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Thermodynamic dissociation constants of risedronate using spectrophotometric and potentiometric pH-titration

100%
Meloun M., Ferenčíková Z., Málková H., Pekárek T.
Open Chemistry
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2012
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vol. 10
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issue 2
338-353
EN
Risedronate inhibits bone resorption in diseases like osteoporosis, Paget’s disease, tumor bone diseases or the malfunction of phosphocalcium metabolism. The acid-base properties of risedronate in an aqueous solution have been studied in a pH range from 2 to 12 and can be described in terms of four dissociation steps: pK a,2, pK a,4, pK a,5 (related to the dissociation of POH groups) and pK a,3 related to the dissociation of protonated amino group NH3+. The mixed dissociation constants were determined at different ionic strengths I = 0.02 to 0.20 mol dm−3 KCl and of 25°C and 37°C using pH-spectrophotometric and pH-potentiometric titration methods. Determination of group parameters L 0, H T might lead to false estimates of common parameters p K a;therefore, the computational strategy employed is important. A comparison between the two programs ESAB and HYPERQUAD demonstrated that the ESAB program provides a better fit of potentiometric titration curve. The thermodynamic dissociation constants pK aT were estimated by a nonlinear regression of (pK a, I) data and a Debye-Hückel equation at 25°C and 37°C, pK a,2T = 2.37(1) and 2.44(1), pK a,3T = 6.29(3) and 6.26(1), pK a,4T = 7.48(1) and 7.46(2) and pK a,5T = 9.31(7) and 8.70(3) at 25°C and 37°C using pH-spectroscopic data and pK a,2T = 2.48(3) and 2.43(1), pK a,3T= 6.12(2) and 6.10(2), pK a,4T = 7.25(2) and 7.23(1) and pK a,5T = 12.04(5) and 11.81(2) at 25°C and 37°C. The ascertained estimates of three dissociation constants pK a,3, pK a,4, pK a,5 are in agreement with the predicted values obtained using PALLAS [...]
2
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Investigations of ternary complexes of Co(II) and Ni(II) with oxydiacetate anion and 1,10-phenanthroline or 2,2′-bipyridine in solutions

100%
Wyrzykowski D., Pranczk J., Jacewicz D., Tesmar A., Pilarski B., Chmurzyński L.
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issue 1
107-114
EN
Potentiometric (PT) and conductometric (CT) titration methods have been used to determine the stoichiometry and formation constants in water for a series of ternary complexes of Co(II) and Ni(II) involving the oxydiacetate anion (ODA) and 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) ligands, namely [Co(ODA)(phen)(H2O)], [Co(ODA)(bpy)(H2O)], [Ni(ODA)(phen)(H2O)] and [Ni(ODA)(bpy)(H2O)]. The ternary complex formation process was found to take place in a stepwise manner in which the oxydiacetate ligand acts as a primary ligand and the phen or bipy ligands act as auxiliary ones. The stability of the ternary complexes formed is discussed in the relation to the corresponding binary ones. Furthermore, the kinetics of the substitution reactions of the aqua ligands in the coordination sphere of the Ni-ODA and Co-ODA complexes to phen or bipy were studied by the stopped-flow method. The kinetic measurements were performed in the 288–303 K temperature range, at a constant concentration of phen or bipy and at seven different concentrations of the binary complexes (4–7 mM). The influence of experimental conditions and the kind of the auxiliary ligands (phen/bipy) on the substitution rate was discussed. [...]
3
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Complexation of nickel (II) ion with B3 vitamin in aqueous dimethyl sulfoxide

88%
Gamov G., Dushina S., Sharnin V., Zavalishin M.
Open Chemistry
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2013
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vol. 11
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issue 12
1959-1963
EN
The stability change of nickel(II) ion complexes including one and two nicotinamide (B3 vitamin) molecules in aqueous dimethyl sulfoxide (XDMSO = 0–0.85 m.f.) was studied at 298.2±0.1 K and 0.25 ionic strength value (NaClO4) using the potentiometric method. The first stage constant of complexation increased until organic solvent concentration was 0.5 m.f. and reduced at higher DMSO content. The difference between complex and central ions solvation is a dominating contribution into the Gibbs energy change of mononicotinamide complex formation reaction. When the second ligand molecule was bonded into the coordination compound, the nicotinamide contribution to ΔtrGr rose and became prevailing at XDMSO = 0.7–0.85. The ligand was found to replace a water molecule in the coordination sphere of the cation according to spectrophotometric study results. [...]
4
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The thermodynamic dissociation constant of naphazoline by the regression analysis of potentiometric data

88%
Meloun M., Ferenčíková Z., Netolická L., Vrána A.
Open Chemistry
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2011
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vol. 9
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issue 1
66-74
EN
The mixed dissociation constant of naphazoline is determined at various ionic strengths I [mol dm−3] in the range of 0.01 to 0.26 and at temperatures of 25°C and 37°C using ESAB and HYPERQUAD regression analysis of the potentiometric titration data. A strategy of efficient experimentation is proposed in a protonation constant determination, followed by a computational strategy for the chemical model with a protonation constant determination. Two group parameters, L 0 and H T were ill-conditioned in the model and their determination is therefore uncertain. These group parameters, L 0 and H T, can significantly influence a systematic error in the estimated common parameter pKa and they always should be refined together with pK a. The thermodynamic dissociation constant pK aT was estimated by nonlinear regression of {pK a, I} data at 25°C and 37°C: for naphazoline pK alT = 10.41(1) and 10.13(2). Goodness-of-fit tests for various regression diagnostics enabled the reliability of the parameter estimates to be found. [...]
5
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Construction of different types of ion-selective electrodes and validation of direct potentiometric determination of phenytoin sodium

88%
El-Tohamy M., Razeq S., El-Maamly M., Shalaby A.
Open Chemistry
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2010
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vol. 8
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issue 4
937-945
EN
The construction and performance characteristics of phenytoin sodium selective electrodes are detailed. Two types of electrodes: plastic membrane I and coated wire II, were constructed based on the incorporation of phenytoin sodium with tungstosilicic acid. The influence of membrane composition, kind of plasticizer, pH of the test solution, soaking time and the electrodes’ foreign ions were investigated. The electrodes showed a Nernstian response with a mean calibration graph slope of 30.9±0.1 and 28.9±0.1 mV decade−1 at 25°C for electrode I and II respectively, over a phenytoin sodium concentration range of 5×10−3−5×10−6 M and 1×10−3−1×10−6 M with a detection limit 1.3×10−6 M and 2.5×10−7 M for electrode I and II, respectively. The electrodes gave average selective precision and were usable within the pH range 6–10. Interference studies from common cations, alkaloids, sugars, amino acids and drug excipients are reported. The results obtained by the proposed electrodes were also applied successfully for the determination of the drug in pharmaceutical preparations and biological fluids.
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