Potentiometric Curves Research Articles

Importance in ligand exchange chromatography of stereoselectivity in the ternary complexes copper(II)-N-substituted l-proline- d/ l-α amino acids

Resolution of d,l-valine, d,l-phenylalanine and d,l-proline by ligand exchange chromatography has been achieved by using polyacrylamide resins containing the l-proline-copper(II) complex as stationary phases. Ternary complexes, models of the associations occurring in the chromatographic process, have been studied in aqueous solution. The stability constants of the ternary complexes involving a copper(II) ion, N-substituted l-proline as soluble model of the active site of the resin and a l or d-α-amino acid have been calculated from potentiometric curves. The significant stereoselectivity in the ternary complexes allow to explain the chromatographic results. Possible structures for the complexes are presented.

Kinetics and mechanism of phenylacetylene hydrogenation under hydrogen pressure

Kinetic and potentiometric curves of phenylacetylene hydrogenation on a 5% Pt/C catalyst in ethanol under hydrogen pressure have been studied. On the basis of chromatographic data a possible scheme of the process is suggested.

Coordination complexes and catalytic properties of proteins and related substances. 104. Electrostatic effects in hemoglobin: hydrogen ion equilibriums in human deoxy- and oxyhemoglobin A

The modified Tanford-Kirkwood theory of Shire et al. [Shire, S. J., Hanania, G.I.H., & Gurd, F.R.N. (1974) Biochemistry 13, 2967] for electrostatic interactions was applied to the hydrogen ion equilibria of human deoxyhemoglobin and oxyhemoglobin. Atomic coordinates for oxyhemoglobin were generated by the application of the appropriate rigid rotation function to alpha and beta chains of the deoxyhemoglobin structure [Fermi, G. (1975) J. Mol. Biol. 97, 237]. The model employs two sets of parameters derived from the crystalline protein structures, the atomic coordinates of charged amino acid residues and static solvent accessibility factors to reflect their individual degrees of exposure to solvent. Theoretical titration curves based on a consistent set of pKint values compared closely with experimental potentiometric curves. Theoretical pK values at half-titration for individual protein sites corresponded to available observed values for both quaternary states. The results bring out the cumulative effects of numerous electrostatic interactions in the tetrameric structures and the major effects of the quaternary transition that result from changes in static solvent accessibility of certain ionizable groups.

Determination of carbonate in the presence of hydroxide : Part I. Analysis of first-derivative potentiometric curves

Mathematical consideration of the course of neutralization of a solution containing a moles of a strong base (e.g. sodium hydroxide) and a moles of a salt of a weak dibasic acid (e.g. sodium carbonate) with a strong acid (e.g. hydrochloric acid) shows that the first derivative of the titration curve should contain three maxima. The first, corresponding to neutralization of most of the hydroxide, is small (with height proportional to a 1 2 ), displaced slightly from the equivalence point, and disappears for a less than 0.01 mol l -1. The second corresponds to the conversion of most of the CO 3 2- to HCO 3 - and its height is almost independent of a. The third corresponds to the conversion of HCO 3 - to H 2CO 3, with height proportional to a 1 2 . The two minima are independent of a and of the dissociation constants of the weak acid. These conclusions were examined experimentally and were extended to the titration of hydroxide contaminated with a small amount of carbonate.

Determination of residual chlorine in water with computer automation and residual-chlorine electrode

A method of determining active chlorine in water in the concentration range 3 to 100 ppB with an accuracy of 2 ppB has been developed. The method uses the Orion model 97-70 residual-chlorine electrode and the known standard-addition potentiometric technique to measure the potential difference between iodide and iodine that is generated by active chlorine. The assay is computer-automated to carry out the multiple addition of a standard; measure the potentials; plot the potentiometric curve, its Gran version, and an error function; and calculate the concentration of active chlorine in the assay solution. The electrode, the chemical reactions that occur during the assay, the preparation and storage of standards, and the procedure are discussed. Data obtained from the assay of standards prepared from deionized water, two sources of public water supply, and one swimming pool are presented. 1 table, 8 figures.

Discrete charge calculations of potentiometric titrations for globular proteins: Sperm whale myoglobin, hemoglobin alpha chain, cytochrome c

The modified Tanford-Kirkwood theory of Shire et al. for intramolecular electrostatic interactions has been applied to hydrogen ion equilibria of sperm whale ferrimyoglobin, human hemoglobin α-chain and horse cytochrome c. The model employs two sets of parameters derived from the crystalline protein structures, first, the atomic coordinates of charged amino acid residues and, second, static accessibility factors to reflect their solvent exposure. In addition, a consistent set of intrinsic pK values (pK int) for the individual groups is employed. The theoretical pK values at half-titration for individual groups in each protein correspond to the available observed pK values, and the theoretical titration curves compare closely with experimental potentiometric curves.

Equivalence-point determinations for automated potentiometric titrations with ion-selective electrodes

An improved method is presented for determining the equivalence point in potentiometric titrations. Titration rate and data acquisition are under computer control, and ion-selective electrodes are used as indicators. As the titration proceeds, the potentiometric curve and Gran (antilog) plot are displayed on a CRT. Upon completion of the titration, the operator uses interactive graphic techniques to interrogate the Gran plot and develop an error function. This error function identifies the period during the potentiometric titration when the cell output is most nearly Nernstian. Data obtained during the identified period are used to estimate the equivalence point. Data are presented together with a discussion of the characteristic features of the error function.

Mixed ligand derivatives of Th(IV)-EDTA,-CDTA or-DTPA chelate with glycollic and malic acids

The interaction of 1∶1 Th(IV)-EDTA,-CDTA or-DTPA chelate with glycollic and malic acids has been investigated potentiometrically and the formation of 1∶1∶1 mixed ligand chelates inferred from the potentiometric curves. The hydrolysis and dimerization constants of the binary chelates and the equilibrium constants of the ternary derivatives have been evaluated at 30±1° and 35±1°C and also the thermodynamic functions, viz. ΔF0, ΔH and ΔS.

ChemInform Abstract: PH‐METRIC STUDIES ON THE BINARY AND TERNARY COMPLEXES OF OXOVANADIUM(IV)

AbstractDie Komplexbildung in den Systemen VO2 /Salicylsäure, VO′ /5‐Sulfosalicylsäure und VO"/8‐Hydroxychino1in‐5‐sulfonsäure wird potentiometrisch untersucht.

Zero-current bipotentiometric end-point indication with pretreated electrodes—III: Mathematical simulation of the titration curves

A mathematical model is described which simulates the appearance of the bipotentiometric titration curves obtained experimentally with two differently pretreated indicator electrodes. Simplified equations are used to calculate the individual potentiometric curves, and the bipotentiometric curve is obtained by difference, as corresponds to the experimental technique.

PH-Metric studies on the binary and ternary complexes of oxovanadium(IV)

The interaction of oxovanadium(IV) with some hydroxy acids, salicylic (SA), 5-sulphosalicylic (SSA) and 8-hydroxyquinoline-5-sulphonic (HQSA) acids, has been studied potentiometrically. Further, pH-metric studies of the ternary systems, VO2+−HQSA-dicarboxylic or hydroxy acids (where dicarboxylic acids = phthalic and maleic acids and hydroxy acids =SA andSSA) have been carried out and the formation of 1∶1∶1 mixed complexes inferred from the potentiometric curves. The equilibrium, chelate formation and hydrolysis constants have been calculated in the case of binary systems. The ternary complexes have been found to be more stable as indicated by their formation constants.

Differential electrolytic potentiometry with periodic polarisation. Part XXIII. The effect of bias and distortion on periodic differential electrolytic potentiometry, the d.c. output produced and time-biassed differential electrolytic potentiometry in oxidation-reduction titrations

Any departure from the pure, symmetrical, bias-free input waveform; external d.c. bias; internal d.c. offset, distortion, amplitude or mark-to-space (time) bias; produces a deterioration in the periodic differential electrolytic potentiometric titration curve. The peak potential is decreased, the peak becomes less sharp, the discrimination becomes worse, errors are introduced and the electrodes more quickly become deactivated when any bias or distortion is present in the input current waveform, and this effect is manifest with 2 per cent. contamination of the waveform and destructive at 5 per cent. A d.c. bias causes the peak to split into two peaks. At the same time, the electrodes produce a d.c. output; the symmetrical polarisation shows no d.c. component in the output; and for d.c. offset and amplitude bias, this d.c. output is the same as for classical d.c. differential electrolytic potentiometry. With a time-biassed periodic input of any waveform, a d.c. output of unique properties is produced. This output has the same forms as the classical d.c. differential electrolytic potentiometric curves, but the end-points are sharper, the discrimination is better, the end-points are error-free with dichromate and cerium(IV) titrants, the d.c. potential stabilises very quickly and remains drift-free, even for type II (b) curves, the high-quality end-point persists to very low concentrations, the electrodes retain their activity for a long time and the process is independent of frequency. Such titrations can be performed as fast as titrations with visual indicators. The positive errors in classical d.c. differential electrolytic potentiometric titrations with cerium(IV), chromium(VI) and in zero-current potentiometric titrations with vanadium(V) are explained.

Studies on the polymer + plasticizer membrane-electrode—VIII : Use of the PVC + tricresyl phosphate membrane as indicating electrode in potentiometric precipitation titration

Results obtained by use of the PVC + tricresyl phosphate membrane-electrode in potentiometric precipitation titration of some halides and pseudohalides are given. Solutions of Ag +, Tl + and Hg 2 2+ were used as titrants, to which the membrane is responsive. The fast response to Ag + ions allowed the determination of iodide by automatic recording of the potentiometric curve (or its derivative).

Linear potentiometric titrations

A method for graphical representation of potentiometric titration curves is proposed. It permits determination of the end-point even if the conventional potentiometric curve, pH-volume, fails to define exactly an inflection point. The limits and the possibilities of the method are discussed and some examples of practical applications are illustrated.

Physico chemical studies on the composition of complex arsenites of metals Part IV: conductometric and potentiometric studies on the composition of cadmium arsenite

The formation and precipitation of cadmium arsenite has been studied by conductometric and potentiometric titrations between cadmium nitrate and sodium arsenite (meta) at different concentrations with either of the substances used as the reagent in titration. In the case of direct titrations (cadmium nitrate added to sodium arsenite in the conductivity cell), one distinct break in the curves is observed corresponding to the formation of the Cd (AsO/sub 2/)/sub 2/ where the molecular ratio is 2:1. The direct and reverse potentiometric titrations curves give one maxima in dE/dV at point corresponding to the formation of the complex Cd (AsO/sub/2)/sub/2 where the molecular ratio of reactants Cd:AsO/sub/2 is 1:2. The composition has been arrived at by comparing the calculated values with observed values by conductometric and potentiometric titrations. The composition of cadmium arsenite arrived at both by conductometry and potentiometry is best representative as Cd(AsO/sub/2)/sub/2

Contributions au probleme de la determination du point d'equivalence—II: Une nouvelle methode pour determiner le coefficient reel d'asymetrie dans les titrages potentiometriques

A new method is described for determining the real coefficient of asymmetry, u, in a potentiometric titration. For this purpose, the angular coefficients, A 1 and A 1' of the two straight lines obtained by linearising the potentiometric curve before and after the point of equivalence by the method of least squares are calculated. Using the method described, the real coefficient of asymmetry is calculated by the method of least squares with the help of titration results obtained some distance from the equivalence point. The value of the coefficient has the nature of an arithmetic mean.

Potentiometric studies on bismuth ferrocyanide and the determination of bismuth

The reaction between bismuth nitrate and ferrocyanides of alkali metals such as sodium, potassium, lithium and ammonium, has been studied potentiometrically using platinum as an indicator electrode in conjunction with a saturated calomel electrode through KNO3 bridge. The equivalence point obtained from the maximum value of dE/dV corresponds to the formation and complete precipitation of a compound having molecular formula BiMFe(CN)6, where M represents an alkali metal. The potentiometric curves are symmetrical on both sides of the end point and the results obtained are very accurate and reproducible. The method of potentiometric titrations, though simple, offers a rapid and accurate means for the determination of bismuth solutions at fairly low concentrations. The effect of addition of ethanol and electrolytes on the course of the reaction has been studied.

Potentiométrie à intensité constante

Potentiometry at constant intensity is a generalization of classical potentiometry. A. study of variations of the polarization curve of the solution to be analysed, during the addition of the titrating solution, allows one to forecast the general shape of the potentiometric curves and the experimental conditions necessary for detection of the equivalence point with sufficient precision. This study has been made quantitatively in a simple case of a precipitation reaction. The results are applicable — at least qualitatively — to other types of reactions. The influence of the operative conditions' is shown by the example of the determination of complexon by copper in the presence of thiocyanate. In particular, the importance of the electrolysis circuit resistance is shown for the shape of the determination curve, this being able to pass progressively from the potentiometric curve into the amperometric curve. The repercussions of this deformation for the detection of the equivalence pointarc demonstrated, particularly when using automatic titrimeters.

Reduction potentials of various aromatic hydrocarbons and their univalent anions

AbstractPotentiometric titrations of solutions of various aromatic hydrocarbons in 1,2‐dimethoxyethane and tetrahydrofuran with a solution of biphenylsodium have shown chemical reduction to proceed in two stages: equation image The reduction potentials of the aromatic hydrocarbons, calculated from the potentiometric curve, are in good agreement with the half‐wave potentials obtained from polarographic reductions in 96% dioxan‐water. The standard potentials of the first reduction step expressed in Volts against the standard potential of biphenyl fit the following equation: with γ = ‐ 2.19 ± 0.059 e.V. (2 × stand. dev. from mean) in which a stands for the root of the m.o. secular equation (overlap S = 0.25 included) corresponding to the lowest non‐occupied π‐orbital of the hydrocarbon and γ is the resonance‐parameter.The electrochemical series of aromatic hydrocarbons including a rough estimate of the standard potential of sodium in 1,2‐dimethoxyethane and the mechanism of chemical reduction in inert and proton‐active solvents are discussed.

Étude de la phényl-2 oxine et de ses chélates: Préparation et propriétés des phényl-2 oxinates

The authors have studied the potentiometric curves obtained during the neutralization, in dilute alcoholic media, of acid solutions containing 2-phenyl-8-quinolinol and metallic cations, with sodium hydroxide. By thermogravimetric analysis the number of water molecules and regions of stability for hydrates and anhydrous compounds were also established. Moreover these results were compared with those previously obtained for oxinates and methyl-2 oxinates.