Equilibrium constants and assay of bases in acetonitrile

The acid-base equilibria of a series of bases of pharmaceutical interest have been studied in acetonitrile medium and used to investigate the possibility of the use of this solvent to determine basic substances with complex and diverse structure. The pK(HB+) values of a series of cardiovascular compounds have been determined. From these values, simple potentiometric and visual titrations in acetonitrile media are proposed for the assay of bases in pharmaceutical formulations.     

Potentiometric determination of acids and bases using a silica gel based carbon-epoxy indicator electrode

The construction and the application of a silica gel based carbon-epoxy indicator electrode for the potentiometric determination of acids and bases are described. The effect of composition of silica gel and carbon-epoxy, slope (mV/pH), linear response (pH range) and the use for acid-base titrations were investigated. The data obtained for the acid-base titrations were compared with those obtained using a glass electrode in the same conditions. The electrode showed a linear response in the pH 2 to 13 range with a slope of -40.5 +/- 0.4 mV/pH (at 25 degrees C) and a response time of less than 15 s. The lifetime of the electrode was higher than one year (over 6000 determinations) with a decrease of only 5% of the initial potentiometric response. The silica gel based carbon-epoxy electrode showed excellent results in the end-point indication potentiometric titrations in determination of acids and bases. The miniaturization of the proposed electrode for flow injection analysis was investigated.     

A non-linear least-squares approach to the refinement of all parameters involved in acid-base titrations

A non-linear least-squares computer program has been written for the refinement of the parameters involved in potentiometric acid-base titrations. The program ACBA (ACid-BAse titrations) is applicable under quite general conditions to solutions containing one or more acids or bases. The method of refinement used gives the program several advantages over the other programs described previously.     

Potentiometric determination of acids and bases using a silica gel based carbon-epoxy indicator electrode

The construction and the application of a silica gel based carbon-epoxy indicator electrode for the potentiometric determination of acids and bases are described. The effect of composition of silica gel and carbon-epoxy, slope (mV/pH), linear response (pH range) and the use for acid-base titrations were investigated. The data obtained for the acid-base titrations were compared with those obtained using a glass electrode in the same conditions. The electrode showed a linear response in the pH 2 to 13 range with a slope of –40.5 ± 0.4 mV/pH (at 25 °C) and a response time of less than 15 s. The lifetime of the electrode was higher than one year (over 6000 determinations) with a decrease of only 5% of the initial potentiometric response. The silica gel based carbon-epoxy electrode showed excellent results in the end-point indication potentiometric titrations in determination of acids and bases. The miniaturization of the proposed electrode for flow injection analysis was investigated. Received: 11 August 1999 / Revised: 4 October 1999 / Accepted: 7 October 1999     

Linear titration plots for polyfunctional weak acids and bases

Procedures are derived for obtaining the equivalence volumes in the potentiometric titrations of polyfunctional weak acids and weak bases by a linear titration plot method. The effect of errors in the equilibrium constants on the accuracy is considered. A Fortran program is available to do the calculations.     

交流示波极谱滴定的研究(ⅩⅢ)——有机碱及有机酸盐类药物在水溶液中的中和滴定

本文用交流示波极谱中和滴定技术,在水溶液中直接滴定有机碱和有机酸的盐类药物,简便、快速、准确。     

Acid-base strengths in 1,2-dichloroethane

The pK_a value of hydriodic acid in 1,2-dichloroethane was determined from conductivity measurements. A glass electrode was calibrated for dichloroethane in the potentiometric titration of hydriodic acid with tetramethylguanidine. From potentiometric titrations, the pK_a values in dichloroethane of hydrobromic acid, hydrochloric acid, picric acid and some sulfonphthaleins as well as some protonated nitrogen bases were determined. In the curves of the titrations of the carboxylic acids and the hydrogen halides with TMG, evidence was found for the formation of the complex B(HX)₂.     

聚苯胺修饰电极酸碱示波电位滴定法——Ⅰ.水溶液中的酸碱滴定

示波电位滴定具有终点直观、仪器便宜、操作简便、快速、准确等特点。本文将对氢离子响应敏锐、使用寿命长、制备简单的聚苯胺修饰电极(PAME)^[4,5]应用于酸碱示波电位滴定中,使电位突跃更加明显,化学计量点更易判断(与双铂电极体系比较),可滴定多元弱酸、弱碱、混合酸和混合碱.与pH玻璃电极比较,具有阻抗低,响应灵敏、迅速,终点电位突跃大,不易损坏,不需预处理,制备简单等特点.     

Thermometric and other titrations of sparingly soluble compounds in aqueous micellar media

Several organic acids and bases that are very sparingly soluble in water have been determined by acid-base and redox titrimetry after solubilizing them in micellar suspensions of sodium dodecylsulfate or cetyltrimethylammonium bromide. Thermometric titrations are much more rapid, convenient, accurate, and precise than the corresponding potentiometric ones. Titrations in such media can effect some differentiations that cannot be made in water and are also much more economical than titrations in non-aqueous solvents.     

In Process Citation

A first attempt to apply inorganic membranes made from parchment paper impregnated with alkaline earth precipitates, as indicating electrodes for potentiometric acid-base titrations in nonaqueous media, is presented. Mixtures of acids can be titrated and the course of the titration is a function of the nature of the solvent and the titrant.     

Potentiometric titrations in methanol/water medium: Intertitration variability

Homogeneous sets of data from strong acid-strong base potentiometric titrations in a mixed solvent medium (9:1 v/v methanol/water), performed by means of the glass electrode, at various constant ionic strengths and with different reference electrodes, have been analysed by statistical criteria. A standardized procedure has been established to obtain reliable potentiometric data in mixed solvents. It has been demonstrated how, with the aid of a proper linearized model, analysis of variance (ANOVA) applied to the standardization titrations can be used to test the reliability of a potentiometric chain in a medium other than pure water. The error expected in the stability constants thus evaluated is related to the intertitration error of the operational pK*(')(w), for the same medium and the same chain. The results obtained by applying ANOVA to the mixed solvent data substantially confirm the trend found for aqueous media, the intertitration error being larger than the intratitration error for all the parameters investigated (E(')(0), pK*(')(w), mean equivalence volume). The stochastic error thus obtained depends on the ionic medium used and on the kind of reference electrode employed in the electrochemical chain.     

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.     

Potentiometric Studies of Copper(II) Complexes of Some Substituted Benzylidene-2-Hydroxyanilines in Dioxane–Water Media

The formation and protonation constants of 17 Schiff bases-derived 2-hydroxyaniline with some substituted benzaldehydes, and the stability constants of Cu(II) complexes of these Schiff bases, have been determined potentiometrically in 20, 40, and 60% dioxane–water media. The data from the potentiometric titrations were evaluated with the BEST computer program. For all Schiff bases studied, it was observed that the log K_OH values related to the protonation equilibria of the phenolic oxygen are increased, and the log K_NH values related to the protonation equilibria of the azomethine nitrogen are decreased, as the dioxane content is increased. The variation of these constants is discussed on the basis of specific solute–solvent interactions and structural changes of Schiff bases from water to the dioxane–water media. Also, titrimetric-pH investigation of substituted benzilidene-2-hydroxyaniline systems has revealed the formation of stable mono-Schiff base complexes with the metal ion Cu(II).     

Determination of organic bases in non-aqueous solvents by catalytic thermometric titration

Catalytic thermometric titrations have been developed for bases (brucine, diethylaniline, potassium acetate and triethylamine) in acetic acid by continuous and discontinuous addition of the standard solution and automatic temperature recording. The determination of weak bases, e.g., antipyrine, unsuccessful in acetic acid by catalytic thermometric titration, has been achieved by using nitromethane or acetic anhydride as solvent. Catalytic thermometric titrations were also performed by coulometric generation of hydrogen ions for the determination of micro amounts of weak bases in a mixture of acetic anhyride and acetic acid.     

Bis(trifluoroacetoxy)iodobenzene,a new oxidant in potentiometric titrations

A new oxidimetric titrant, bis(trifluoroacetoxy)iodobenzene is proposed for potentiometric titrations in acetonitrile and water-acetonitrile media. The general analytical conditions for using this titrant and the results for the potentiometric titrations of various inorganic and organic reductants are described.     

交流示波极谱滴定的研究(Ⅻ)——中和滴定

本文系统地报导了交流示波极谱滴定在中和滴定中的应用,解决了弱酸弱碱在水溶液中直接滴定等问题。同时提出了中和滴定的两大类指示剂的概念,并且总结了它们的性质。因此,示波极谱中和滴定将大大地丰富中和滴定的内容,扩大其应用范围。     

COMPLEXATION OF ALKALINE EARTH CATIONS BY NONCYCLIC POLYETHERS,CROWN ETHERS,AND CRYPTANDS IN ACETONE

Abstract

The complexation of akaline earth cations by non-cyclic polyethers, crown ethers and cryptands was studied in acetone by means of potentiometric and calorimetric titrations. The results show that the stabilities of the complexes decrease in the order: cryptands, crown ethers and noncyclic polyethers. Interactions between the different ligands and solvent molecules influence the stability of the complexes formed when compared with methanol as solvent.     

The complexation of alkaline cations by crown ethers and cryptands in acetone

Stability constants and thermodynamic values for the complex formation of alkali ions by crown ethers, diaza crown ethers and cryptands have been measured by means of potentiometric and calorimetric titrations in acetone as solvent. The interactions between the ligands and solvent molecules play an important role for the complex formation. Cryptands form the most stable complexes with alkali ions if inclusion complexes are formed. Even in the case that the salts are not completely dissociated in acetone the presence of ion pairs does not influence the calculated values of the stability constants.     

A Simple Sensor for Potentiometric Titrations

Abstract

A sensor for potentiometric titrations was prepared by coating a spectroscopic graphite rod with a solution of poly(vinyl chloride) and dioctylphthalate in tetrahydrofuran. The reference was a Ag/AgCl single-junction electrode. The sensor was used in the following potentiometric titrations:

1. precipitation titrations 2. acid-base titrations, 3. compleximetric titrations, and 4. redox titrations.

A survey of its use in such titrations is presented.

Preparation of the coated-graphite sensor is simple and rapid. Moreover, it is quite inexpensive. A limitation is its applicability in aqueous media only, because organic solvents will dissolve the membrane.     

An Apparatus for Titrations in Liquid Ammonia

An apparatus for titrations in liquid ammonia is described. The titrations can be carried out in the absence of air, and the utility of the apparatus is demonstrated for a number of conductometric and potentiometric titrations.