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The kinetics and mechanism of formation of [Fe(CN)5INH]3- by a ligand substitution reaction between aquapentacyanoferrate(II) ion and isoniazid, (pyridine-4-carboxylic acid hydrazide) (INH) have been investigated spectrophotometrically in acidic media at 435 nm ( lmax of product), under pseudo-first order conditions, using excess of INH over [Fe(CN)5H2O]3-. The reaction was studied as a function of [Fe(CN)5 H2O3-], [INH], pH, ionic strength and temperature, by varying only one of these variables while other variables were held constant. The reaction exhibited first-order kinetics in [INH] while it was independent of [Fe(CN)5H2O3-] in the chosen concentration range. The effect of temperature on the reaction rate yielded the activation parameters. The most plausible mechanism consistent with the experimental results has been proposed. [ABSTRACT FROM AUTHOR]

The kinetics of the palladium(II)-catalysed ligand exchange of the hexacyanoferrate(II) complex ion with 4-Cyanopyridine (4-CNpy) in aqueous acidic medium have been investigated at 25.0 + 0.1 °C, pH 3.2 + 0.02 and ionic strength, I = 0.1 M (KNO3). The reaction was followed spectrophotometrically by measuring the increase in absorbance of the intense cherry red complex [Fe(CN)54-CNpy]3-, corresponding to metal-to-ligand charge transfer (MLCT) transitions, at λmax 477 nm. The effects of pH, ionic strength, the concentrations of [Fe(CN)64-], Pd2+ and 4-CNpy on the reaction rate have been studied and analysed. The kinetic data suggest that substitution follows an interchange dissociative (Id) mechanism and occurs via formation of a solvent-bound intermediate. The effects of the dielectric constant of the medium on the reaction rate have been used to visualize the formation of a polar activated complex and an interchange dissociative mechanism for the reaction. A most probable mechanism has been proposed in order to interpret the kinetic data. Kinetic evidence is reported for the displacement of CN by 4-CNpy in [Fe(CN)64-]. Activation parameters for the proposed reaction have been evaluated using an Eyring plot, and provided further support to the proposed mechanistic scheme. [ABSTRACT FROM AUTHOR]

The kinetics and mechanism of formation of [Fe(CN)5D-PA]3− by ligand substitution reaction between aquapentacyanoferrate(II) ion, and D-pencillamine (D-PA) has been investigated spectrophothmetrically in presence of anionic surfactant micelle, sodium dodecylsulphate (SDS) at 421 nm (λmax of product) under pseudo-first order conditions using excess of D-PA over [Fe(CN)5H2O]3−. The reaction was studied as a function of [Fe(CN)5H2O3−], [D-PA], [SDS], pH, ionic strength, and temperature, by varying only one of these variables at a time while the other variables held constant. The reaction exhibited first order kinetics in [D-PA] while it was independent of [Fe(CN)5H2O3−] in the chosen concentration range. The sodium dodecylsulphate (SDS) micelle, being of negatively charged, does not reveal any regular effect except at or near its critical micelle concentration (cmc). The effect of temperature on the reaction rate was also studied to calculate the activation parameters, computed by Arrhenius and Eyring plots. The most plausible mechanism consistent with the experimental results has also been proposed. [ABSTRACT FROM PUBLISHER]

Abstract: A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described. Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 488nm (λ max of [Fe(CN)5PhNHNH2]3−, complex) under the optimum reaction conditions at: 2.5×10−3 M [Fe(CN)6]4−, 1.0×10−3 M [PhNHNH2], 8.0×10−7 M [Ag+], pH 2.8±0.02, ionic strength (μ) 0.02M (KNO3) and temperature 30±0.1°C. A linear relationship obtained between absorbance (measured at 488nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [thiocyanate] in the range of 0.8–8.0×10−8 M with a detection limit of 2×10−9 M. The standard deviation and percentage error have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst–inhibitor (K CI), catalyst–substrate (K s) and Michaelis–Menten constant (K m) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of thiocyanate and their limits has been investigated. [Copyright &y& Elsevier]

The kinetics and mechanism of Hg2+-catalyzed substitution of cyanide ion in an octahedral hexacyanoruthenate(II) complex by nitroso-R-salt have been studied spectrophotometrically at 525 nm (λmax of the purple-red–colored complex). The reaction conditions were: temperature = 45.0 ± 0.1°C, pH = 7.00 ± 0.02, and ionic strength (I) = 0.1 M (KCl). The reaction exhibited a first-order dependence on [nitroso-R-salt] and a variable order dependence on [Ru(CN)64-]. The initial rates were obtained from slopes of absorbance versus time plots. The rate of reaction was found to initially increase linearly with [nitroso-R-salt], and finally decrease at [nitroso-R-salt] = 3.50 × 10-4 M. The effects of variation of pH, ionic strength, concentration of catalyst, and temperature on the reaction rate were also studied and explained in detail. The values of k2 and activation parameters for catalyzed reaction were found to be 7.68 × 10-4 s-1 and Ea = 49.56 ± 0.091 kJ mol-1, ΔH≠ = 46.91 ± 0.036 kJ mol-1, ΔS≠ = -234.13 ± 1.12 J K-1 mol-1, respectively. These activation parameters along with other experimental observations supported the solvent assisted interchange dissociative (Id) mechanism for the reaction. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 215–226, 2009 [ABSTRACT FROM AUTHOR]

Abstract: A kinetic analytical method using uncatalyzed ligand substitution reaction and spectrophotometeric monitoring for the quantification of D-pencillamine (D-PA) i.e. 2-amino-3-mercapto-3-methylbutanoic acid or β,β-dimethylcysteine or 3-mercaptovaline or 3-mercapto-D-valine in pure as well as dosage forms has been validated and applied. The method is based on the uncatalyzed ligand substitution reaction between [Fe(CN)5(H2O)]3− and D-PA which produces the complex [Fe(CN)5(D-PA)]3−. The substituted complex [Fe(CN)5(D-PA)]3− shows an absorption maximum at 421nm. The proposed method shows good linear dynamic range of 14.92–149.21μgmL−1. By measuring the fixed time absorbance as a measure of initial rate of the complex formation during the course of the reaction at this wavelength, D-PA can be determined in the range 14.92–149.21μgmL−1. The recoveries of D-PA from various samples spiked at the usage level have been quantitative. The limit of quantification (3σ) was determined as 5.16, 3.68 and 3.13μgmL−1 of D-PA corresponding to the fixed time of 2, 3 and 5min respectively. The common excipients used as additives in pharmaceuticals did not interfere in the proposed method. The method was successfully employed for the analysis of D-PA content in commercial pharmaceutical preparations (tablets) and revealed quantities almost equal to those measured using the standard method, and demonstrated good accuracy and precision. The proposed method is rapid, simple, accurate and precise without the need for authentic analyte standards. It could therefore be used as an alternative to the quantification of D-PA in various samples. [Copyright &y& Elsevier]