M. Inés Toral

Polarographic behaviour and determination of ranitidine in pharmaceutical formulations and urine

A polarographic method for the determination of ranitidine is described, based on the reduction of the nitro group at a dropping-mercury electrode. The proposed method permits the drug to be determined, without any prior separation or extraction, in pharmaceutical formulations and urine at levels at which the unchanged drug is excreted. The current is diffusion controlled and proportional to concentration from 3.58 x 10(-3) to 1.50 mmol l-1. The detection limit is 1.07 x 10(-3) mmol l-1. The reduction process was studied at different pH values and a reduction mechanism is proposed in which the importance of the homogeneous chemical reactions associated with the electron-transfer steps are indicated.

Simultaneous determination of amiloride and furosemide in pharmaceutical formulations by first digital derivative spectrophotometry

This work presents a simple and fast method for the simultaneous determination of amiloride and furosemide by digital derivative spectrophotometry. HCl 1 x 10(-2) mol/l dissolved in ethanol was used as solvent and to extract drugs from formulations. Subsequently the samples were evaluated directly by first digital derivative spectrophotometry, using a smoothing factor of 8 and scale factor of 1 x 10(-4). The simultaneous determination of furosemide and amiloride can be carried out at 241.4 and 343.6 nm, respectively. In both cases, the zero crossing approach was used. When both compounds are present together in a sample, it is possible to quantify one in the presence of the other, without mutual interference. The determination range was found to be of 6.9 x 10(-8) to 16 x 10(-5) and 6.8 x 10(-8) to 8 x 10(-5) mol/l, for amiloride and furosemide, respectively. A good level of repeatability (RSD) of 0.9 and 0.6% was observed for amiloride and furosemide, respectively. The ingredients commonly found in commercial pharmaceutical formulations do not interfere. The proposed method was applied to the determination of these drugs in pharmaceutical formulations.

Voltammetric behaviour of nitrofurazone, furazolidone and other nitro derivatives of biological importance

In pyridine-formic acid buffer and tetramethylammonium chloride solution of pH 4.5 at a dropping mercury or a glassy carbon electrode, nitrofurazone, furazolidone and nitrofurantion are reduced in a single six-electron wave, while chloramphenicol and other structurally related nitro derivatives are reduced in only one four-electron wave, the nitro group being reduced to the amine or to the hydroxylamine, respectively. The electrochemical behaviour of these compounds depends mainly on the nature and position of the substituents. Reduction to the primary amine occurs when the substituents possess available π electrons to conjugate with the nitro group of the aromatic ring, which determines the transformation of the hydroxylamine into the amine via formation of a highly reducible intermediate imine or a quinonoid structure. In contrast, if the formation of the intermediate imine is made impossible by an adverse effect of the substituent, the hydroxylamine does not undergo further reduction.Cyclic voltammograms were recorded at different pH values and at different scan rates in order to identify certain relatively unstable species. The effect of pH on the diffusion-limited current and on the E½ values of the polarographic waves was also studied and the results obtained were compared with those obtained by cyclic voltammetry.On this basis, and according to the polarographic and cyclic voltammetric data, a reduction mechanism for the nitrofuran derivatives is suggested, in which the importance of the homogeneous chemical reactions associated with the electron-transfer steps is examined.

Flow Injection Photometric Determination of Zinc and Copper With Zincon Based on the Variation of the Stability of the Complexes With pH

A flow injection photometric method for the sequential determination of zinc and copper in mixtures was developed based on the variation of the stability of the chromogenic complexes between the analytes and the reagent zincon with pH. At pH 5.0 only the Cu-zincon complex exists, whereas at pH 9.0 the copper and zinc chelates co-exist. A three-channel manifold was implemented containing two alternating buffer streams (pH 5 and 9) which permit the colored reaction products to be formed sequentially at both pH values, and consequently the mixtures can be resolved. A continuous preconcentration unit (Chelex-100) was used in order to increase the sensitivity of the method, thus allowing the analysis of water samples in which the analytes are present at the ng ml-1 level. On the other hand, preconcentration was not required when the analytes were determined in brass. Under the optimum conditions and using a preconcentration time of 2 min, the detection limits (3 sigma) were found to be 0.35 and 0.80 ng ml-1 for zinc and copper, respectively. The repeatability of the method, expressed as the RSD, was in all instances less than 3.1%. Considering the sequential determination of both species, a sampling rate of 70 h-1 was obtained if preconcentration of the samples was not required.

Determination of crystal violet in water by direct solid phase spectrophotometry after rotating disk sorptive extraction

The microextraction of crystal violet (CV) from water samples into polydimethylsiloxane (PDMS) using the rotating disk sorptive extraction (RDSE) technique was performed. The extracting device was a small Teflon disk that had an embedded miniature magnetic stirring bar and a PDMS (560 μL) film attached to one side of the disk using double-sided tape. The extraction involves a preconcentration of CV into the PDMS, where the analyte is then directly quantified using solid phase spectrophotometry at 600 nm. Different chemical and extraction device-related variables were studied to achieve the best sensitivity for the determination. The optimum extraction was performed at pH 14 because under this condition, CV is transformed to the neutral and colorless species carbinol, which can be quantitatively transferred to the PDMS phase. Although the colorless species is the chemical form extracted in the PDMS, an intense violet coloration appeared in the phase because the -OH bond in the carbinol molecule is weakened through the formation of hydrogen bonds with the oxygen atoms of the PDMS, allowing the resonance between the three benzene rings to compensate for the charge deficit on the central carbon atom of the molecule. The accuracy and precision of the method were evaluated in river water samples spiked with 10 and 30 μg L(-1) of CV, yielding a relative standard deviation of 6.2% and 8.4% and a recovery of 98.4% and 99.4%, respectively. The method detection limit was 1.8 μg L(-1) and the limit of quantification was 5.4 μg L(-1), which can be decreased if the sample volume is increased.

Simultaneous determination of iron and ruthenium as ternary complexes by extractive second derivative spectrophotometry.

A highly sensitive and selective second derivative spectrophotometric method has been developed for the determination of ruthenium and iron in mixtures. The method is based on the formation of the binary complexes of iron and ruthenium with 4,7-diphenyl-1,10-phenanthroline (bathophenanthroline) in the presence of ethyleneglycol. These complexes are formed at pH 4.0-6.0 upon heating at 90 degrees C for 60 min. The ternary perchlorate complexes are then separated by liquid-liquid extraction. The extracts were evaluated directly by derivative spectrophotometric measurement, using the zero-crossing approach for determination of both analytes. Ruthenium and iron were thus determined in the ranges 9.6-450 and 16.3-280 ng/ml, respectively, in the presence of one another. The detection limits achieved (3sigma) were found to be 2.9 ng/ml of ruthenium and 4.9 ng/ml of iron. The relative standard deviations were in all instances less than 1.5%. The proposed method was applied to the determination of both analytes in synthetic mixtures.

Simultaneous determination of chlordiazepoxide and clidinium bromide in pharmaceutical formulations by derivative spectrophotometry

A direct and simple first derivative spectrophotometric method has been developed for the simultaneous determination of clidinium bromide and chlordiazepoxide in pharmaceutical formulations. Acetonitrile was used as solvent for extracting the drugs from the formulations and subsequently the samples were evaluated directly by derivative spectrophotometry. Simultaneous determination of the drugs can be carried out using the zero-crossing method for clidinium bromide at 220.8 nm and the graphical method for chlordiazepoxide at 283.6 nm. The calibration graphs were linear in the ranges from 0.983 to 21.62 mg/l of clidinium bromide and from 0. 740 to 12.0 mg/l of chlordiazepoxide. The ingredients commonly found in commercial pharmaceutical formulations do not interfere. The proposed method was applied to the determination of these drugs in tablets.

Determination of mefenamic acid and paracetamol by first derivative spectrophotometry

Abstract A direct and simple first derivative spectrophotometric method has been developed for the determination of mefenamic acid and paracetamol in pharmaceutical formulations. A methanolic hydrochloric acid solution was used as solvent for extracting the drugs from the formulations and subsequently the samples were evaluated directly by derivative spectrophotometry. Simultaneous determination of both drugs can be carried out using the zero-crossing and the graphical methods. The methods do not require simultaneous equations to be solved. The calibration graphs were linear in the ranges from 1.8 × 10−6 to 1.6 × 10−4 M of mefenamic acid and from 4.1×10−6 to 1.4 × 10−4 M of paracetamol. The ingredients commonly found in commercial pharmaceutical formulations do not interfere. The proposed method was applied to the determination of these drugs in tablets.

SIMULTANEOUS DETERMINATION OF ACETAMINOPHEN AND TRAMADOL BY SECOND DERIVATIVE SPECTROPHOTOMETRY

A rapid method for the simultaneous determination of acetaminophen (A) and tramadol (T) by second derivative spectrophotometric has been developed. From a solvent effect studies and the spectral behaviours of A and T, ethanol was selected as solvent. For a Δλ value of 210 nm a smoothing factor of 8,000 and scale factor of 1,000,000 were selected, because in these conditions the signal/noise ratio was favoured in order to avoid error. In these conditions also it is possible the simultaneous determination of A/T in a molar relation of 17/1 contained in pharmaceutical formulations. At 285.7 nm the second derivative value is T concentration dependent, corresponding to zero-crossing point of A. On the other hand, T does not absorb between 296.0 to 400.0 nm, thus 308.0 nm was selected for A determination by graphic method. The determination ranges for A and T were 8.1x10-7 - 5 1x10-5 mol/L and 3.4 x10-7 - 5.0 x10-5 mol/L, respectively and can be determined with good precision and accuracy, without previous separation. A study of interferents, was carried out by using the excipients Zafin® tablet. The recoveries were 97.4 ± 2.0% and 100.6 ± 1.7% for A and T, respectively. This tablet containing both drugs was assayed using the methods developed, showing a good accuracy and precision.

Simultaneous determination of platinum and palladium by second derivative spectrophotometry using 3-(2'-thiazolylazo)-2,6- diaminopyridine as chromophore ligand

ABSTRACT A second derivative spectrophotometric method has been developed for the determination of palladium and platinum in mixtures. The method is based on the formation of the platinum and palladium complexes with 3-(2-thiazolylazo)-2, 6-diaminopyridine, (2, 6-TADAP), in the presence of 1.7 M perchloric acid solution, upon heating at 90° C for 30 min and on the subsequent direct derivative spectrophotometric measurement. The zero-crossing approach and the graphic method were used for determination of platinum and palladium, respectively. Each analyte was determinated in the presence of one another in the ranges 8.9×10-7 -3.1×10-5 M for platinum and 4.6×10-7 - 6.8×10-5 M, for palladium. The detection limits achieved (3a) were found to be 2.7×10-7 M of platinum and 1.4×10-7 M of palladium. The relative standard deviations were in all instances less than 1.0%. In this work is included a study of effect of interferents and the application of the proposed method in synthetic mixtures.