Kinetic study of the fast step of the alkaline hydrolysis of p-chloranil using stopped flow technique

The alkaline hydrolysis of p-chloranil or 2,3,5,6-tetrachloro-1,4-benzoquinone (C₆Cl₄O₂, Q) was studied, using stopped flow spectrophotometry and Electron Spin Resonance techniques (E.S.R.). In the present study it was shown for the first time, that a free radical is produced chemically and that it can account for the propagation of the reaction. It was found that in alkaline conditions chloranil in a “Michael” fashion undergoes 1,2 addition being hydrolyzed and in turn produces a chloranil free radical (Q^•) The hydrolysis then proceeds via a number of intermediates yielded by this radical and a number of different products is formed. The formation of these products, both quantitatively and qualitatively has a strong dependence on the concentration of the OH⁻ species and chloranil. The various possible routes of the hydrolysis are studied either spectrophotometrically or by E.S.R. Two different intermediates are observed absorbing at 426 nm and at 540 nm, respectively. Each species was formed and destroyed within 10 s to 30 min depending on the exact conditions. The reaction rate constants for the formation and the decay of the intermediates was estimated using the Guggenheim method. At both wavelengths the rate constants seem to have a complex relation to the concentration of the anion. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 385–391, 1997     

Kinetic study of the acid hydrolysis of sucrose and lactose and kinetic determination of sucrose using a periodate-selective electrode

A novel procedure for the kinetic study of the acid hydrolysis of sucrose and lactose has been developed. The parameter observed is the rate of the reaction of periodate with quenched aliquots of the solution in which hydrolysis takes place. This rate measured with a periodate-selective electrode, and is expressed in terms of rate of increase in potential (gDE/Δt). Based on the observed differences in the hydrolysis rate, a kinetic method has been developed for the determination of sucrose in the presence of other carbohydrates in the concentration range 0.01–0.10 M with an average error of 1.4%. The method has been applied to the determination of sucrose in milk products and soft drinks.     

Potentiometric microdetermination of catechol in the presence of its isomers using a lead-ion selective electrode

Summary Catechol was determined on the microscale by potentiometric titration with lead nitrate. Solutions were buffered to pH 9.5 with borate buffer. Emf's were monitored with a lead ion-selective electrode and a single junction reference electrode. Up to an eightfold excess of hydroquinone or resorcinol can be tolerated.

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Potentiometrische Mikrobestimmung von Brenzcatechin in Gegenwart seiner Isomeren mit Hilfe einer blei-spezifischen Elektrode

Zusammenfassung Mikromengen Brenzcatechin wurden mit Bleinitrat potentiometrisch titriert. Die Lösungen wurden mit Borat auf pH 9,5 gepuffert. Die EMK wurde mit einer blei-spezifischen Elektrode gemessen. Hydrochinon und Resorcin stören bis zum achtfachen Überschuß nicht.

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Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore Laboratory under Contract W-7405-Eng-48.     

Kinetic study of N-bromosuccinimide reactions and kinetic determination of pyridoxine using a bromide-selective electrode

The bromide released by the alkaline hydrolysis of N-bromosuccinimide and its reactions with bromide, pyridoxine and thiamine have been studied potentiometrically using a solid-state bromide-selective electrode to monitor the consumed or produced bromide ion. Potential-time indications are obtained using a microcomputer-controlled potentiometric system. The overall rate constants and the activation energies have been calculated. A kinetic-potentiometric procedure for the determination of pyridoxine in the presence of thiamine in pharmaceutical preparations, based on its reaction with N-bromosuccinimide, is presented.     

Potentiometric stripping determination of heavy metals using a graphite-reinforcement carbon vibrating electrode

Potentiometric stripping determination of Cd, Cu and Zn using a vibrating electrode (VE) is presented. A simple VE was constructed by using a piezoelectric bimorph oscillator and an inexpensive graphite-reinforcement carbon (GRC) rod (a mechanical pencil). Experimental results obtained with the simple VE follow the equation valid for quantitative application of potentiometric stripping analysis (PSA) in large solutions. It was found that the GRC vibrating electrode is suitable for multielement trace analysis of small samples of 0.05 mL. The relative standard deviations for divalent metals are better than 2%.     

Potentiometric stripping determination of heavy metals using a graphite-reinforcement carbon vibrating electrode

Potentiometric stripping determination of Cd, Cu and Zn using a vibrating electrode (VE) is presented. A simple VE was constructed by using a piezoelectric bimorph oscillator and an inexpensive graphite-reinforcement carbon (GRC) rod (a mechanical pencil). Experimental results obtained with the simple VE follow the equation valid for quantitative application of potentiometric stripping analysis (PSA) in large solutions. It was found that the GRC vibrating electrode is suitable for multielement trace analysis of small samples of 0.05 mL. The relative standard deviations for divalent metals are better than 2%. Received: 3 February 1997 / Revised: 2 June 1997 / Accepted: 7 June 1997     

Potentiometric determination of acids and bases using a silica gel based carbon-epoxy indicator electrode

The construction and the application of a silica gel based carbon-epoxy indicator electrode for the potentiometric determination of acids and bases are described. The effect of composition of silica gel and carbon-epoxy, slope (mV/pH), linear response (pH range) and the use for acid-base titrations were investigated. The data obtained for the acid-base titrations were compared with those obtained using a glass electrode in the same conditions. The electrode showed a linear response in the pH 2 to 13 range with a slope of -40.5 +/- 0.4 mV/pH (at 25 degrees C) and a response time of less than 15 s. The lifetime of the electrode was higher than one year (over 6000 determinations) with a decrease of only 5% of the initial potentiometric response. The silica gel based carbon-epoxy electrode showed excellent results in the end-point indication potentiometric titrations in determination of acids and bases. The miniaturization of the proposed electrode for flow injection analysis was investigated.     

Potentiometric determination of acids and bases using a silica gel based carbon-epoxy indicator electrode

The construction and the application of a silica gel based carbon-epoxy indicator electrode for the potentiometric determination of acids and bases are described. The effect of composition of silica gel and carbon-epoxy, slope (mV/pH), linear response (pH range) and the use for acid-base titrations were investigated. The data obtained for the acid-base titrations were compared with those obtained using a glass electrode in the same conditions. The electrode showed a linear response in the pH 2 to 13 range with a slope of –40.5 ± 0.4 mV/pH (at 25 °C) and a response time of less than 15 s. The lifetime of the electrode was higher than one year (over 6000 determinations) with a decrease of only 5% of the initial potentiometric response. The silica gel based carbon-epoxy electrode showed excellent results in the end-point indication potentiometric titrations in determination of acids and bases. The miniaturization of the proposed electrode for flow injection analysis was investigated. Received: 11 August 1999 / Revised: 4 October 1999 / Accepted: 7 October 1999     

Potentiometric determination of potassium cations using a nickel(II) hexacyanoferrate-modified electrode

Electroactive nickel(II) hexacyanoferrate (NiHCF) thin film modified electrodes are effective potentiometric sensors for the determination of potassium ions. The NiHCF films are deposited onto glassy carbon electrodes by repetitive potential cycling in K(3)Fe(CN)(6)/NaNO(3)/Ni(NO(3))(2) solution. The modified electrodes exhibit a linear response to potassium ions in the concentration range 1x10(-3) to 2.0 mol dm(-3), with a near-Nernstian slope (45-49 mV per decade) at 25 degrees C. In the determination of potassium ion in syrups used for treatment of potassium deficiency, the NiHCF-modified electrode gave comparable results to those obtained using flame emission spectrophotometry.     

Potentiometric determination of sulfonamides with a silver sulfide electrode

The silver sulfide membrane electrode shows well-defined responses to several pharmaceutically important sulfonamides over the concentration range 10^-1–10^-4 M. Both direct and titrimetric determinations of sulfonamides are described and applied to pharmaceutical dosage forms.     

Spectrophotometric and spectrodensitometric determination of Clopidogrel Bisulfate with kinetic study of its alkaline degradation

Four sensitive, selective and precise stability-indicating methods for the determination of Clopidogrel Bisulfate (CLP) in presence of its alkaline degradate and in pharmaceutical formulations were developed and validated. Method A is a second derivative (D²) spectrophotometric one, which allows the determination of CLP in presence of its alkaline degradate at 219.6, 270.6, 274.2 and 278.4 nm (corresponding to zero-crossing of the degradate) over a concentration range of 4-37 μg mL⁻¹ with mean percentage recoveries 99.81 ± 0.893, 99.72 ± 0.668, 99.88 ± 0.526 and 100.46 ± 0.646, respectively. CLP can be determined in the presence of up to 65% of its degradate. D² method was used to study the kinetic of CLP alkaline degradation that was found to follow a first-order reaction. The t_1/2 was 6.42 h while K (reaction rate constant) was 0.1080 mol/h. Method B is the first derivative of the ratio spectra (DD¹) spectrophotometric method, by measuring the peak amplitude at 217.6 and 229.4 nm using acetonitrile and CLP can be determined in the presence of up to 70% of its degradate. The linearity range was 5-38 μg mL⁻¹ with mean percentage recoveries 99.88 ± 0.909 and 99.70 ± 0.952, respectively. Method C based on the determination of CLP by the bivariate calibration depending on simple mathematic algorithm which provides simplicity and rapidity. The method depends on quantitative evaluation of the absorbance at 210 and 225 nm over a concentration range 5-38 μg mL⁻¹ with mean percentage recovery 99.27 ± 1.115. CLP can be determined in the presence of up to 70% of its degradate. Method D is a TLC-densitometric one, where CLP was separated from its degradate on silica gel plates using hexane:methanol:ethyl acetate (8.7:1:0.3, v/v/v) as a developing system. This method depends on quantitative densitometric evaluation of thin layer chromatogram of CLP at 248 nm over a concentration range of 0.6-3 μg/band with mean percentage recovery 99.97 ± 1.161. CLP can be determined in the presence of up to 90% of its alkaline degradate. The selectivity of the proposed methods was checked using laboratory prepared mixtures. The proposed methods have been successfully applied to the analysis of CLP in pharmaceutical dosage forms without interference from other dosage form additives and the results were statistically compared with the official method.     

Potentiometric determination of cetylpyridinium chloride using a new type of screen-printed ion selective electrodes

A new type of screen-printed ion-selective electrode for the determination of cetylpyridinium chloride (CPC) is presented. These new electrodes involve in situ, modified and unmodified screen-printed ion-selective electrodes for the determination of CPC. The screen-printed electrodes (SPEs) show a stable, near-Nernstian response for 1 × 10⁻² to 1 × 10⁻⁶ M CPC at 25 °C over the pH range 2-8 with cationic slope 60.66 ± 1.10. The lower detection limit is found to be 8 × 10⁻⁷ M and response time of about 3 s and exhibit adequate shelf-life (6 months). The fabricated electrodes can be also successfully used in the potentiometric titration of CPC with sodium tetraphenylborate (NaTPB). The analytical performances of the SPEs are compared with those for carbon paste electrode (CPE) and polyvinyl chloride (PVC) electrodes. The method is applied for pharmaceutical preparations with a percentage recovery of 99.60% and R.S.D. = 0.53. The frequently used CPC of analytical and technical grade as well as different water samples has been successfully titrated and the results obtained agreed with those obtained with commercial electrode and standard two-phase titration method. The sensitivity of the proposed method is comparable with the official method and ability of field measurements.     

Direct determination of benzylnitriles using the cyanide ion selective electrode

The response characteristics of the solid state cyanide ion selective electrode towards some benzylnitriles are investigated. In 10M KOH solution, the electrode exhibits nearly Nernstian response over the concentration range of 10^–2 to 10^–4 M of various substituted benzylnitriles with an anionic slope of 53–59 mV/concentration decade. The response time varies from 10 to 20 min depending on both the nature of the substituent group and the concentration of the nitrile compound. Direct potentiometric measurement of some nitrile compounds at the concentration level of 0.01 to 1 mg/ml shows an average recovery of 98.2% and a mean standard deviation of 2.3%. Many nitrogen functional groups do not interfere.     

Potentiometric determination of some common alkaloids with a picrate-selective electrode

Potentiometric methods are described for the determination of alkaloids, based on the formation of insoluble alkaloid picrate salts, using a picrate ion-selective indicating electrode. Micro-amounts of strychnine, papaverine, quinine and cocaine were determined by direct potentiometry and titrimetrically, using Gran plots, with average errors of about 4% and 2%, respectively. The method was successfully applied to pharmaceutical preparations.     

Potentiometric detection of ascorbate using a graphite carbon electrode

A graphite carbon electrode was used for the potentiometric detection of ascorbate. The electrode exhibits a linear response with a slope of -42+/-1.0 mV decade(-1) in concentrations ranging from 5x10(-4) to 5.0x10(-2) M in 0.1 M NaOH solution with a detection limit of 5.0x10(-6) M. The response mechanism of this electrode was investigated by potentiometry, voltammetry, and scanning electron micoscropy (SEM), and it suggests that the electrode potential change resulted from the ion-exchange adsorption and subsequent oxidation of ascorbate on the electrode surface at pH 12-13. The electrode exhibits high sensitivity, selectivity and reproducibility.     

A kinetic study of soluble glucose oxidase using a rotating-disc electrode

In order to determine the kinetic parameters of glucose oxidation catalysed by the enzyme glucose oxidase (GO) the initial velocity of hydrogen peroxide formation was measured using a rotating disc electrode (RDE). The major advantage of this method is the possibility of continuous measurement of the increase in hydrogen peroxide concentration. This means that the real initial reaction rate V₀ can be determined, which is required for constructing a double-reciprocal plot. Several combinations of substrate concentrations (glucose and oxygen) were used. The method, in which a platinum black RDE was used, appeared to be very useful. Product inhibition experiments showed that the ping-pong mechanism is valid for GO. The three kinetic parameters of this mechanism were determined by initial velocity experiments.     

Oxidation with alkaline permanganate using formic acid for the back-titration : Potentiometric determination of chromium

Determination of chromium by oxidation of chromite with permanganate does not give accurate results. KmnO₄ is reduced to MnO₂. Titration of KmnO₄ with Cr⁺³ solution in the presence of 0.8–1.5N NaOH and Ba⁺² ions yields manganate and gives good results. In the absence of Ba⁺² ions and in the presence of 0.5–2N NaOH reduction of KmnO₄ passes quantitatively to MnO₂.Cr⁺³ can be determined by adding the chromic solution to KmnO₄ while stirring in presence of 1N NaOH and Ba⁺² ions, or a. 2.5N NaOH in the absence ofBa⁺² ions. The excess KmnO₄ is then back-titrated with formic acid.     

Potentiometric stripping determination of nickel at a dimethylglyoxime-containing graphite paste electrode

Potentiometric stripping determination of nickel(II) can be performed at zero current with a graphite paste electrode chemically modified with dimethylglyoxime. Among oxidants investigated, atmospheric oxygen present in solution was selected as the best, providing sharp and reproducible analytical signals.