Potentiometric and Coulometric Determination of Carbimazole

ABSTRACT

A method for the determination of carbimazole involving its reaction with iodine in an alkaline medium is presented. In volumetric titration with potentiometric end-point detection the range of determination is 30–500 μmol (5.6–93 mg). In coulometric titration using biamperometric end-point detection, 0.5–20 μmol (0.09–3.7 mg) of carbimazole was successfully determined. The coulometric titration was applied to the determination of carbimazole in tablets.     

Maßanalyse und Katalyse

The application of automatic titration instruments for volumetric determinations using catalytic indicator reactions is described. Examples for a photometric (Mn²⁺ with EDTA), potentiometric (Hg²⁺ and Ag⁺ with KI, I^? with Hg²⁺) and biamperometric (Cu²⁺ with EDTA) regulation of the automatic titrator are given. In some of these methods, contrary to the usual way, it is the inhibitor which serves as a titrant (“titration with the brake”) and not the catalyst.     

Reaktion von Vanadium(IV) mit 3′-Pyridylfluoron und seine photometrische Bestimmung

The application of automatic titration instruments for volumetric determinations using catalytic indicator reactions is described. Examples for a photometric (Mn²⁺ with EDTA), potentiometric (Hg²⁺ and Ag⁺ with KI, I^? with Hg²⁺) and biamperometric (Cu²⁺ with EDTA) regulation of the automatic titrator are given. In some of these methods, contrary to the usual way, it is the inhibitor which serves as a titrant (“titration with the brake”) and not the catalyst.     

Potentiometric and Coulometric Determination of 5-Phenyl-1,3,4-Oxadiazole-2-Thiol

A method for the determination of 5-phenyl-1,3,4-oxadiazole-2-thiol involving its reaction with iodine in an alkaline medium has been presented. In volumetric titration with potentiometric endpoint detection, the determination range is 25–500 μmol. In coulometric titration using biamperometric endpoint detection, 0.5–5 μmol of 5-phenyl-1,3,4-oxadiazole-2-thiol were successfully determined. The errors and the relative standard deviations were below 1%. The shapes of the obtained potentiometric titration curves of the tested compound are unusual at higher concentrations of sodium hydroxide and depend on the material of the indicator electrode (platinum and gold). Addition of iodine resulted in a significant potential drop. The studied system did not exhibit Nernstian behavior. __________ From Zhurnal Analiticheskoi Khimii, Vol. 60, No. 10, 2005, pp. 1094–1097. Original English Text Copyright ? 2005 by Ciesielski, Krenc. The text was submitted by the authors in English.     

Potentiometric and coulometric titration of 2-thiobarbituric acid

A new method for the determination of 2-thiobarbituric acid, using its reaction with iodine in an alkaline medium is presented. In the volumetric titration with potentiometric end-point detection, the determinability range is 10-400 micromol (1.4-58 mg). In coulometric titration using the biamperometric end-point detection, 0.1-20 micromol (1.4 x 10(-2)-2.9 mg) of 2-thiobarbituric acid was successfully determined.     

Flow-injection single-point titration of acids with biamperometric detection at polarized platinum electrodes

Flow-injection single-point titration of acids is based on biamperometric measurement of iodine formed quantitatively in the reaction of iodide with iodate. Results are presented for determination of sulphuric, hydrofluoric, monochloroacetic, formic and acetic acids. The slopes of the calibration plots, which are linear for about 1–10 rum acid, depend on the strength of the acid, decreasing from sulphuric acid to acetic acid. With microcomputer on-line data processing, relative standard deviations were about 0.2%.     

Direct determination of hypoxen and its analogs by galvanostatic coulometry

A method is developed for determining microgram amounts of antioxidants, i.e., hypoxen and its analogs in model solutions by coulometric titration with electrogenerated halogens using the biamperometric indication of the titration endpoint. The relative standard deviation of the determination varies from 1 to 5%.     

Electrogenerated iodine as a reagent for coulometric titrations in alcoholic media

Conditins are established for the generation of iodine with high current efficiency (>99%) in solutions of potassium acetate in methanol or ethanol, in the presence of potassium iodide or tetraethylammonium iodide at platinum or glassy carbon working electrodes, respectively. Iodine solutions in methanol are stable, but solutions in ethanol are less so. Procedures are given for coulometric titration of various thiols with anodically generated iodine in these solvents; biamperometric or potentiometric end-point detection can be used. The error of the titrations did not exceed ± 2.0% for ca. 1-mg samples.     

In-capillary derivatization and determination of iodine in sodium chloride solution

A new capillary electrophoretic (CE) method was developed for the simple and selective determination of iodine in 0.5 mol l(-1) NaCl. The proposed method is based on the in-capillary derivatization of iodine with thiosulfate ions using the zone-passing technique and direct photometric detection of the iodide and tetrathionate formed. The optimal conditions for the separation and derivatization reaction were established by varying the concentration of iodine, electrolyte pH and applied voltage. The optimized separations were carried out in phosphate electrolyte (pH 6.86) using direct photometric detection at 253.7 nm. Common photometric detection absorbing anions such as Cl(-), NO(2)(-), S(2)O(3)(2-) did not give any interference. Valid calibration (r(2) = 0.994) is demonstrated in the range 16.5-198.1 mg l(-1) of iodine. The detection limit (calculated according to K. Doerffel, Statistik in der analytischen Chemie, 1990) was 11.53 mg l(-1) (by iodide peak area) and 8.45 mg l(-1) (by tetrathionate peak area). The proposed system was applied to the determination of iodine after oxidation of iodide in underground water.     

High-precision gravimetric coulometry using the silver-perchloric acid coulometer: Titration of arsenious oxide with electrogenerated iodine

High-precision gravimetric coulometry with a silver-perchloric acid coulometer is evaluated as an alternative to the conventional titrimetric method. The loss of weight (caused by electrolytic dissolution) of a highly pure silver anode in series with the cathode of a conventional constant-current titration system is measured and related to the number of equivalents of substance titrated. The precision of the method is determined by titrations of the Standard Reference Material 83C arsenious oxide (99.99% pure) with electrogenerated iodine, using biamperometric end-point detection. Depending on the size of the sample, an ultimate precision of 25 ppm is obtained. The assay for 0.5-g samples of the SRM material is 99.993(9) +/- 0.002(5)% purity.     

Determination of methylmercuric ions by photometric titration with dithizone

A rapid photometric titration titration technique, which makes it possible to determine methylmercuric ions with a sensitivity of 15mug, is described. The titration takes place in 80% aqueous ethanol, which is capable of dissolving the titrant, a chloroform solution of dithizone. It has thus been possible to avoid the difficulties arising from co-extraction phenomena that occur when the extractive titration procedures developed for mercury(II) are applied to the determination of methylmercuric ions. A formate buffer is used to keep the pH in the range 2.5-3.0. The attainable precision is considerably better than 1 %.     

Biamperometric end-point detection with quinhydrone electrodes in coulometric neutralization titrations in nonaqueous media

Some new possibilities for continuous coulometric titration of bases, alone and in mixture, as well as of mineral acids, alone and in mixture, were investigated with acetic anhydride-anhydrous acetic acid as solvents. The titration end point was determined biamperometrically. It was established that quinhydrone electrodes with platinum, palladium, and gold as the metal phase can be equally satisfactorily applied to the end-point determination of both the titrations of bases or acids alone, and of their two-component mixtures. Amounts of 6–10 μequiv. of bases, i.e., 20–47 μequiv. of mineral acids, were determined with the maximal average deviation of 1.4%. Results of biamperometric determinations of bases are compared to those of catalytic thermometric determinations, while those of acids are compared with the results of photometric titrations. The agreement between the methods is satisfactory.     

Coulometric titration with air transported sample

Air transported samples of a mixture containing the analyte and a supporting electrolyte are consecutively propelled into a microconduit system through a titration cell, a coil and a detector cell. Different amounts of electrochemically generated titrant are obtained in the separate samples while they are passing through the titration cell. Various quantities of the titrant are achieved by changing the duration of the titrant generating process that is done by various time for the sample solution staying in the titration cell (different sample movement rate). The titrated sample is then homogenised in the coil and detected in an electrochemical detector cell that supplies necessary data for the analyte determination. The declared method is experimentally verified on coulometric titration of hydrochloric acid (HCl) with electrogenerated sodium hydroxide and a titration of aniline with bromine generated in a bromide solution.     

Thin-layer hydrodynamic biamperometric end-point detection: application to the coulometric titration of arsenic(III) with bromine

A sandwich-type thin-layer cell has been applied to hydrodynamic biamperometric end-point detection in coulometric titrations. The detector cell, constructed from two pieces of teflon, has platinum indicator electrodes and is placed in a flow loop attached to the coulometric titration cell. The thin-layer cavity has a volume of 6.5 μl and a thickness of 51μm, which are obtained using teflon tape as a spacer. A peristaltic pump maintains a continual flow through the loop. The titration cell is a closed vessel completely filled with the solution to be titrated. Operational amplifier circuits control the potential difference applied across the two thin-layer electrodes, and to measure the current developed in the thin-layer cell. Noise arising from the pulsed nature of the flow is reduced by a factor of 25 by using an electronic filter. The titration of arsenic(III) with electrogenerated bromine was used to study the performance of the system. Concentrations from 24 ppb to 1.6 ppm can be determined with relative inaccuracies of +4 and +0.1%, respectively.     

Electrochemical Determination of Glutathione

Procedures were developed for determining glutathione by voltammetry and coulometric titration with electrogenerated oxidants using the biamperometric indication of the titration end-point. Possible mechanisms of the glutathione reaction with electrogenerated halogens are discussed. Microgram amounts of glutathione can be determined in model solutions with an RSD of 1–2%. The oxidation wave of glutathione in the voltammogram is observed at 0.95 V. At higher glutathione concentrations, the wave takes the shape of a peak. Glutathione concentration in the range between 9.15 × 10^–5 and 2.14 × 10^–3 M is a linear function of its oxidation wave height at a stationary platinum electrode in a 0.05 M H₂SO₄ solution. The determination limit for glutathione is 1.9 × 10^–5 M. The procedures for determining glutathione in human blood were proposed.     

Robust flow-batch coulometric/biamperometric titration system: determination of bromine index and bromine number of petrochemicals

A flow–batch system was constructed and evaluated to perform coulometric titrations with biamperometric end point detection. The flow section of the system is employed for sampling by injecting a sample volume (50–300 μL) in a flow injection-like system. About 1.5 mL of a suitable carrier solution is delivered by a peristaltic pump in order to quantitatively transfer the sample to the system titration cell (2.0 mL total inner volume). The carrier contains the coulometric precursor for the titrant species. The cell contains two pairs of platinum electrodes used for coulometric generation of reagent and biamperometric detection and is actively stirred. The titrant species is generated and the titration is performed by the usual batch procedure with the excess of titrant being detected by biamperometry following the analysis of the titration curve. System operation is computer controlled and all operations are automated, including titration curve analysis and cell cleaning after the titration is ended. The system is characterized by its robustness because its operation does not depend on flow rates, and the work using coulometric methods which generate gases at the counter-electrode is not troublesome. The flow–batch system has been evaluated for determination of bromine index and bromine number (relative to the total reactive olefin content) in petrochemicals according to an ASTM procedure. Typical precision (R.S.D.) is between 0.5 and 6% for different petrochemicals whose bromine number/index vary from 1000 to 10 mg of bromine per 100 g of sample, respectively. Recoveries for standard additions are between 92 and 123% for 10 mg of Br₂ per 100 g increments and 98 to 101% for 100 mg per 100 g increments. Accuracy of the proposed system was evaluated against results obtained by the standard ASTM with no significant difference detected at 95% confidence level.     

Practical method for evaluation of linearity and effective pathlength of on-capillary photometric detectors in capillary electrophoresis

The optical characteristics of on-capillary photometric detectors for capillary electrophoresis were evaluated and five commercial detectors were compared. Plots of sensitivity (absorbance/concentration) versus absorbance obtained with a suitable testing solution yield both the linear range and the effective path length of the detector. The detector linearity is a crucial parameter when using absorbing electrolytes, such as for indirect photometric detection, and especially for highly absorbing electrolyte probe ions. The upper limits of the linear ranges (determined as 5% decline in sensitivity) for five commercial detectors ranged from 0.175 to 1.2 AU. The effective pathlength reflects the quality of the optical design of the detector and is equal to the capillary internal diameter only for a light beam passing exactly through the capillary centre, but becomes progressively shorter for imperfect optical designs. The determined effective pathlength for the five investigated detectors ranged from 49.7 to 64.6 microm for a 75 microm I.D. capillary.     

Iodimetric determination of 2-mercaptopyrimidines

A new method for the determination of 2-mercaptopyrimidines, using their reaction with iodine in neutral and alkaline medium, is presented. The determinability range in the volumetric titration, in phosphate buffer with starch as an indicator, was found to be equal to 40-1000 mumol for 2-mercaptopyrimidine (I) and 100-2000 mumol for 2-mercapto-4-methylpyrimidine (II), and 200-2000 mumol for 4,6-dimethyl-2-mercaptopyrimidine (III). In the volumetric titration in alkaline medium, with the potentiometric end-point detection, the determinability range is 50-250 mumol for 2-mercapto-4-methylpyrimidine (II), 50-500 mumol for 4,6-dimethyl-2-mercaptopyrimidine (III), 20-250 mumol for 4,5-diamino-2,6-dimercaptopyrimidine (IV), and 20-1000 mumol for 2-thioorotic acid (V). In the coulometric titration, using the biamperometric end-point detection, 0.1-4.0 mumol of 2-mercaptopyrimidine (I) and 0.1-5.0 mumol of 2-thioorotic acid (V) were successfully determined.     

Determination of cysteine, 3-mercaptopropionic, and mercaptosuccinic acid with neutral hexacyanoferrate(III)

Two procedures are proposed for the determination of -SH groups. In both a known amount of hexacyanoferrate(III) is added to the sample and the excess is then determined. In one procedure an excess of ascorbic acid is added and the excess of that is titrated with iodine. In the other, the excess is determined by direct titration with ascorbic acid. Both procedures avoid errors resulting from side-reactions of iodine which takes place if the excess of hexacyariofcrrate(III) is reacted with potassium iodide.     

Determination of the drug content of pharmaceuticals containing phenothiazine compounds by triangle programmed flow titration

Summary A flow-through triangle programmed coulometric titration technique is worked out for the determination of phenothiazine compounds. The oxidation reaction of phenothiazines with hypobromite ion served as titration reaction. A biamperometric detector cell has been used for following the titration process. The titration technique has been used for the determination of the drug content (average and individual) of chlorpromazine, promethazine, diethazine and trifluoperazine in pharmaceutical preparations. The relative standard deviation was in the range of 2.0 to 3.5%.

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Bestimmung des Wirkstoffgehaltes von Phenothiazin-haltigen Pharmazeutica mit Hilfe einer Dreieck-programmierten Durchflu\-Titration