Calculated equivalence volume in potentiometric acid-base titrations

Summary Using a pH meter, a motorized burette and a programmable pocket calculator potentiometric acidbase titrations may be carried out with a high degree of precision, a high degree of accuracy and in a time comparable to the time needed to perform a titration with a visual indicator. With the exception of the lowest concentration of the weak acid a precision higher than 0.01 ml of the reagens is easily obtainable, whereas the accuracy may be as high as 0.1 %. The method is suitable over a wide range of concentrations of either strong or weak acids.

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Berechnetes Äquivalenzvolumen bei potentiometrischen Säure-Base-Titrationen

Zusammenfassung Mit Hilfe eines pH-Meßgeräts, einer Motorbürette und einem programmierbaren Taschenrechner können potentiometrische Säure-Base-Titrationen mit hoher Reproduzierbarkeit und Richtigkeit in einer mit der Verwendung von visuellen Indicatoren vergleichbaren Zeit durchgeführt werden. Mit Ausnahme der niedrigsten Konzentration einer schwachen Säure wurde eine Reproduzierbarkeit von besser als 0,01 ml und eine Richtigkeit von 0,1 % erhalten. Das Verfahren ist für einen weiten Konzentrationsbereich von starken und schwachen Säuren geeignet.

     

Rapid determination of the equivalence volume in potentiometric acid-base titrations to a preset pH-II Standardizing a solution of a strong base, graphic location of equivalence volume, determination of stability constants of acids and titration of a mixture of two weak acids

A newly proposed method of titrating weak acids with strong bases is applied to standardize a solution of a strong base, to graphic determination of equivalence volume of acetic acid with an error of 0.2%, to calculate the stability constants of hydroxylammonium ion, boric acid and hydrogen ascorbate ion and to analyse a mixture of acetic acid and ammonium ion with an error of 0.2-0.7%.     

Method for determination of the equivalence point in potentiometric titrations

A method is developed for calculating the end-point and the ionization constant in a potentiometric titration. The influence of dilution is studied. The method is compared with the procedure of KOLTHOFF and that of HAHN.     

The aluminium indicator electrode in acid-base potentiometric titrations

Aluminium has been employed as an indicator electrode in potentiometric acid-base titrations. The titration curves obtained were approximately 3.6 times broader than those obtained with the antimony electrode.     

A modified gran method for determination of equivalence points in potentiometric precipitation titrations

Gran's method is frequently applied in potentiometric titration to obtain a more precise estimate of the equivalence point, particularly when the analyte concentration is too low to give well defined end-points. With precipitation reactions, however, when the analyte concentration is comparable to the solubility of the precipitate, even the use of Gran's method will not avoid significant errors. A correction term is introduced and a computer program outlined to overcome this problem. Titrations of 10^-4 M chloride are used for illustration.     

Evaluation of potentiometric acid-base titrations by partial-least-squares calibration

Partial-least-squares calibration is used for the evaluation of acid-base titration curves. The method is advantageous because no physical model is assumed and only information from the titration curves is used. In the procedure, a model is developed from samples of known composition (calibration set) and is then used to predict unknowns (test set). The only assumption is that the two sets are similar. The method was evaluated first by titrating mixtures of two monoprotic acids with a small difference in pK_a values and then by titrating samples containing four or five diprotic acids. The results show that it is possible to determine monoprotic acids having a difference in pK_a of only 0.1, or up to five diprotic acids having pK_a values in the range 1.8–7.2, simultaneously.     

Calculated equivalence volume in complexometric titrations

Summary Using a digital pH-meter, a motorized burette and a programmable pocket calculator, complexometric titrations were executed with a high degree of precision, a high degree of accuracy and in a time comparable to the one needed for performing a titration with a visual indicator. If a suitable sensing electrode is at hand, concentrations as low as 0.001 M may be analyzed without difficulties.

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Berechnetes Äquivalenzvolumen bei komplexometrischen Titrationen

Zusammenfassung Mit Hilfe eines Digital-pH-Meßge-räts, einer Motorbürette und einem programmierbaren Taschenrechner konnten komplexometrische Titrationen mit hoher Präzision und Genauigkeit in einer der visuellen Indikation vergleichbaren Zeit durchgeführt werden. Mit einer geeigneten Meßelektrode können Konzentration bis herab zu 0,001 M ohne Schwierigkeiten erfaßt werden.

     

A source of error in the determination of equivalence volumes of carboxylic acids from potentiometric titrations

Emf measurements on mixtures of carboxylic acids, HA, and the corresponding bases, A, containing a high and constant salt background, show that the equation E = E₀ + glnh + j_hh commonly used in the calculation of equivalence volumes is not strictly followed. Terms linear in [HA] and [A] should be added. The origin of these terms is discussed briefly and their influence on the accuracy of equivalence-volume determinations is evaluated.     

Calculated equivalence volume in precipitation titrations

Summary Using a digital pH meter, a motorized burette and a programmable pocket calculator, precipitation titrations may be carried out with a high degree of accuracy and in a time comparable to the time needed to perform a titration with a visual indicator. Not too diluted solutions (not below 0.001 M) may be analyzed with a 95% confidence interval of less than ±0.01 ml, whereas the accuracy amounted to 0.1%.

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Berechnetes Äquivalenzvolumen bei Fällungstitrationen

Zusammenfassung Mit Hilfe eines Digital-pH-Meßgeräts, einer Motorbürette sowie einem programmierbaren Taschenrechner können Fällungstitrationen mit hoher Genauigkeit in einer der visuellen Titration vergleichbaren Zeit durchgeführt werden. Nicht zu verdünnte Lösungen (nicht unter 0,001 M) können im 95%-Vertrauensbereich auf ±0,01 ml und mit einer Genauigkeit von 0,1% analysiert werden.

     

Analysis of variance in determinations of equivalence volume and of the ionic product of water in potentiometric titrations

Homogeneous sets of data from strong acid-strong base potentiometric titrations in aqueous solution at various constant ionic strengths have been analysed by statistical criteria. The aim is to see whether the error distribution matches that for the equilibrium constants determined by competitive potentiometric methods using the glass electrode. The titration curve can be defined when the estimated equivalence volume VEM, with standard deviation (s.d.) sigma (VEM), the standard potential E(0), with s.d. sigma(E(0)), and the operational ionic product of water K(*)(w) (or E(*)(w) in mV), with s.d. sigma(K(*)(w)) [or sigma(E(*)(w))] are known. A special computer program, BEATRIX, has been written which optimizes the values of VEM, E(0) and K(*)(w) by linearization of the titration curve as a Gran plot. Analysis of variance applied to a set of 11 titrations in 1.0M sodium chloride medium at 298 K has demonstrated that the values of VEM belong to a normal population of points corresponding to individual potential/volume data-pairs (E(i); v(i)) of any titration, whereas the values of pK(*)(w) (or of E(*)(w)) belong to a normal population with members corresponding to individual titrations, which is also the case for the equilibrium constants. The intertitration variation is attributable to the electrochemical component of the system and appears as signal noise distributed over the titrations. The correction for junction-potentials, introduced in a further stage of the program by optimization in a Nernst equation, increases the noise, i.e., sigma(pK(*)(w)). This correction should therefore be avoided whenever it causes an increase of sigma(pK(*)(w)). The influence of the ionic medium has been examined by processing data from acid-base titrations in 0.1M potassium chloride and 0.5M potassium nitrate media. The titrations in potassium chloride medium showed the same behaviour as those in sodium chloride medium, but with an s.d. for pK(*)(w) that was smaller and close to the expected instrumental noise, whereas the titrations in nitrate medium had a high noise level and even the determination of VEM was less certain. Procedures are also proposed for obtaining reference sets of data and checking the conformity of the solutions and apparatus to the chosen reference.     

Evaluation of the equivalence point in potentiometric titrations with application to traces of chloride

A mathematical model applicable to the determination of the equivalence point (V_eq) is described. The regression equation is (1 + V/V₀)³E = Σ³_i= _0AiVⁱ, where E is the e.m.f. corresponding to volume V of titrant added, V₀ is the initial volume of titrand, and A_i are the regression coefficients. On the basis of A_i values obtained by the least-squares method, the algorithm for V_eq and the criterion of correctness of results obtained from measurements are presented. The method is applied to titrations of chloride with silver nitrate solutions.     

Graphic determination of equivalence volumes in potentiometric titrations of mixtures of weak acids-I Two monobasic acids

A titration method for analysis of binary mixtures of weak acids is given. The equivalence volumes of the individual acids can be determined graphically from only two titration points. However, use of more than two points gives a more accurate result. Mixtures of acetic acid with ascorbic acid, hydroxyacetic and monochloroacetic (Delta log K(H)(HA), = 0.5-1.9) are titrated with an error of approximately, 0.5-2.5% in all three cases.     

Approach to accuracy assessment of the glass-electrode potentiometric determination of acid-base properties

Thermodynamic data are suitable subject for investigating strategies and concepts for the evaluation of complete measurement uncertainty budgets in situations where the measurand cannot be expressed in a mathematical formula. Some suitable approaches are the various forms of Monte Carlo simulations in combination with computer-intensive statistical methods that are directed to an evaluation of empirical distribution curves for the uncertainty budget. Basis of the analysis is a cause-and-effect diagram. Some experience is available with cause-and-effect analysis of thermodynamic data derived from spectrophotometric data. Another important technique for the evaluation of thermodynamic data is glass-electrode potentiometry. On basis of a newly derived cause-and-effect diagram, a complete measurement uncertainty budget for the determination of the acidity constants of phosphoric acid by glass-electrode potentiometry is derived. A combination of Monte Carlo and bootstrap methods is applied in conjunction with the commercially available code SUPERQUAD. The results suggest that glass-electrode potentiometry may achieve a high within-laboratory precision because major uncertainty contributions become evident via interlaboratory comparisons. This finding is further underscored by analysing available literature data.     

On the accuracy of acid-base determinations from potentiometric titrations using only a few points from the titration curve

There are several procedures which use only a few points on the titration curve for the calculation of equivalence volumes in acid-base titrations. The accuracy of such determinations will depend on the positions of the points on the titration curve. The effects of errors in the stability constants and in the pH measurements on the accuracy of the analysis have been considered, and the results are used to establish the conditions under which these errors are minimized.     

Determination of the equivalence point in potentiometric titrations with gran's first method used to test the electrode response

Gran's first method is compared to the standard addition (subtraction) method. It is shown that Gran's first method canbe used to test the electrode response of, for example, a copper-selective electrode.     

Sensitivity of the acid-base properties of clays to the methods of preparation and measurement. 2. Evidence from continuous potentiometric titrations

The effects of experimental procedures on the acid-base consumption titration curves of montmorillonite suspension were studied using continuous potentiometric titration. For that purpose, the hysteresis amplitudes between the acid and base branches were found to be useful to systematically evaluate the impacts of storage conditions (wet or dried), the atmosphere in titration reactor, the solid-liquid ratio, the time interval between successive increments, and the ionic strength. In the case of storage conditions, the increase of the hysteresis was significantly higher for longer storage of clay in suspension and drying procedures compared to "fresh" clay suspension. The titration carried out under air demonstrated carbonate contamination that could only be cancelled by performing experiments under inert gas. Interestingly, the increase of the time intervals between successive increments of titrant strongly emphasized the amplitude of hysteresis, which could be correlated with the slow kinetic process specifically observed for acid addition in acid media. Thus, such kinetic behavior is probably associated with dissolution processes of clay particles. However, the resulting curves recorded at different ionic strengths under optimized conditions did not show the common intersection point required to define point of zero charge. Nevertheless, the ionic strength dependence of the point of zero net proton charge suggested that the point of zero charge of sodic montmorillonite could be estimated as lower than 5.     

Construction of a permanganate ion-selective electrode and its application to potentiometric titrations

The construction of a liquid-membrane type permanganate ion-selective electrode and its application to potentiometric titrations are described. The benzylcetyldimethylammonium-permanganate ion-pair in the aqueous phase is easily extracted into nitrobenzene and the extract is employed as the liquid ion-exchange membrane of the ion-selective electrode. The electrode gives Nernstian response to permanganate in the concentration range from 10(-6) to 10(-1)M, and the potential is almost independent of pH over the range from 3.0 to 10.5. The electrode can be used as indicator electrode in potentiometric titrations with permanganate.     

Rapid potentiometric determination of the iron oxidation state in silicates

The rapid potentiometric determination of the oxidation state of iron in a wide range of silicates is described. A platinum/platinum—rhodium (20%) bimetallic electrode with a small constant applied current is used for end-point detection. Samples are decomposed in 10–60 min by boiling with hydrofluoric—sulphuric acid in special PTFE decomposition—titration vessels in an atmosphere of nitrogen. Total iron is reduced with mercury-treated zinc or nickel reductors. Sulphides or other constituents do not interfere significantly and the procedures are relatively insensitive to aerial oxidation. The average standard deviation is 0.35% for iron(II) and 0.22% for total iron. The accuracy compares well to the available data for international standard rocks.