Spectrophotometric reaction rate method for determination of barbituric acid by inhibition of the hydrochloric acid-bromate reaction

A new kinetic-spectrophotometric method was developed for the determination of barbituric acid. The method is based on its inhibition effect on the reaction between hydrochloric acid and bromate. The decolorization of methyl orange by the reaction products was used to monitor the reaction spectrophotometrically at 510 nm. The variable affecting the rate of the reaction was investigated. The method is simple, rapid, relatively sensitive and precise. The limit of detection is 7.9x10(-7) M and calibration rang is 1x10(-6)-6.0x10(-4) M barbituric acid. The linearity range of the calibration graph is depends on bromate concentration. The relative standard deviation of seven replication determination of 5.6x10(-6) M barbituric acid was 1.8%. The influence of potential interfering substance was studied.     

Kinetic determination of ascorbic acid by the BZ oscillating chemical system

A new analytical method for the determination of ascorbic acid by the perturbation caused by different amounts of ascorbic acid on the BZ oscillating chemical system involving the Ce(IV)-catalyzed reaction between potassium bromate and malonic acid in a acidic medium is proposed. The method relies on the linear relationship between the change in the oscillation amplitude of the chemical system and the concentration of ascorbic acid, which is in this work exposed for the first time. The calibration curve is linearly proportional to the concentration of ascorbic acid over the range 3.5x10(-6)-4.7x10(-4) M, with the regression coefficient is 0.9975. Two different methodologies were used to address the determination. Some aspects of the potential mechanism of action of ascorbic acid on the BZ oscillating chemical system are discussed in detail.     

Flow-injection determination of ascorbic acid and cysteine simultaneously with spectrofluorometric detection.

A simple and sensitive spectrofluorometric method was developed for the simultaneous determination of ascorbic acid and cysteine by a flow-injection system. This method is based on the reduction of Tl(III) with ascorbic acid or cysteine in acidic media, producing fluorescence reagent, TlCl3(2-) (lambdaex = 227 nm, lambdaem = 419 nm). The injected sample solution was divided into two separate streams. The first stream was treated with Tl(III) at pH 3.0 and then passed through a 270 cm reaction coil to the flow cell of the spectrofluorometer, where the fluorescence intensity was measured. This signal is related to ascorbic acid and cysteine concentration. The second part of the injected sample solution was treated with Tl(III) in HCl solution and then passed through a 50 cm reaction coil to the flow cell and the fluorescence intensity was measured. This signal is related only to cysteine. Thus, the ascorbic acid content was determined directly by the difference according to the calibration curve. Ascorbic acid and cysteine can be determined in the range of 1 x 10(-6) to 5.0 x 10(-5) M, at a rate of 16 samples per hour. The limits of detection (S/N = 3) were 8 x 10(-7) M for ascorbic acid and 7 x 10(-7) M for cysteine. The influence of potential interfering substances was studied. The proposed method was successfully applied to the simultaneous determination of both analytes in real samples.     

Spectrophotometric determination of a nanomolar amount of ascorbic acid using its catalytic effect on copper(II) porphyrin formation

A simple, fast and sensitive flow-injection method is proposed for the determination of nanomolar amounts of ascorbic acid in tea, urine and blood. The procedure is based on the accelerating effect of a nanomolar level of ascorbic acid on the reaction of cooper(II) with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H(2)tmpyp(4+). Ascorbic acid reduces Cu(II) to Cu(I) which catalyzes the incorporation of Cu(II) into H(2)tmpyp(4+) to form Cu(II)(tmpyp)(4+). In this method two solutions, one containing ascorbic acid and H(2)tmpyp(4+) and the other containing copper(II) and acetate buffer (pH 5.0), were injected into two flowing streams of water through two sample injectors of 120 mu1 sample volume. The mixture was allowed to react in a 2 m reaction coil and the colored solution of Cu(II)(tmpyp)(4+) was monitored at 550 nm (epsilon = 2.01 x 10(4)M(-1)cm(-1)). The present method was applied to the determination of ascorbic acid in tea, urea and blood. Reducing agents such as sugars and vitamins B(1), B(2), B(6) and B(12) did not give serious errors at a concentration of 10(-6) M for the determination of 1.0 x 10(-8)M ascrobic acid. The relative standard deviation of the present method was 2.8% for the determination of 1.0 x 10(-8)M ascorbic acid. The reaction mechanism was clarified from the kinetic results of the formation of Cu(II)(tmpyp)(4+) in the presence of various concentrations of ascorbic acid, copper(II) and hydrogen ion.     

Determination of ascorbic acid in pharmaceuticals and urine by reverse flow injection.

Two reverse flow injection (FI) methods, using spectrophotometric detection, are proposed for the determination of ascorbic acid. Both methods are based on its reaction with the ethylenediaminetetraacetic acid-CoIII complex in a medium of 5% diethylamine. In the first method, using the peak-height FI technique, ascorbic acid is determined over the range from 2 x 10(-4) to 5 x 10(-3) mol dm-3 and in the second, using the peak-width FI method, the working range is extended (2 x 10(-3)-5 x 10(-2) mol dm-3). Both FI methods were applied to the determination of ascorbic acid in pharmaceuticals while the peak-height FI technique was also used to determine ascorbic acid in urine.     

Flow injection amperometric detection of ascorbic acid using a Prussian Blue film-modified electrode

The PB film-modified electrode was used as an amperometric detector for flow injection analysis of ascorbic acid. The modified electrode detector showed good sensitivity, stability and reproducibility. The calibration curve for ascorbic acid was linear over the concentration range from 5.0x10(-6) to 1.0x10(-3) mol l(-1) with a slope of 19.9 mA mol(-1) per litre and a correlation coefficient of 0.999. The detection limit of this method was 2.49x10(-6) mol l(-1). The relative standard deviation of six replicate injections of 2.5x10(-4) mol l(-1) ascorbic acid was 2.5%. The results obtained for ascorbic acid determination in pharmaceutical products are in good agreement with those obtained by using the procedure involving the reaction between triiodide and ascorbic acid.     

Kinetic determination of thiocyanate on the basis of its catalytic effect on the oxidation of methylene blue with potassium bromate.

This study reports a sensitive kinetic spectrophotometric method for the determination of trace amounts of thiocyanate. In acidic solution, Methylene Blue (MB) is oxidized by bromate to form a colorless compound. The reaction is accelerated by trace amounts of thiocyanate and can be followed by measuring the absorbance at 664 nm. The absorbance of the reaction decreased with an increase in the reaction time. Under the optimum experimental conditions (0.56 M of sulfuric acid, 3.9 x 10(-5) M of MB, 3.0 x 10(-3) M of bromate, 180 s, 25 degrees C), thiocyanate can be determined in the range 5.0 - 180 ng/ml. The relative standard deviations (n = 8) are 2.81 and 1.43% for 10.0 and 150 ng/ml thiocyanate, respectively. The detection limit of this method is (3sigma) 3.8 ng/ml. This method was successfully applied to the determination of thiocyanate in real samples.     

A new spectrofluorometric method for the determination of ascorbic acid based on its activating effect on a hemoglobin-catalyzed reaction.

A spectrofluorometric method for the determination of ascorbic acid (AA) based on its activation on the hemoglobin-catalyzed reaction was proposed. The fluorescence intensity of the product was measured under the optimal experimental conditions, i.e. 4.0 x 10(-6) M H2O2, 6.0 x 10(-5) M p-cresol, 1.2 M NH3-NH4Cl (pH 10.4) and 2.0 x 10(-7) M hemoglobin. The order of additions of the reagents was also studied. The activation of AA was found to be associated with a high ammonia concentration. The linear range of the method was 9.0 x 10(-10)-3.6 x 10(-8) M of AA. The detection limit was calculated to be 3.0 x 10(-10) M. The relative standard deviation of this method is 1.6% at 7.0 x 10(-9) M for 11 determinations.     

Determination of ultratrace amounts of ruthenium(III) by the delayed autocatalytic reaction using citric acid.

A method for determination of ultratrace amounts (ppq levels) of ruthenium(III) was developed using a copper(II)-phthalocyanine-3,4',4",4"'-tetrasulfonic sodium salt (Cu-PTS) as an indicator in a potassium bromate autocatalytic reaction system. A satisfactory calibration curve of ruthenium(III) ion was obtained by the time measurement in the concentration range of 1 x 10(-13) M to 5 x 10(-12) M with the relative standard deviation (RSD) of 2.8% (n=5). The determination limits (3sigma) were 3.30 x 10(-14) M (3.34 ppq).     

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.     

Liquid-phase microextraction-gas chromatography-mass spectrometry for the determination of bromate, iodate, bromide and iodide in high-chloride matrix

In the determination of bromate and iodate, any free bromide and iodide present was quantitatively removed by anion exchange with silver chloride exploiting the differences in silver salts solubility product, being AgCl, 1.8 x 10(-10), AgBr, 5.0 x 10(-13), AgI, 8.3 x 10(-17), AgBrO(3), 5.5 x 10(-5) and AgIO(3), 3.1 x 10(-8). The oxyhalides were reduced with ascorbic acid to halides and converted to 4-bromo-2,6-dimethylaniline and 4-iodo-2,6-dimethylaniline by their reaction with 2-iodosobenzoate in the presence of 2,6-dimethylaniline at pH 6.4 and 2-3, respectively. Single drop microextraction (SDME) of the haloanilines in 2 microl of toluene and injection of the whole extract into GC-MS, or liquid-phase microextraction (LPME) into 50 microl of toluene and injection of 2 microl of extract, resulted in a sensitive method for bromate and iodate. The latter method of extraction has been found more robust, sensitive and to give better extraction in shorter period than SDME. Total bromine/iodine was determined without any treatment with silver chloride. High concentration of chloride in the matrix did not interfere. A rectilinear calibration graph was obtained for 0.05 microg-25 mg l(-1) of bromate/bromide and iodate/iodide, the limit of detection were 20 ng l(-1) of bromate, 15 ng l(-1) of iodate, 20 ng l(-1) of bromide and 10 ng l(-1) of iodide (by LPME in 50 microl of toluene). The method has been applied to seawater and table salt. From the pooled data, the average recovery of spiked oxyhalide/halide to real samples was in range 96.7-105.7% with RSD in range 1.6-6.5%.     

Electrocatalytic characteristics of a 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone modified carbon-paste electrode in the oxidation of ascorbic acid.

A carbon-paste electrode spiked with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone was constructed by the incorporation of 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone in a graphite powder silicon oil matrix. It shown by cyclic voltammetry and double potential-step chronoamperometry, which this ferrocene derivative modified a carbon-paste electrode, can catalyze the ascorbic acid oxidation in an aqueous buffered solution. It has been found that under the optimum conditions (pH 7.00), the oxidation of ascorbic acid at the surface of this carbon paste modified electrode occurs at a potential of about 260 mV less positive than that of an unmodified carbon-paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration, and a linear calibration curve was obtained in the range of 6 x 10(-5) M-7 x 10(-3) M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (2sigma) was determined to be 6.3 x 10(-5) M. This method was also used for the determination of ascorbic acid in some pharmaceutical samples, such as effervescent tablets, ampoules and multivitamin syrup, by using a standard addition method. The reliability of the method was established by a parallel determination against the official method.     

Catalytic spectrophotometric determination of vanadium in seawaters based on the bromate oxidative coupling reaction of metol and 2,3,4-trihydroxybenzoic acid.

A new, simple, sensitive and selective catalytic method is developed for the determination of vanadium in natural and sea waters. The method is based on the catalytic effect of V(V) and/or V(IV) on the bromate oxidative-coupling reaction of metol with 2,3,4-trihydroxybenzoic acid (THBA). The reaction is followed spectrophotometrically by tracing the oxidation product at 380 and/or 570 nm after 5 min of mixing the reagents. The optimum reaction conditions are 6.4 x 10(-3) mol l-1 of metol, 2.0 x 10(-3) mol l-1 of THBA and 0.16 mol l-1 of bromate at 35 degrees C and in the presence of an activator-buffer solution of 1 x 10(-2) mol l-1 of tartrate (pH = 3.10). Following the recommended procedure, V(V) and/or V(IV) can be determined with linear calibration graphs up to 0.75 ng ml-1 and detection limits, based on the 3Sb criterion, of 0.008 and 0.018 ng ml-1 at 380 and 570 nm, respectively. The developed method was successfully applied, without any separation or preconcentration processes, to the determination of vanadium in natural and seawaters following the direct calibration and standard addition techniques, respectively.     

Potentiometric determination of bromate using an Fe(III)-Fe(II) potential buffer by circulatory flow-injection analysis.

A method for the potentiometric determination of bromate by circulatory flow injection analysis (CFIA) is described. The procedure involves the use of an Fe(III)-Fe(II) potential buffer solution, which is recycled via a reservoir. The analytical method is based on a linear relationship between the concentration of bromate and a very transient potential change in the electrode potential due to the generation of intermediate bromine during the reaction of bromate with the Fe(III)-Fe(II) potential buffer solution, which also contains NaBr, (NH4)6Mo7O24 and H2SO4. An aliquot (5 microl) of a bromate sample solution was injected into the stream of the potential buffer solution, 100 ml of which was circulated at a flow rate of 1 ml/min; the potential buffer solution stream was then returned to the reservoir after passing through a flow-through redox electrode detector. A potential change due to the reaction of the injected sample with the potential buffer in a reaction coil was measured with the detector in the form of a peak signal. The effects of the bromide, sulfuric acid and Fe(III)-Fe(II) concentrations in the potential buffer, and length of the reaction coil on the peak heights were examined in order to optimize the proposed CFIA method. The analytical sensitivities to bromate were 5.6 mV/microM for 1 x 10(-2) M and 30.9 mV/microM for 1 x 10(-3) M in the concentration of Fe(III)-Fe(II) in a potential buffer solution containing 0.35 M NaBr, 0.2% (NH4)6Mo7O24 and 1 M H2SO4. The detection limit of bromate obtained by a 1 x 10(-3) M Fe(III)-Fe(II) potential buffer solution was 0.02 microM (2.5 ppb). The numbers of repetitive determinations in which the relative sensitivities within 5% were regarded as being tolerated were ca. 4000 and 2000 for the use of only 100 ml of 1 x 10(-2) M and 1 x 10(-3) M Fe(III)-Fe(II) potential buffer solution, respectively.     

Flow-injection analysis method for the determination of nitrite and nitrate in natural water samples using a chemiluminescence NOx monitor.

A flow-injection analysis method for the determination of nitrite and nitrate in natural water samples has been developed that consists of two systems based on their reduction to NO with hydrazine and/or ascorbic acid, followed by chemiluminescence detection. The procedure of sweeping the generated NO into an NOx monitor, by means of a gas-liquid separating coil consisting of microporous polytetrafluoroethylene (PTFE) tubing, offers practical advantages. The adjustment of the carrier gas-flow rates could yield the same calibration graphs for the two measurement systems, and the accumulation sweeping mode provides a higher sensitivity. Chemiluminescence detection allows a wide linear calibration range of 5 x 10(-8) to 5 x 10(-5) M. The detection limits for nitrate and nitrite, defined as three-times the standard deviation of measurement blanks, are 2 x 10(-8) M and 1 x 10(-8) M, respectively, and the average precision was 3.2% at ambient natural concentration levels. Recovery tests were between 94% and 106% for a variety of natural water samples. The method is relatively free from interferences from the substances normally found in natural water, and only ferric ion has an effect for the nitrite determination.     

Analytical potential of the reaction between <Emphasis Type="Italic">p</Emphasis>-phenylenediamines and peroxodisulfate for kinetic spectrophotometric determination of traces of ascorbic acid

The analytical potential of the redox reactions between N, N-dimethyl- or N, N, N', N'-tetramethyl- p-phenylenediamine (DMPPD or TMPPD, respectively) and peroxodisulfate in acidic media for kinetic spectrophotometric determination of trace amounts of ascorbic acid (AA) has been investigated. The goal was to explore reaction conditions ensuring an excess or a deficit of S(2)O(8)(2-), and to identify some kinetic features of the global process for the purpose of calibration. Because the induction period of the overall reaction increases significantly in the presence of micromolar amounts of AA, the most suitable calibration graphs are generated by plots of induction period against the analyte concentration. The best sensitivity was achieved for oxidation of DMPPD with a deficit of S(2)O(8)(2-). The calibration line obtained has an RSD of 2.6% for 4 x 10(-6) mol L(-1) AA (n=3) and the detection limit is 4 x 10(-8) mol L(-1) (7 micro g L(-1)), a value comparable with the best mentioned by the literature. No systematic study of interferents was performed. Instead, the effect of buffer reagents (phosphate, phthalate, citrate) and dissolved oxygen on the results is discussed. Experiments were performed under closely controlled conditions. This is the first report of ascorbic acid analysis in which "contamination" by environmental oxygen was prevented by rinsing all vessels and saturating all solutions with argon, an approach which enables determination of the actual ascorbic acid content. Because of the effort involved, however, the method might not be suitable for routine analysis. Analysis of a commercially available vitamin C pellet gave good results when a "special" calibration graph, obtained in the presence of all other constituents of the sample except AA, was employed. The AA was "removed" from the pellet by oxidation with environmental air. Although it seems rather elaborate, this procedure did not require prior preparation of the sample nor extraction and/or concentration of ascorbic acid from it.     

Evaluation of luminol-H(2)O(2)-KIO(4) chemiluminescence system and its application to hydrogen peroxide, glucose and ascorbic acid assays

A novel chemiluminescence (CL) system was evaluated for the determination of hydrogen peroxide, glucose and ascorbic acid based on hydrogen peroxide, which has a catalytic-cooxidative effect on the oxidation of luminol by KIO(4). Hydrogen peroxide can be directly determined by luminol-KIO(4)-H(2)O(2) CL system. The detection limit was 3.0x10(-8) mol l(-1) and the calibration graph was linear over the range of 2.0x10(-7)-6.0x10(-4) mol l(-1). The relative standard deviation of H(2)O(2) was 1.1% for 2.0x10(-6) mol l(-1) (N=11). Glucose was indirectly determined through measuring the H(2)O(2) generated by the oxidation of glucose in the presence of glucose oxidase at pH 7.6. The present method provides a source for H(2)O(2), which, in turn, coupled with the luminol-KIO(4)-H(2)O(2) CL reaction system. The CL was linearly correlated with glucose concentration of 0.6-110 mug ml(-1). The relative standard deviation was 2.1% for 10 mug ml(-1) (N=11). Detection limit of glucose was 0.08 mug ml(-1). Ascorbic acid was also indirectly determined by the suppression of luminol-KIO(4)-H(2)O(2) CL system. The calibration curve was linear over the range of 1.0x10(-7)-1.0x10(-5) mol l(-1) of ascorbic acid. The relative standard deviation was 1.0% for 8.0x10(-7) mol l(-1) (N=11). Detection limit of ascorbic acid was 6.0x10(-8) mol l(-1). These proposed methods have been applied to determine glucose, ascorbic acid in tablets and injection.     

Catalytic oxidation of ascorbic acid by some ferrocene derivative mediators at the glassy carbon electrode. Application to the voltammetric resolution of ascorbic acid and dopamine in the same sample

Direct-current cyclic voltammetry is used to investigate the suitability of some ferrocene derivatives such as ferrocenecarboxylic acid, ferroceneacetic acid and ferrocenemethanol as mediators for ascorbic acid oxidation in aqueous solutions with low pH. The ascorbic acid coupled catalytically to three ferrocene derivatives exhibiting homogeneous second-order rate constants k(s), in the range 7.36 x 10(5) - 1.23 x 10(7). The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration and the linearity range obtained in the presence of ferrocenecarboxilic acid, having the largest second-order rate constant, was 5 x 10(-5) - 1.5 x 10(-3) M. The catalytic effect of the ferrocene derivatives on the electrochemical oxidation of ascorbic acid reduced the oxidation potential of ascorbic acid, resulting in the separation of the overlapping voltammograms of ascorbic acid and dopamine at the glassy carbon electrode in a mixture. This allowed the determination of ascorbic acid in the presence of dopamine. The calibration graph obtained by linear sweep voltammetry for ascorbic acid in the presence of dopamine of fixed concentration is linear in the range 5 x 10(-5) - 1.5 x 10(-3) M. In a similar manner, dopamine is determined in the presence of a high concentration of ascorbic acid, up to 100 times that of dopamine, using ferroceneacetic acid as the most suitable mediator for this purpose.     

Development of a spectrophotometric determination of siderophores using flow-injection analysis

A method has been developed for the spectrophotometric determination of siderophores using flow-injection analysis (FIA) based on the reaction of siderophores with the ternary complex Eriochrome Cyanine R-Fe(III)-cetyltrimethylammonium bromide. 2,3-Dihydroxybenzoic acid, 2,3-dihydroxynaphthalene, and tolypocine were used as the model iron-binding ligands. The calibration curve for one of the siderophores (tolypocine) is linear in the concentration range 2.6 x 10(-6)-1.5 x 10(-4)M. The determination limit (10sigma) for tolypocine was 2.6 x 10(-6)M. The applicability of the method was demonstrated on the determination of the complexation ability of siderophores produced by some entomopathogenic fungi. Samples can be analysed at a rate of 30 samples per hour.     

Catalytic kinetic determination of ultratrace amounts of nitrite with detection by linear scan voltammetry at a DME

Nitrite has a very strong catalytic effect on the bromate oxidation of Methyl Orange in dilute sulphuric acid medium. The oxidation product of methyl orange exhibits a well derivative voltammetric wave at -0.41 V vs. SCE in sodium hydroxide medium. The linear scan voltammetric behaviour for the product at a DME has been studied, and it was selected as indicator component for the indicator reaction. Based on these studies, a novel and highly sensitive and selective catalytic reaction-voltammetric method for nitrite is proposed. A detection limit of 2 x 10(-9)M and calibration graph from 4 x 10(-7) to 4 x 10(-7)M nitrite are obtained. Nitrite in water samples was determined by this method, with satisfactory results.