods, the pK(a), values of the conjugate acids of two bases are evaluated by the methods described.     

A computational method for approximate evaluation of ionization constants from potentiometric data

A general equation is given for the evaluation of proton dissociation constants from potentiometric data. The method is applicable to monobasic acids, and also polybasic acids, where the successive dissociation steps may or may not overlap. A computer program was employed for the calculations. The method was tested by evaluation of the constants for 22 acids, from monobasic to octobasic, with satisfactory agreement with literature data.     

Luminol as a fluorescent acid-base indicator

The acid and base dissociation constants of luminol are determined at various ionic strengths. The transition interval occurs at pH 7.7-9.0, therefore luminol is a fluorescent indicator for the titration of strong and weak acids and strong bases. Its value as an indicator is established by titrating milk, red wine and cherry juice.     

Volume Changes in the Course of Neutralization of Nitrilotris(methanephosphonic) Acid with Aqueous Ammonia

Volume changes in the course of neutralization of solutions of nitrilotris(methanephosphonic), hydrochloric, and nitric acids with aqueous ammonia solution were measured dilatometrically at 298 K. The changes in the molar volumes were calculated for various stages of the neutralization. The data were com- pared to the results of titration of nitrilotris(methanephosphonic) acid with strong hydroxyl-containing bases. The specific features of reactions of ammonia with acids and the state of nitrilotris(methanephosphonic) acid in various stages of its dissociation were taken into account in data interpretation.     

Für eine korrekte darstellung der ph-lehre

According to the classic theory weak acids and bases dissociate in water whilst their salts are hydrolysed by water; in the first process the water is supposed to perform a merely physical action as its high dielectrical constant diminishes the electrostatic attraction between the ions; in the second process the water is said to participate chemically.We know nowadays these conceptions to be obsolete: there is always a reaction with the water. In these reactions the molecules of acids and the cations of bases are hydrolysed in the same manner, whilst the molecules of bases and the anions of acids react analogously to each other but differently from the first process. The analogies as formulated up to now are incorrect and different ones have to supersede them. We shall not establish dissociation constants of a different kind for acids and bases, but identical equilibrium constants for both of them.By this development the calculation of pH-values is greatly simplified, as more than half of the actual formulae disappear and the meaning and interpretation of the remaining formulae are self-explanatory. The transition from aqueous to non-aqueous solutions is easy. Important analogies to the redox systems are established.     

Conthemporary methods for the experimental determination of dissociation constants of organic acids in solutions

The survey is dedicated to the critical analysis of different experimental methods of determining the dissociation constants (K_a) of organic acids in solutions. Advantages and disadvantages of the contemporary methods of rapid analysis used in biochemistry are considered. Special attention is paid to the problems of measurements in the nonaqueous media and to the reasons for possible incorrect results. The processes in solutions that should be taken into account for obtaining the correct values of dissociation constant are analyzed. On the basis of the consideration the recommendations directed toward an increase in the accuracy of measurements are given.     

Comparison of a single-use pH test strip and a glass electrode for potentiometric titration

The potentiometric titration of a carbonate mixture or an acetate solution is a common experiment in analytical laboratories. Typically, a glass electrode combined with a calomel or Ag/AgCl reference electrode is used to locate the equivalence points in neutralization titrations. The dissociation constants of weak acids and bases can be calculated from the pH at the half-neutralization point. Recently, a new commercial product for measuring pH has been developed. This novel acid–base detection strip is a single-use sensor that requires neither storage in a preservation liquid nor calibration prior to use. This study examined its suitability for the continuous monitoring of pH changes in potentiometric titrations of carbonate mixtures, acetate solutions, or ammonia solutions. There were no significant differences in the concentrations of solutions tested using a glass electrode and a pH test strip. The pK_a, pK_b, and pH values determined using the two systems differed by less than 5%. The results confirmed that the pH strips are suitable for continuously monitoring pH changes during neutralization titrations. However, the strips can only be used once.     

Determination of dissociation constants of amino acids by capillary zone electrophoresis.

A method is proposed for the determination of dissociation constants of amino acids by capillary zone electrophoresis. According to the dissociation equilibrium of amino acids and the basic theory of electrophoresis, the nonlinear relationship between the pH value of the buffer and the effective electrophoretic mobilities of the analyte was obtained. The dissociation constants can be calculated from the pH values and the corresponding effective electrophoretic mobilities using the program written in C++. The dissociation constants, pKa1 and pKa2, of 11 kinds of amino acids were determined successfully by the proposed method. The determined dissociation constants were compared with values in the literature; the differences between them are in the range of -0.03 to 0.06. No significant differences were observed between the determined dissociation constants and the corresponding literature values.     

Interference of elements upon extraction by oxygen-containing extractants: Calculating the distribution coefficients

The extraction of tellurium (IV), tin (IV), and indium (III) by aliphatic alcohols and ketones from hydrochloric acid solutions was studied both upon the individual separation of elements and upon their simultaneous extraction. Electroconductivities of the extracts were measured and, using these data, the dissociation constants of the extracted metal-halogen acids in organic solvents were calculated by Fuoss-Krauss method. A method for calculating the distribution coefficients of elements upon their simultaneous extraction by hydrate-solvate and mixed solvate and hydrate-solvate mechanisms is proposed. It is shown that the calculated distribution coefficients are in good agreement with the experimental data.     

Calculation of stoichiometric dissociation constants of monoprotic carboxylic acids in dilute aqueous sodium or potassium chloride solutions and p[m(H(+))] values for acetate and formate buffers at 25 degrees C

Equations are given for calculation of the stoichiometric (molality scale) dissociation constants, K(m), of weak acids in dilute aqueous electrolyte solutions at 298.15 K from the thermodynamic dissociation constant, K(a), of the acid and the ionic strength, I(m), of the solution. The equations for K(m) were based on the single-ion activity coefficient equations of the Hückel type. The equations were tested with the conductivity data for formic, acetic, propionic, n-butyric, lactic, chloroacetic, alpha-crotonic and cyanoacetic acids, and with data measured by Harned cells for formic, acetic, propionic, n-butyric and glycolic acids. These data were taken from the literature. According to these tests, K(m) can be obtained by the Hückel method within experimental error at least up to I(m) of about 0.1 mol kg(-1). On the basis of the equations for K(m), it is suggested p(m(H)) values {p(m(H))=-lg[m(H(+))/(mol kg(-1))] where m refers to the molality} for buffer solutions containing acetic or formic acid. A new calibration method is suggested for glass electrode cells, and this method is based on the p(m(H)) values instead of pH values (pH=-lg[a(H(+))] where a refers to the activity).     

质子酸的酸性强弱比较

酸碱质子理论是大学无机化学教学中最重要的概念之一,溶液中的酸碱强弱的比较虽然可以通过查找酸碱解离常数来得到,但在数据缺失情况下的定性比较酸碱强弱在教学中也很重要,而且这种比较可以帮助学生理解物质结构与性质之间的相关性。本文根据质子酸碱的化学组成和分子结构分析它们的酸碱性,并主要根据分子结构信息,包括键的极性、键的强度、键的离域、氢键、分子中键的环境等,推导、比较它们酸碱性的相对强弱。