Equilibrium and kinetic studies of electron transfer reactions involving quinolinium dichromate and aminoalcohols in aqueous acid media

Summary Oxidation of the aminoalcohols (AA) such as ethanolamine, diethanolamine and triethanolamine by quinolinium dichromate (QDC) yielded formaldehyde as the main product in aqueous H₂SO₄ and HClO₄ media. The reaction kinetics exhibited a first-order dependence on [QDC]. Plots of 1/k versus 1/[AA] indicated the formation of a QDC-A A adduct prior to the rate limiting step. The equilibrium constants, K, for the formation of QDC-AA adducts were evaluated by kinetic and spectroscopic (Ardon's) methods. The reaction was found to be catalysed by HClO₄ and H₂SO₄. The results were analysed in terms of various acidity function theories.     

Colorimetric determination of isoniazid and its pharmaceutical formulations

Two colorimetric methods for the estimation of isoniazid are developed. The first method depends on coupling of isoniazid with diazotized 1-amino anthraquinone zinc chloride salt (fast red AL salt) to form a red colour (lambda(max) 510 nm). The second one is based on the formation of a green complex (lambda(max) 655 nm) between the acid hydrazide and 2,6-dimethoxy-1,4-benzoquinone (DMBQ). All measurements of the two procedures were carried out in the presence of sodium hydroxide at room temperature (20 +/- 3 degrees C). The two methods are applied for the determination of isoniazid in presence of congenial drugs, vitamins and additives normally encountered with it in pharmaceutical dosage forms. The reliability of these methods was established by parallel determination with the reported and official methods.     

Spectrophotometric determination of etodolac in pure form and pharmaceutical formulations

Background

A routine method for the quantification of beryllium in biological fluids is essential for the development of a chelation therapy for Chronic Beryllium Disease (CBD). We describe a procedure for the direct determination of beryllium in undigested micro quantities of human blood and serum using graphite furnace atomic absorption spectrometry. Blood and serum samples are prepared respectively by a simple 8-fold and 5-fold dilution with a Nash Reagent. Three experimental setups are compared: using no modifier, using magnesium nitrate and using palladium/citric acid as chemical modifiers.

Results

In serum, both modifiers did not improve the method sensitivity, the optimal pyrolysis and atomization temperatures are 1000°C and 2900°C, respectively. In blood, 6 μg of magnesium nitrate was found to improve the method sensitivity. The optimal pyrolysis and atomization temperatures were 800°C and 2800°C respectively.

Conclusion

In serum, the method detection limit was 2 ng l^-1, the characteristic mass was 0.22 (± 0.07) pg and the accuracy ranged from 95 to 100%. In blood, the detection limit was 7 ng l^-1, the characteristic mass was 0.20 (± 0.02) pg and the accuracy ranged from 99 to 101%.     

Extractive spectrophotometric determination of fluoroquinolones and antiallergic drugs in pure and pharmaceutical formulations.

A simple, rapid and sensitive spectrophotometric procedure has been proposed for the assay of fluoroquinolones viz., ciprofloxacin (CPF) and norfloxacin (NRF), and antiallergic drugs viz., methdilazine hydrochloride (MDH) and isothipendyl hydrochloride (IPH) in bulk and pharmaceutical formulations. The method is based on the reaction of selected drugs with Brilliant Blue G (BBG) in NaOAc-AcOH buffer of pH 4.0 for CPF and NRF or in neutral medium for MDH and IPH to give chloroform soluble ion-association complexes. The effects of various parameters have been studied. The ion-association complexes exhibited absorption maxima at 610 nm for CPF, at 614 nm for NRF and MDH, and at 612 nm for IPH. Beer's law plots were obeyed in the concentration ranges of 0.5-6.0, 0.4-8.0, 0.1-6.0 and 0.2-6.1 (mg ml(-1) for CPF, NRF, MDH and IPH, respectively, with correlation coefficients not less than 0.9969. Molar absorptivity values as calculated from the Beer's law data were found to be 2.86 x 10(4), 2.64 x 10(4), 3.13 x 10(4) and 5.51 x 10(4) mol(-1) cm(-1) for CPF, NRF, MDH and IPH, respectively. The influence of commonly employed excipients in the determination of the studied drugs has been studied. The results obtained by the proposed method were statistically validated.     

Indirect spectrophotometric determination of propranolol hydrochloride and piroxicam in pure and pharmaceutical formulations.

Two simple and sensitive indirect spectrophotometric methods for the assay of propranolol hydrochloride (PPH) and piroxicam (PX) in pure and pharmaceutical formulations have been proposed. The methods are based on the oxidation of PPH by a known excess of standard N-bromosuccinimide (NBS) and PX by ceric ammonium sulfate (CAS) in an acidic medium followed by the reaction of excess oxidant with promethazine hydrochloride (PMH) and methdilazine hydrochloride (MDH) to yield red-colored products. The absorbance values decreased linearly with increasing concentration of the drugs. The systems obeyed Beer's law over the concentration ranges of 0.5 - 12.5 and 0.3 - 16.0 microg/ml for PPH, and 0.4 - 7.5 and 0.2 - 10 microg/ml for PX with PMH and MDH, respectively. Molar absorptivity values, as calculated from Beer's law data, were found to be 1.36 x 10(4) and 2.55 x 10(4) l mol(-1) cm(-1) for PPH, and 2.08 x 10(4) and 2.05 x 10(4) l mol(-1) cm(-1) for PX with PMH and MDH, respectively. The common excipients and additives did not interfere with their determinations. The proposed methods have been successfully applied to the determinations of PPH and PX in various dosage forms. The results obtained by the proposed methods compare favorably with those of official methods.     

Spectrophotometric determination of certain cephalosporins in pure form and in pharmaceutical formulations

A simple and reproducible spectrophotometeric method for the assay of cefotaxime sodium, cefuroxime sodium, and ceftriaxone disodium with metol-chromium(VI) reagent has been developed. The procedure is based on direct oxidation of metol by potassium dichromate in presence of drug in acidic medium and subsequent formation of ternary complex. Beer's law is obeyed in the range 0.2-28 microg ml(-1) at lambdamax 520 nm. For more accurate analysis, Ringbom optimum concentration range is found to be 0.8-26.5 microg ml(-1). The molar absorptivity and Sandell sensitivity were calculated. Six replicate analysis of solutions containing seven different concentrations of the examined drugs were carried out and gave a mean correlation coefficient < or =0.9996; the factors of the regression line equation for the three cephalosporins were calculated. The proposed method was applied to the determination of the examined drugs in pharmaceutical formulations and the results demonstrated that the method is equally accurate, precise, and reproducible as the official methods.     

Spectrophotometric determination of pipazethate hydrochloride in pure form and in pharmaceutical formulations

Three simple, sensitive, and reproducible spectrophotometric methods (A-C) for the determination of pipazethate hydrochloride (PiCl) in pure form and in pharmaceutical formulations are described. The first and second methods, A and B, are based on the oxidation of the drug by Fe3+ in the presence of o-phenanthroline (o-phen) or bipyridyl (bipy). The formation of tris-complex upon reactions with Fe3+-o-phen and/or Fe3+-bipy mixture in an acetate buffer solution of the optimum pH values was demonstrated at 510 and 522 nm, respectively, with o-phen and bipy. The third method, C, is based on the reduction of Fe(III) by PiCl in acid medium and subsequent interaction of Fe(II) with ferricyanide to form Prussian blue, which exhibits an absorption maximum at 750 nm. The concentration ranges are from 0.5 to 8, 2 to 16, and 3 to 15 microg/mL for Methods A-C, respectively. For more accurate analysis, Ringbom optimum concentration ranges were calculated. The molar absorptivity, Sandell sensitivity, and detection and quantitation limits were calculated. The developed methods were successfully applied to the determination of PiCl in bulk and pharmaceutical formulations without any interference from common excipients. The relative standard deviations were < or =0.83% with recoveries of 98.9-101.15%.     

Oxidation of nucleic acid bases by hexavalent manganese in aqueous alkaline medium: a kinetic study

Oxidation of uracil or thymine by Mn^VI in aqueous alkali shows first order dependence on [MnO ₄ ²⁻ ] and fractional order dependence on [uracil] or [thymine]. The effect of ionic strength is negligible on the rate of oxidation. The product of oxidation is the 5,6-dihydroxy compound. A mechanism is proposed involving the formation of a 5-membered cyclic complex between the substrate and manganate in a fast step, which subsequently disproportionates to give the final products. The large negative values of the entropy of activation suggest the formation of an intermediate complex. The formation constants of the latter and the rate of its disproportionation have been calculated from the derived rate law. The rate of oxidation of thymine is faster than that of uracil.     

Linear free energy relationships in the kinetic study of oxidation of phenols by quinolinium dichromate

The oxidation of ortho-, meta-, and para-substituted phenols by Quinolinium Dichromate (QDC) to the corresponding Quinones in aqueous acetic acid medium is first-order with respect to [QDC] and [phenol]. There is no effect of added Quinoline on rate. The reaction is acid catalyzed. A medium of low dielectric constant favors the oxidation process, and the rate of the oxidation process changes with change in the ionic strength of the medium. Electron releasing groups on the benzene ring enhance the rate of oxidation, while electron withdrawing groups retard, compared to the unsubstituted phenol. A correlation exists between log k₂ and σ, the Hammett's substituent constant, with a slope of −3.79 at 303 K. Analysis of the rate data using σ_I and σ_R values indicate that both inductive and resonance effects of the substituents equally influence the rate of the reaction. The free energy of activation, ΔG^≠, is linearly correlated with σ. A probable mechanism is proposed. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 437–443, 1997.     

Indirect spectrophotometric determination of diltiazem hydrochloride in pure form and pharmaceutical formulations

Three simple, accurate, and sensitive spectrophotometric methods (A, B and C) have been described for the indirect assay of diltiazem hydrochloride (DIL.HCl), either in pure form or in pharmaceutical formulations. The first method (A) is based on the oxidation of DIL.HCl by N-bromosuccinimide (NBS) and determination of unconsumed NBS by measuring the decrease in absorbance of amaranth dye (AM) at a suitable λ _max =521 nm. Other methods (B) and (C) involve the addition of excess cerric ammonium sulfate (CAS) and subsequent determination of the unconsumed oxidant by a decrease in the red color of chromotrope 2R (C2R) at a suitable λ _max =528 nm or a decrease in the orange-pink color of rhodamine 6G (Rh6G) at λ _max =525 nm, respectively. Regression analysis of Beer-Lambert plots showed good correlation in the concentration ranges 3.0–9.0, 3.5–7.0 and 3.5–6.3 μg ml⁻¹ for methods A, B and C, respectively. The apparent molar absorptivity, Sandell's sensitivity, detection and quantification limits were calculated. The proposed methods have been applied successfully for the analysis of the drug in its pure form and its dosage form. No interference was observed from a common pharmaceutical adjuvant. Statistical comparison of the results with the reference method shows excellent agreement and indicates no significant difference in accuracy and precision.     

Simultaneous determination of ascorbic acid and acetylsalicylic acid in pharmaceutical formulations

A direct, simple, and practical first-derivative spectrophotometric method is described for simultaneous determination of ascorbic acid and acetylsalicylic acid. The effects of the solvent, excipients, and spectral variables on the analytical signal were investigated. The drugs were determined simultaneously with a 0.01 M methanolic hydrochloric acid solution as the solvent, and the signals were evaluated directly by using the zero-crossing method at 245.0 and 256.0 nm for acetylsalicylic acid and ascorbic acid, respectively. The method allows the simultaneous determinations of acetylsalicylic acid and ascorbic acid in the ranges of 6.6 x 10(-6) to 1.5 x 10(-4)M and 3.4 x 10(-6) to 2.0 x 10(-4)M, respectively, with standard deviation of <2.0%. The proposed method was applied to determinations of these drugs in tablets.     

Kinetic spectrophotometric determination of hyoscine butylbromide in pure form and in pharmaceutical formulations

A simple and sensitive kinetic method was described for the determination of hyoscine butylbromide in pharmaceutical preparations. The method is based upon a kinetic investigation of the oxidation reaction of the drug with alkaline potassium permanganate at room temperature for a fixed time of 15 min. The absorbance of the colored manganate ion was measured at 610 nm. The absorbance–concentration plot was rectilinear over the range of 1.0–10 μg mL^?1 (r = 0.9999) and detection limit of 0.092 μg mL^?1. The concentration of hyoscine butylbromide was calculated using the corresponding calibration equation for the fixed-time method. The determination of hyoscine butylbromide by the fixed-concentration and rate constant methods is also feasible with the calibration equations obtained but the fixed-time method has been found to be more applicable. The different experimental parameters affecting the development and stability of the colors were carefully studied and optimized. The proposed method was applied to the determination of hyoscine butylbromide in pharmaceutical formulations. The results obtained were in good agreement with those obtained using the official British Pharmacopeial method (2004).     

Spectrophotometric determination of hyoscine butylbromide and famciclovir in pure form and in pharmaceutical formulations

A simple, rapid, and extractive spectrophotometric method was developed for the determination of hyoscine butylbromide (HBB) and famciclovir (FCV) in pure and pharmaceutical formulations. These methods are based on the formation of yellow ion-pair complexes between the basic nitrogen of the drug and four sulphonphthalein acid dyes, namely; bromocresol green (BCG), bromothymol blue (BTB), bromocresol purple (BCP) and bromophenol blue (BPB) in phthalate buffer of pH range (3.0-3.5). The formed complexes were extracted with chloroform and measured at 420, 412, 409 and 415nm for HBB and at 418, 412, 407 and 414nm for FCV using BCG, BTB, BCP and BPB, respectively. The analytical parameters and their effects on the reported systems are investigated. Beer's law was obeyed in the range 1.0-20mugmL(-1) with correlation coefficient (n=6)>/=0.9997. The molar absorptivity, Sandell sensitivity, detection and quantification limits were also calculated. The composition of the ion pairs was found 1:1 by Job's method in all cases and the conditional stability constant (K(f)) of the complexes have been calculated. The free energy changes (DeltaG) were determined for all complexes formed. The proposed methods have been applied successfully for the analysis of the studied drugs in pure and pharmaceutical formulations with percentage recoveries ranges from 99.84 to 100.26. The results were in good agreement with those obtained by the official methods.     

Oxidation of isoniazid by N‐haloarenesulfonamidates in alkaline medium: A kinetic and mechanistic study

The kinetics of oxidation of Isoniazid (INH) by sodium N‐haloarenesulfonamidates, chloramine‐T (CAT), bromamine‐T (BAT), chloramine‐B (CAB), and bromamine‐B (BAB), has been studied in alkaline medium at 303 K. The oxidation reaction follows identical kinetics with a first‐order dependence on each [oxidant] and [INH] and an inverse fractional‐order on [OH^−:]. Addition of the reaction product (p‐toluenesulfonamide or benzenesulfonamide) had no significant effect on the reaction rate. Variation of ionic strength and addition of halide ions have no influence on the rate. There is a negative effect of dielectric constant of the solvent. Studies of solvent isotope effects using D₂O showed a retardation of rate in the heavier medium. The reaction was studied at different temperatures, and activation parameters have been computed from the Arrhenius and Eyring plots. Isonicotinic acid was identified as the oxidation product by GC‐MS. A two‐pathway mechanism is pro‐posed in which RNHX and the anion RNX⁻ interact with the substrate in the rate‐limiting steps. The mechanism proposed and the derived rate laws are consistent with the observed kinetics. The rate of oxidation of INH increases in the order: BAT > BAB > CAT > CAB. This effect is mainly due to electronic factors. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 221–230, 2000     

Oxidation of diclofenac sodium by diperiodatoargantate(III) in aqueous alkaline medium and its determination in urine and blood by kinetic methods

The kinetics and oxidation of diclofenac sodium (DFS) by diperiodatoargentate(III) (DPA) in alkaline medium at 298 K and at a constant ionic strength of 0.60 mol dm^?3 were studied spectrophotometrically. The oxidation products were [2‐(2,6‐dicloro‐phynylamino)‐phenyl]‐methenol and Ag(I), identified by LC‐ESI‐MS and IR spectral studies. The reaction between DFS and DPA in alkaline medium exhibits 1:1 stoichiometry. The reaction is first order in [DPA] and has a less than unit order dependence each in [DFS] and [alkali]. Increasing concentrations of IO^?₄ retard the reaction. The active species of DPA proposed to be monoperiodatoargentate(III), and a mechanism is suggested. The rate constants involved in the different steps of the mechanism were determined and are discussed. The activation parameters with respect to a rate‐limiting step of the mechanism were determined. The thermodynamic quantities were also determined. Using the oxidation of DFS by DPA, DFS was analyzed by kinetic methods in urine and blood sample. The proposed method enables DFS analysis in the range from 5.0 × 10^?5 to 5.0 × 10^?3 mol dm^?3. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 336–346, 2010     

Validated kinetic spectrophotometric method for the determination of metoprolol tartrate in pharmaceutical formulations

A kinetic spectrophotometric method has been described for the determination of metoprolol tartrate in pharmaceutical formulations. The method is based on reaction of the drug with alkaline potassium permanganate at 25+/-1 degrees C. The reaction is followed spectrophotometrically by measuring the change in absorbance at 610 nm as a function of time. The initial rate and fixed time (at 15.0 min) methods are utilized for constructing the calibration graphs to determine the concentration of the drug. Both the calibration graphs are linear in the concentration range of 1.46 x 10(-6)-8.76 x 10(-6) M (10.0-60.0 microg per 10 ml). The calibration data resulted in the linear regression equations of log (rate)=3.634+0.999 log C and A=6.300 x 10(-4)+6.491 x 10(-2) C for initial-rate and fixed time methods, respectively. The limits of quantitation for initial rate and fixed time methods are 0.04 and 0.10 microg ml(-1), respectively. The activation parameters such as E(a), DeltaH(double dagger), DeltaS(double dagger) and DeltaG(double dagger) are also evaluated for the reaction and found to be 90.73 kJ mol(-1), 88.20 kJ mol(-1), 84.54 J K(-1) mol(-1) and 63.01 kJ mol(-1), respectively. The results are validated statistically and through recovery studies. The method has been successfully applied to the determination of metoprolol tartrate in pharmaceutical formulations. Statistical comparison of the results with the reference method shows excellent agreement and indicates no significant difference in accuracy and precision.     

Oxidation of aniline and some para-substituted anilines by benzimidazolium fluorochromate in aqueous acetic acid medium – A kinetic and mechanistic study

The oxidation kinetics of some para-substituted anilines by benzimidazolium fluorochromate (BIFC) have been studied in aqueous acetic acid media in the presence of perchloric acid. The reaction is first order with respect to both aniline and BIFC and is catalysed by a hydrogen ion. The rate data obey Hammett relationship. The products of oxidation are the corresponding azo benzenes. Based on the kinetic results and product analysis, a suitable mechanism has been proposed for the reaction of BIFC with anilines.     

Sequential injection technique for the determination of chlorpromazine hydrochloride in pure form and pharmaceutical formulations

A simple, economical, and automated spectrophotometric method for the determination of chlorpromazine hydrochloride by sequential injection analysis using ammonium metavanadate as colorimetric reagent is proposed. The various chemical and physical conditions that affected the reaction have been thoroughly investigated. The calibration curve was linear within the range 10–100 μg/mL. The detection limit (S/N = 3) was 0.7 μg/mL and the limit of quantification (S/N = 10) was 2.3 μg/mL. The sampling frequency was 22 h⁻¹. The method has been used for the determination of chlorpromazine hydrochloride in pure form and pharmaceutical formulations. The t-test has revealed that there is no evidence of significant differences between the obtained results at the 95% confidence level. The method can be applied to the quantitative determination of chlorpromazine hydrochloride. It is also applicable in the quality control of chlorpromazine hydrochloride preparations. The text was submitted by the authors in English.     

Spectrophotometric determination of paracetamol in drug formulations by oxidation with potassium dichromate

A rapid spectrophotometric method for determination of paracetamol is described, based on oxidation with dichromate for 15 min in 6M sulphuric acid at 80 degrees , and measurement (at 580 nm) of the chromium(III) formed. The method is applied to the determination of paracetamol in drugs prescribed for colds, coughs and flu. Of the common pharmaceuticals associated with paracetamol, only ascorbic acid and acetylsalicylic acid interfere. The results have been statistically compared with those obtained by the official (BP) and cerium(IV) methods.