Miniquad-A general computer programme for the computation of formation constants from potentiometric data

A new computer programme for the computation of formation constants of complex species in solution is presented. The programme can be applied to all kinds of potentiometric titration data, including multi-reactant and multi-electrode systems. A statistical analysis is made of the results in order to assess their validity, and to assist in hypothesis testing.     

Miniglass,an interactive program for the evaluation of stability constants of metal/ligand complexes from potentiometric data

MINIGLASS is a microcomputer program for the treatment of pH titrations for evaluation of stability constants and adjustment of the parameters defining the titration curve. The program is written in PASCAL for the HP-200 computer series, and contains procedures for blank titrations, acidity constants and metal/ligand stability constants. Graphics support is provided for easy interaction with the user. The data files can be written by the editor of the operating system or can be created automatically from an automatic titration system. The program is tested for the determination of the formation constants of the well-studied Ni²⁺/glycine system. The results obtained agree well with literature values.     

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.     

Spectrophotometric evaluation of overlapping ionization constants of bisbiacetylmonoxime-o-phenylenediimine

The acid-base properties of bisbiacetylmonoxime-o-phenylenediimine were studied spectrophotometrically. The ionization constants of the acid-base equilibria involved were calculated from the A = f(pH) dependence for the aqueous ethanolic solutions (2% ethanol, I = 0.1, room temperature). The pK_a values were evaluated by means of a variety of methods. Results obtained were compared with values reported in the literature for other dioximes derivatives.     

A new method to evaluate force constants from experimental spectral data

An iterative procedure is proposed to facilitate the determination of molecular vi-brational force constants from the experimental fundamental frequencies. Proper restrictions are introduced to the force constants based on physical considerations for getting reasonable results. The experimental data of Coriolis coupling coefficients and isotopic frequency shifts are utilized to reduce the uncertainty of the calculated force constants when they are available. A series of various kinds of molecules have been calculated by this method and the results are satisfactory.     

A desk-computer program for calculation of the parameters of acid-base titration curves and protonation or metal-complex stability constants from potentiometric data

The program MINIPOT is designed to run on a desk-computer with 16-kbyte of memory. It can calculate the optimum values of the parameters of a blank acid-base titration in any solvent, and the protonation/deprotonation or overall stability constants of a maximum of four species with compositions H(q)L(p) or M(q)L(p) from potential, titrant volume and analytical concentration data. The program, written in BASIC, is based on the least-squares principle and is capable of simultaneous refinement of a maximum of four parameters.     

Numerical evaluation of complex stability constants from polarographic data for quasi-reversible processes

Stability constants of copper(II) butyrate complexes were determined by polarography. Numerical treatment of polarographic data for quasi-reversible electrode processes was developed and yielded good estimates of reversible E_{$\text{1}\text{2}$} values. An improved three-electrode polarographic apparatus was constructed based on operational amplifiers.     

A combined SAMO-ZDO method for the evaluation of ionization potentials

A scheme for the ab initio calculation of vertical ionization potentials without the necessity to compute two-electron repulsion integrals is discussed. The method employs the simulated ab initio molecular orbital (SAMO) method to generate Koopman's theorem eigenvalues. These are then corrected for the change in relaxation and correlation effects due to ionization by a Green's function perturbation scheme, in which all necessary integrals are evaluated using the zero differential overlap (ZDO) approximations, in this case the complete neglect of differential overlap (CNDO) method.     

A general method to determine ionization constants of responsive polymer thin films

A general method has been developed to determine the ionization constants of polymer thin films based on the stimuli-responsiveness of the polymer. Robust polymer films were fabricated on silicon wafers and gold slides using perfluorophenyl azide (PFPA) as the coupling agent. The ionization constants were measured by a number of techniques including ellipsometry, dynamic contact angle goniometry, and surface plasmon resonance imaging (SPRi). Using poly(4-vinylpyridine) (P4VP) as the model system, P4VP thin films were fabricated and the ionization constants of the films were measured taking advantage of the pH responsive property of the polymer. The pK(a) determined by ellipsometry, ~4.0, reflects the swelling of the polymer film in response to pH. The pK(a) value calculated from the dynamic contact angle measurements, ~5.0, relies on the change in hydrophilicity/hydrophobicity of the films as the polymer undergoes protonation/deprotonation. The pK(a) value measured by SPRi, ~4.9, monitors in situ the change of refractive index of the polymer thin film as it swells upon protonation. This was the first example where SPRi was used to measure the ionization constants of polymers.     

Multiparametric curve fitting-XI: POLET computer program for estimation of formation constants and stoichiometric indices from normalized potentiometric data

The FORTRAN computer program POLET(84) analyses a set of normalized potentiometric titration curves to find a chemical model, i.e., the number of species present and their stoichiometry, and to determine the corresponding stability constants log beta(pqrs) and unknown stoichiometric indices p, q, r, and s of up to quaternary M(p)L(q)Y(r)H(s) complexes. The program belongs to the ABLET family, based on the LETAG subroutine, and can use an algorithmic and/or heuristic minimization strategy, or a beneficial combination of both. The data, a set of potentiometric titration curves plotted as volume and potential, are converted into normalized variables (formation function, pH) and then a computer-assisted search for a chemical model by POLET(84) is applied. The procedure for efficient application of POLET(84) in an equilibrium analysis is described and the program is validated by use of literature and simulated data. The reliability of the chemical model and its parameters is established by the degree-of-fit achieved, and the closeness of the stoichiometric indices to integral values.     

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.     

A critical comparison of computer programs for the potentiometric determination of stability constants

The performance of five of the best-known computer programs LETAGROP, SCOGS, MINIQUAD, SUPERQUAD and ESTA, is critically discussed. All these programs are based on least-squares refinement procedures but differ in some important aspects: (a) the definition of the residuals function; (b) the minimization procedure; (c) the statistical and weighting parameters. Each of these aspects is evaluated and the programs are compared on this basis.     

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.     

Critical comparison of evaluation methods of potentiometric titration data using a microcomputer-controlled automatic potentiometric titration system

A microcomputer-controlled automatic potentiometric titrator withP controlled intelligent auto-sampler has been built for serial analysis. Controlling and evaluating programs have also been written. In order to use the most powerful evaluating method, a comparison was made between the methods. The data of the titration curves were obtained by the implicit relation characterizing the curve. The different evaluating methods were tested by using these data. The effects of random errors stemming from measurements were also investigated. The methods used were: two point differentiating formula, differentiation of interpolating and smoothing spline functions, the Gran method, and implicit regression with the Gauss-Newton-Marquardt method. Regression is outstanding among the methods, but it needs much more computing time compared with others. The second-best method is the differentiation with smoothing spline function with end point determination based on finding the sign reversal of the second derivative. This method is much faster than regression.     

Thermodynamic dissociation constants of isocaine, physostigmine and pilocarpine by regression analysis of potentiometric data

Concentration and mixed dissociation constant(s) of three drug acids, H(J)L, isocaine, physostigmine and pilocarpine, at various ionic strengths, I, in the range 0.03-0.81 and 25 degrees C have been determined with the use of regression analysis of potentiometric titration data when common parameter, pK(a), and group parameters E'(0), L(0), and H(T) are simultaneously refined. Internal calibration of the glass electrode cell in the concentration scale [H(+)] performed during titration was used. The estimate of ill-conditioned group parameters has a great influence on a systematic error in estimated pK(a) and therefore it makes the computational strategy important. As more group parameters are refined and a better fit achieved, a more reliable estimate of dissociation constants results. The thermodynamic dissociation constant, pK(a)(T), an ill-conditioned ion-size parameter, ?, and the salting-out coefficient, C, were estimated by non-linear regression of {pK(a), I} data and an extended Debye-Hückel equation. The goodness-of-fit test based on regression diagnostics is a measure of the reliability of parameters, and proves that reliable estimates for isocaine pK(a)(T)(=)8.96(1), ?=8(3) A and C=0.50(3) at 25 degrees C, for physostigmine pK(a)(T)(=)8.07(3), ?=19(26) A and C=0.64(3) at 25 degrees C, and for pilocarpine pK(a)(T)(=)7.00(1), ?=7(1) A and C=0.53(2) at 25 degrees C were found.     

Empirical method for description of solvent effect on the ionization constants of NH acids

An empirical method was used to calculate 192 ionization constants of 24 NH acids in 8 solvents. The relative error in the calculations did not exceed 10%, and the average error was no higher than 3.5%. The calculated values were compared with those obtained by quantum-chemical and other up-to-date methods for the determination of acid ionization constants in various solvents.     

A comparative study of the application of the method of least-squares in the potentiometric determination of protonation constants

Methods of simple and multiple linear regression applied to the potentiometric determination of protonation constants of diprotic and triprotic acids are studied critically. The best way of fitting the data, according to the order of magnitude of the constants, is established. The conclusions are checked by calculating the protonation constants of succinic and citric acids.     

A new potentiometric method for the estimation of vanadium

Summary A new potentiometric method is given for the accurate determination of small amounts of vanadium after reduction to the 4-valent state, involving back-titration of an added excess of EDTA by mercuric solution. The fact that 5-valent, but not 4-valent, vanadium has no influence on the course of the back-titration of excess EDTA with mercuric ions was used in the analysis of various binary, ternary and quaternary mixtures with requisite accuracy by two, three or four potentiometric titrations of identical mixtures at different pH-values. The method, slightly modified, may be applied to the analysis of cobalto-and nickelo-vanadium alloys.

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Zusammenfassung Zur genauen Bestimmung kleiner Mengen Vanadium nach, der Reduktion zur 4 wertigen Stufe wird ein potentiometrisches Verfahren beschrieben, bei dem ein zugesetzter überschu\ an ÄDTA mit Quecksilber(II)-Lösung zurücktitriert wird. Die Tatsache, da\ 5 wertiges (jedoch nicht 4wertiges) Vanadium die Rücktitration von ÄDTA mit Hg²⁺-Ionen nicht stört, wurde zur Analyse von binÄren, ternÄren und quaternÄren Gemischen ausgenutzt, wobei 2, 3 oder 4 potentiometrische Titrationen identischer Proben bei verschiedenen pH-Werten durchgeführt wurden. Kobalt- und Nickel-Vanadiumlegierungen können nach geringer Modifizierung des Verfahrens ebenfalls analysiert werden.