Determination of weak acids by indirect potentiometry without titration

The simple determination of the sum of weak (polycarboxylic) acids in aqueous solutions and titratable acidity of wines based on indirect iodate-iodide potentiometry without titration was described. Received: 17 July 1997 / Revised: 20 October 1997 / Accepted: 26 October 1997     

Determination of weak acids by indirect potentiometry without titration

The simple determination of the sum of weak (polycarboxylic) acids in aqueous solutions and titratable acidity of wines based on indirect iodate-iodide potentiometry without titration was described.     

Determination of protonation constants by coulometric titration

Application of coulometric titration to the determination of the protonation constants of acids and bases offers several advantages because of its simplicity, precision and accuracy. This procedure is rapid and requires only one calibration solution of strong acid in the same solvent and at the same ionic strength as the solution of acid (or base) being investigated. The procedure seems to be especially advantageous in the case of non-aqueous or mixed solvents having amphiprotic character. The validity of the method has been checked with several substances in water, 95% ethanol and ethylene glycol.     

Determination of stability constants of complexes from the titration curve by the maximum likelihood principle

The numerical methods used to determine stability constants from titration data are analysed. A new method based on the maximum likelihood principle is proposed.     

Direct titration of glycerol by constant current potentiometry

A direct, constant-current potentiometric titration method allows quantitative determination of 0.50-5.00 mg samples of glycerol in aqueous solution with errors ranging from 0.2% for the large samples to 2.0% for the smaller samples. Perchloratocerate is used as the standard oxidant and pyrolytic graphite as the electrode material. A direct determination is made possible by elevated temperatures, high acidity, and a relatively large constant current. Sources of error and possible interferences are discussed.     

Determination of silver in dore metal by weight titration with equivalence-point detection by differential electrolytic potentiometry

Use of weight titrimetry, with differential electrolytic potentiometry for detection of the equivalence point, instead of gravimetry, has been critically tested for the determination of silver in doré metal. The electrode response in the presence of possible interferences (Sb, As, Se, Cu, Ni, Fe, Pb and Co) was studied by voltammetry. Titration curves for six different samples were obtained and their differences from those for pure silver were interpreted. Positive errors were found to occur in the presence of 0.1% Fe(III) and Se(IV) but not in the presence of 1% Cu(II). The results obtained by gravimetry and weight titrimetry were compared. The imprecision of the proposed method was between 0.03 and 0.05% (relative standard deviation); that of gravimetry was between 0.05 and 0.08%. The absolute difference from the gravimetric results was between + 0.05 and + 0.08%. The time for analysis of one sample was between 60 and 80 min.     

Autoprotolysis constants by coulometric titration

A procedure is described for the evaluation of autoprotolysis constants in which a strong acid is titrated with coulometrically generated strong base. A two-compartment cell is used, and the acid may be added as such to the solvent under study, or generated in situ in the cell. When a silver auxiliary electrode can be used, as with solutions containing bromide ion, a single-compartment cell may be used, which seems to give more accurate results because it avoids the errors caused by diffusion of solution through the diaphragm. The results for the constants obtained for ethylene glycol, for methanol and for water are in reasonable agreement with values in the literature.     

Determination of N-acetyl-L-cysteine by flow-injection potentiometry

A multi-purpose tubular flow-through sensor was constructed with an AgI-based membrane. The membrane was prepared by pressing silver salts (AgI, Ag₂S) and powdered Teflon. This membrane was incorporated in the tubular flow-through sensor body. A 2-mm diameter hole was drilled through the center of the tubular sensor and the membrane, thus determining the active sensor volume of about 4 μL. The tubular sensor with reference electrode was placed into a complex flow-injection system and used for the flow-through determination of N-acetyl-L-cysteine, (NAC), in perchloric acid medium, pH = l. Linear dependence was established between the recorded signal height and the concentration of NAC in the injected sample. The recorded change in potential for a decade change in concentration, 62 mV {p (NAC)}^–1, in the concentration range from 1 × 10^–4 to 1 × 10^–1 mol L^–1, was based on the formation mechanism of the sparingly soluble deposit between silver and NAC on the surface of the sensitive part of the membrane. Received: 7 July 1997 / Revised: 22 September 1997 / Accepted: 26 September 1997     

Determination of acid dissociation constants based on continuous titration by feedback-based flow ratiometry

We propose a method for the determination of acid dissociation constants based on the rapid detection of the equivalence point (EP) by feedback-based flow ratiometry and the subsequent estimation of the half equivalence point (EP_1/2). The titrant (e.g., NaOH) flow rate F_B was varied in response to a control voltage V_c from a controller, while the titrand (e.g., CH₃COOH) flow rate F_A was held constant. The pH of the mixed solution was monitored downstream from the confluence point of the solutions following a knotted tubular mixer. Initially, V_c was increased linearly. At the instance the detector sensed EP, the ramp direction of V_c changed downward. The pH increased further because of the lag time between the mixing of solutions and the sensing of pH. Following the pH maximum, the pH decreased. The EP was sensed again in this downward scan. The V_c that gives EP_1/2 was computed from the V_c just at the time of the EP detection. The V_c was held constant at this level for 18 s, and the plateau pH value thus obtained was taken to be the pK_a of the analyte subject to activity corrections. Studies on the dependence of the pK_a on the ionic strength or dielectric constant of the solution were conducted in an automated fashion by delivering NaCl solution or acetonitrile through an additional channel. Satisfactory results were obtained with good throughput (53 s per determination) and precision (R.S.D.≈0.3%) for various acids.     

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.     

Determination of fluoride by analate additions potentiometry

Summary A novel direct potentiometric technique called analate additions potentiometry has been applied to the determination of fluoride using a fluoride ion selective indicator electrode. As expected, the analytical results are comparable to those obtained by the standard additions technique. An error of less than 1 part in 1000 and a precision of about 0.5% was achieved. The technique should be most useful in the determination of small sample volumes which cannot be done by the standard additions method without prior dilution.

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Zusammenfassung Ein neues, direktes potentiometrisches Verfahren wurde für die Fluoridbestimmung angewendet, wobei eine für Fluoridionen selektive Indikatorelektrode verwendet wurde. Wie erwartet sind die Ergebnisse dieses Probenzusatzverfahrens mit denen der Standardzusatzmethode vergleichbar. Der Fehler ist kleiner als l, die Genauigkeit beträgt etwa 0,5%. Das angegebene Verfahren eignet sich vor allem zur Analyse kleiner Probevolumina, die ohne vorherige Verdünnung mit der Standardzusatzmethode nicht analysiert werden können.

     

Thermodynamic protonation constants of vardenafil by the nonlinear regression of multiwavelength pH-spectrophotometric titration data

pH-spectrophotometric titration data were used to determine protonation constants of vardenafil at different ionic strengths I and temperatures of 25°C and 37°C. The use of two different multiwavelength and the multivariate treatment of spectral data, SPECFIT32 and SQUAD(84) nonlinear regression analyses and INDICES factor analysis is presented. The reliability of the protonation constants of the drug was proven with goodness-of-fit tests of the pH-spectra. The thermodynamic protonation constants log K _T ⁱ were estimated by a nonlinear regression of (log K, I) data using the Debye-Hückel equation, yielding log K ₄ ^T = 3.59(1) and 3.26(1), log K ₃ ^T = 5.64(1) and 5.81(1), log K ₂ ^T = 9.41(1) and 8.59(2), log K ₁ ^T = 10.92(2) and 10.05(1) at 25°C and 37°C, where the figure in brackets is the standard deviation in last significant digit. Concurrently, the experimental determination of four thermodynamic protonation constants was combined with the computational prediction of the MARVIN program based on knowledge of the chemical structures of the drug and was in good agreement with its experimental value. The factor analysis of spectra in the INDICES program predicts the correct number of light-absorbing components when the instrument error is known and when the signal-to-error ratio SER is higher than 10.      

Calculation of stability constants from photometric data

A method originally designed for the determination of stability constants by using a wedge calorimeter has been adapted to normal photometric measuring techniques. The calculations are simple and the method is especially suitable for use when the absorptivities of the absorbing species are unavailable because these species cannot be obtained alone in solution. Results of some determinations are given.     

Determination of autoprotolysis constants of some non-aqueous solvents using coulometric titration

Autoprotolysis constants of acetonitrile, propionitrile, nitromethane, ethylene carbonate and dimethyl sulphoxide were determined using a coulometric — potentiometric method with a hydrogen/palladium electrode as generator. The method is based on the titration of a strong base, tetrabutylammonium hydroxide, with H⁺ ions generated by anodic oxidation of hydrogen dissolved in palladium. The titration was carried out in a galvanic cell with glass and calomel electrodes at 25°C. The pK_s values for the investigated solvents are: acetonitrile, 28.8; propionitrile, 24.6; nitromethane, 23.7; ethylene carbonate. 21.5; and dimethyl sulphoxide 29.1. These data are in accordance with those reported in the literature.     

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.     

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.