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.     

Simultaneous determination of halide and thiocyanate ions by potentiometric precipitation titration and multivariate calibration

Halide and thiocyanate ions can be determined by a precipitation titration with silver nitrate as the titrant, and the end-point can be evaluated by a potentiometric method, in which generally a silver indicator electrode is used as the indicator electrode and a double-junction Ag–AgCl electrode as the reference electrode. However, when mixtures of halide and thiocyanate are titrated, it is difficult to determine these components individually for there are overlapping steps in the potentiometric titration curves, especially in the case that there are obvious differences between concentrations of the components. In this paper, the linear equation for the potentiometric precipitation titration of a mixture of halide and thiocyanate ions was developed and it was then used for determining the components in the mixtures simultaneously with the aid of multivariate calibration methods. By application of this model, 27 synthetic mixtures with three- and four-component combinations of chloride, bromide, iodide and thiocyanate with low concentration levels from 1.8×10⁻⁴ to 6.2×10⁻⁴ mol l⁻¹ were analyzed and acceptable results were obtained.     

自动电位滴定法快速测定钾盐中的钾含量

钾含量的测量常用火焰光度法和四苯硼钾重量法。火焰光度法适用于微量分析 ,对较高含量钾的测定误差较大 ,四苯硼钾重量法的测定准确度高 ,但操作繁琐 ,测定周期长 ,易出错。本文采用四苯硼钠溶液电位滴定法 ,测定高、中含量组分钾 ,测定周期短 ,所用试剂少 ,且不受样品中水不溶物的干扰 ,选择性好 ,测定的准确度和精密度均在 0 .2 %以下 ,适用于钾盐、卤水、复合肥和钾肥中钾的测定。1　实验部分1 .1　仪器和药品ZD 2型自动电位滴定仪 (上海雷磁仪器厂 ) ,PK 1型钾离子选择电极 ,2 1 7型饱和甘汞电极 (双盐桥 )。四苯硼钠标准液 :0 .1mol…     

Development of a novel potentiometric titration method for determination of polypropylene degradation

In this study, a novel potentiometric titration of hydroperoxide in degraded polypropylene (PP) is proposed. This titration is quite sensitive compared with the conventional ones such as UV and manual titrations, and its detection limit was about 2 meq/kg. The sensitivity was equal to that of molecular weight measurement by GPC for the degraded PP and, in addition, the volatilization behavior of the hydroperoxide could be detected. This titration was found to be very effective for the determination of PP degradation.     

Development of quinhydrone electrode in chloroform. Application to the determination of equilibrium constants of acid-base reactions and potentiometric titration of some amine drugs

The optimum conditions for the functioning of quinhydrone electrode as acid indicator in chloroform was determined by voltammetric and potentiometric studies. This electrode was used for the determination of equilibrium constants of acid-base reactions and the potentiometric determination of the end point of acidimetric titrations of some amine drugs such as phenothiazinic derivatives in pharmaceutical preparations, after extraction into chloroform. The relative standard deviations of 20 mg of aliphatic amines, piperidine and piperazine substituted phenothiazines were 0.5, 0.7 and 1 % respectively.     

Simultaneous determination of ascrobic,citric, and tartaric acids by potentiometric titration with PLS calibration

The partial least squares (PLS) modeling method was used in the analysis of mixtures of ascorbic, citric, and tartaric acids by potentiometric titration. Binary mixtures of tartaric and citric acids, as well as ternary mixtures of tartaric, citric, and ascorbic acids, were titrated by sodium hydroxide pH-metrically. The linear relationship between the volumes of titrant and the concentrations of analytes was obtained by PLS regression. The designed model was then used to predict the concentrations of components in unknown samples. The practical utility of this method was demonstrated for the simultaneous determination of acids in binary and ternary mixture systems within concentration range from 4 × 10⁻⁴ to 2 × 10⁻³ M, and acceptable results were obtained. The text was submitted by the authors in English.     

Differential electrolytic potentiometric titration method for the determination of ciprofloxacin in drug formulations

A simple and rapid differential electrolytic potentiometric titration method for the determination of ciprofloxacin was developed. The work is based on the fast complexation reaction between iron(III) and ciprofloxacin in a ratio of 1:3, respectively, in sulfuric acid media of 0.09 mol dm⁻³. Among the electrodes tested silver amalgam electrodes were found to be a suitable indicating system. By applying a current density of 0.5 μA cm⁻² to these electrodes and using iron(III) solution of 0.097 mol dm⁻³ as a titrant, normal titration curves were obtained. The method was successfully applied for the determination of ciprofloxacin in drug formulations as low as 4.0 ppm.     

Polyacrylamide. I. Polymer content and hydrolysis level determination by potentiometric titration

A method of determining the degree of hydrolysis by potentiometric titration of polyacrylamide (PAA) with HCl and NaOH was developed. From the titration curves and the control of the degree of purity it is possible to calculate the level of hydrolysis and the quantity of pure acid contained in the PAA of a commercial product.     

The determination of the stability constants of mixed ligand complexes of adenine and amino acids with Ni(II) by potentiometric titration method

A potentiometric titration technique has been used to determine the stability constants for the various complexes of Ni(II) with adenine (A) as primary ligand and selected amino acids (L) as secondary ligands. Ternary complexes of amino acids are formed in a stepwise mechanism, whereby (A) binds to Ni(II), followed by interaction with ligand (L), whereas thiol-containing ligands form ternary complexes through a simultaneous mechanism. The formation constants of the complexes were determined at 25 °C and ionic strength 0.1 M NaNO₃. The relative stabilities of the ternary complexes are compared with those of the corresponding binary complexes in terms of Δlog K values. The concentration distribution of the complexes are evaluated.     

Multiparametric curve fitting-XIII Reliability of formation constants determined by analysis of potentiometric titration data

The formation (protonation) constants log K_i, of the acid H_jL are determined by regression analysis of potentiometric titration data when common parameters (log K_i, i = 1,…, j) and group parameters (E⁰′, L₀, H_T) are refined. The influence of three kinds of error on the protonation constants has been investigated: error from the strategy of minimization, random error, and error from uncertain estimates of group parameters. An analysis of variance of the log K_i, matrix was made for 7 identical titrations and 8 computational strategies, or of 7 identical titrations and 8 different options of group parameters to be refined. The influence of the standard potential E⁰ of the glass-electrode cell on the systematic error in log K is greater than that of the acid concentration (L₀) or the concentration of titrant used (H_T). The ill-conditioned group parameters should be refined together with the common parameters (K_i), otherwise the estimates of log K_i, are not accurate enough. Two ways of calibrating the glass electrode cell were compared. Internal calibration (performed during titration) was more accurate than external calibration done separately. Of the programs tested ESAB and ACBA are the most powerful because they permit refinement of group parameters and internal calibration. Citric acid was chosen as model substance.     

Simultaneous determination of lead and thallium by potentiometric stripping

A potentiometric stripping analysis procedure has been developed for the simultaneous determination of lead and thallium in chloroacetate-chloride medium. This procedure allows the determination of lead or thallium concentrations as low as 10 ng ml . The method is precise, and applicable to the determination of lead and thallium in sea-water.     

On the liquid junction potential for the determination of equilibrium constants by means of the potentiometric technique without constant ionic strength

A modified method for the calculation of liquid junction potentials based on the Henderson equation is proposed. The estimation of the activity coefficients and the conductivities is performed by means of the Modified Bromley Methodology [G. Borge, R. Castaño, M.P. Carril, M.S. Corbillón, J.M. Madariaga, Fluid Phase Equilibria 121 (1996) 85–98;G. Borge, N. Etxebarria, L.A. Fernández, M.A. Olazabal, J.M. Madariaga, Fluid Phase Equilibria 121 (1996) 99–109.] and the Extended Falkenhagen Equation [A. De Diego, Conductivity of concentrated electrolytic solutions: study of the dependence with concentration and temperature, Ph.D. Thesis, University of the Basque Country, Bilbao, 1996.]. The efficiency of the method has been tested in potentiometric titrations without constant ionic strength. Its applicability to potentiometric titrations with constant ionic strength is also discussed.     

Theoretical aspects of the indirect redox potentiometric method for determination of oxidizers and reducers without titration

The theoretical analysis of the redox potentiometry of reducers and oxidizers is carried out, which involves the electrochemical characteristics of generalized analytical reactions and the development of simplified equations for calculating the sought concentration and the sensitivity of the method. The use of degrees of oxidation of atoms made it possible to elucidate the reaction stoichiometry and determine which atoms in complex compounds are involved in the electron transfer. The terminology of the redox potentiometric method is improved.     

Projection maps for the determination of equilibrium constants

Projection maps are suggested for the construction of model functions to be compared with potentiometric measurements of the free ligand concentration. The theoretical curves are found to be a valuable guide in the choice of the optimal conditions for the evaluation of complex formation equilibria. As an example, measurements of [H⁺], with a quinhydrone electrode, have been planned with the help of model maps for the study of the protolysis of oxalic acid in 1 M(KCl) at 25°C.Projection maps are also proposed to interpret data obtained by the method of competitive reactions.     

A new method for the determination of thermodynamic equilibrium constants in separation processes

A new approach to the determination of thermodynamic equilibrium constants in ion-exchange and solvent extraction processes is described. A plot of the logarithm of the distributing species activity in the aqueous phase as a function of its concentration in the non-aqueous phase has an inflection point. The ordinate of the inflection point gives directly the logarithm of the thermodynamic equilibrium constant of the reaction considered. This was made possible by finding that the ratio between n power of the free extractant molecule (or free site) activity and the activity of the distributing species-extractant complex is unity as this point.     

Simultaneous determination of cationic surfactants and nonionic surfactants by ion-association titration.

A simultaneous determination of cationic and nonionic surfactants has been developed using ion-association titration. Tetrabromophenolphthalein ethyl ester (TBPE) was used as an indicator. Benzalkonium reacted with TBPE to form a blue ion-associate in the organic phase. When tetrakis(4-fluorophenyl)borate was added dropwise to the solution, the color of the organic phase turned to yellow at the equivalence point. In addition, when a large amount of potassium ion was added to a solution including Triton X-100, Triton X-100 could be determined by the same technique as described above because of formation of the K+-Triton X-100 cation. The proposed method is available for the stepwise determination of cationic and nonionic surfactants in mixtures.     

用氟钽酸根指示电位滴定钽的新方法

A new potentiometric titration method for the determination of tantalum in sulfuric acid and hydrofluoric acid media is described. EMF's were monitored with a double-junction reference electrode and a flurortantalate ion selective indicator electrode made in this laboratory previously. Five titrants have been evaluated for the precipitation titration. CPC, BDTAC and TPA yield the highest precision and largest potentiometric breaks, then rank CETAB and Hyamine 1622 in turn. CPC is considered to be the most satisfactory owing to the most sharp end-point break yielded when 10^-^4M of Ta is titrated with it. The concentration of H2SO4 suitable for the titration is 0.18 to 1.8M and that of HF is 0.23 to 1.7M. The method is suitable for Ta content in the range of 0.5 to 150 mg with a coefficient of variation of 0.15% (9 results) for 20 mg of Ta. HCl, H3PO4, tartaric acid and (NH4)2C2O4 as wellas a large amount of Fe^3^+, Ni^2^+, Cr^3^+, WO2^-^4, Zr^4^+, La^3^+, Al^3^+, MoO2^-^4 and Nb^5^+ do not interfere, while ClO^-^4, H2O2 and BF^-^4 must be avoided. The method has been found to be very useful for the determination of Ta in steels and crystalline lithium tantalate. The results obtained are in good agreement with those of comparable methods. This method is very simple and provides a fast determination process.     

电位溶出分析法同时测定锡, 铅, 铜

本文应用电位溶出分析法进行同时测定锡、铅和铜的研究。结果发现在草酸溶液中锡、铅和铜有分离清晰的平台,其溶出讯号与浓度呈线性关系,可用于定量分析。用这一方法测定了某些罐头食品中痕量锡、铅和铜,获得满意的结果。     

Simultaneous potentiometric and fluorimetric determination of diclofenac in a sequential injection analysis system

Two independent methods for the determination of diclofenac were simultaneously implemented in an automated analytical system, based on the concept of sequential injection analysis, providing real-time assessment of results quality. The potentiometric detection was carried out with an ion-selective electrode based on a cyclodextrin while for the fluorimetric determination the sample was previously subject to in-line irradiation with UV light. The potentiometric and photochemical-fluorimetric determinations exhibited linear working ranges of 5×10⁻⁶ to 1×10⁻² and 1×10⁻⁶ to 1×10⁻⁴ mol dm⁻³, respectively. Relative standard errors of 0.5% for the potentiometric determination and 0.6% for the photochemical-fluorimetric determination were obtained after 10 consecutive injections of a 5×10⁻⁵ mol dm⁻³ diclofenac standard solution. The sampling rate was about 32 samples h⁻¹. Both methods were applied in the analysis of pharmaceutical formulations. The quality of results obtained was evaluated by comparison to the reference method, with no statistically significant differences for a 95% confidence level.     

Spectrophotometric determination of equilibrium constants of two mutual competitive reactions by the method of isosbestic points

We have developed a method that makes use of dual isosbestic points in a dual ligand, single metal system that allows the determination of the equilibrium constants of complexes in which two ligands compete for the same metal ion, and one complex is colourless. The competition of methyl thymol blue and citrate was used to test the model.