Simultaneous determination of Cd, Pb, Cu, Sb, Bi, Se, Zn, Mn, Ni, Co and Fe in water samples by differential pulse stripping voltammetry at a hanging mercury drop electrode

A highly sensitive and selective voltammetric procedure is described for the simultaneous determination of eleven elements (Cd, Pb, Cu, Sb, Bi, Se, Zn, Mn, Ni, Co and Fe) in water samples. Firstly, differential pulse anodic stripping voltammetry (DPASV) with a hanging mercury drop electrode (HMDE) is used for the direct simultaneous determination of Cd, Pb, Cu, Sb and Bi in 0.1 M HCI solution (pH = 1) containing 2 M NaCl. Then, differential pulse cathodic stripping voltammetry (DPCSV) is used for the determination of Se in the same solution. Zn is subsequently determined by DPASV after raising the pH of the same solution to pH 4. Next, the pH of the medium is raised to pH 8.5 by adding NH3/NH4Cl buffer solution for the determination of Mn by DPASV. Ni and Co are determined in the same solution by differential pulse adsorptive stripping voltammetry (DPAdSV) after adding DMG (1 x 10(-4) M). Finally, 1 x 10(-5) M 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is added to the solution for the determination of Fe by DPAdSV. The optimal conditions are described. Relative standard deviations and relative errors are calculated for the eleven elements at three different concentration levels. The lower detection limits for the investigated elements range from 1.11 x 10(-10) to 1.05 x 10(-9)M, depending on the element determined. The proposed analysis scheme was applied for the determination of these eleven elements in some ground water samples.     

PVC membrane ion selective electrode for the determination of pentachlorophenol in water, wood and soil using tetrazolium pentachlorophenolate

A novel PVC membrane electrode selective for the determination of pentachlorophenolate (PCP) ion based on 2,3,5-triphenyl-2H-tetrazolium (TPT) pentachlorophenolate ion pair as an electroactive material is described. The electrode shows a linear response for PCP ion over the concentration range of 6x10(-5) to 1x10(-2) M with lower detection limit of 4x10(-5) M at 25 degrees C. The electrode posses a Nernstian slope of -55.0+/-1.0 mV/decade and a fast potential response of 45 s, which is almost constant over a pH range of 5.5-10.5. Selectivity coefficient data for some common ions show negligible interference, however, 2,4,6-trichlorophenolate and cetylpyridinium ions interfere. An average recovery of 96.2% with relative standard deviation of 2.3% has been achieved for the determination of 75.0 ppm PCP in wastewater samples. The determination of PCP in soil and wood samples gave results that compare favorably with those obtained by gas chromatography. The electrode has been utilized as an end point indicator electrode for the determination of PCP using potentiometric titration.     

Determination of thiosulfate at the 10(-6) M level by its oxidation with iodine in an organic phase and spectrophometric measurement of triiodide

A highly sensitive method is proposed for the determination of thiosulfate based on the oxidation of aqueous thiosulfate (100 or 200 ml) by iodide in 4 ml of carbon tetrachloride. The excess of iodine was extracted into 8 ml of aqueous iodide solution as triiodide to be measured spectrophotometrically; the thiosulfate could therefore be indirectly highly concentrated and determined selectively. The side-reaction of thiosulfate in a large volume of solution with the hypoiodite formed from the iodine in carbon tetrachloride could be compensated for by adding a certain amount of extra thiosulfate. A linear calibration graph with a negative slope was obtained over the concentration ranges 1.1 x 10(-7)-1 x 10(-5) M (12 ppb-1.12 ppm) for 100 ml of thiosulfate solution and 6 x 10(-8) - 5 x 10(-6) M (6.7 ppb-0.56 ppm) for 200 ml of thiosulfate solution. The proposed method was successfully applied to the determination of various amounts of thiosulfate in hot-spring and lake-water samples.     

Amperometric lysine bioprobes analysis in feeds

Amperometric enzyme electrode probes have been constructed for the specific determination of L-lysine and used in batch and flow analysis. The enzyme lysine oxidase was immobilized on a preactivated polymer support which was placed on a platinum electrode. Additional blocking membranes conferred high stability, reproducibility and avoided electrochemical and enzyme interferences. Parameters including pH, temperature, storage and operational times were optimized. Lysine was determined in the range 10(-6)-2.10(-3)M with a detection limit of 5 x 10(-7)M. The Michaelis constant was 2 x 10(-3)M. This value was approximately two order of magnitudes higher than that reported in literature for the free enzyme. The response time of the probe was about 2 min in batch and flow analysis and 30 sec in flow injection analysis (FIA). The resulting probes were stable for more than three months with more than 300 analyses performed. The determination of lysine was carried out by both flow-through analysis and FIA. Analysis in feeds was carried out by acid hydrolysis to liberate lysine; then the solution was analyzed by the bioprobe and HPLC procedures. Results by the two methods correlated well.     

High-resolution determination of H+ by ion chromatography. Application to the simultaneous determination of H+, Na+, NH4+ and K+ in acid rain.

An ion chromatographic (IC) method was developed for the high-resolution determination of a sample's free hydrogen ion concentration (H+). Highly purified lithium dodecyl sulfate was used as the stationary phase, a slightly acidified aqueous LiCl solution was used as the mobile phase and conductivity was used for analyte detection. An electrical double layer (EDL) containing H+ was established on the stationary phase by using a slightly acidified electrolyte solution as the eluent. H+ in the EDL protonated any weak acid groups (i.e., silanols) on the stationary phase so that H+ from the sample could be retained/separated purely by dodecyl sulfate. The optimum molar ratio of H+:Li+ in the EDL for this IC system was obtained by using an aqueous solution containing 40.0 mM LiCl and 0.07 mM H2SO4 as the eluent. After separation, H+ was detected by direct conductimetric measurement. An H+ detection limit of better than 8.2 x 10(-6) M was obtained from the analysis of standard aqueous H2SO4 solutions. Other monovalent cations could also be separated with this method, giving detection limits of 7.4 x 10(-5), 4.3 x 10(-5) and 4.2 x 10(-5) M for Na+, NH4+ and K+, respectively. The method was applied to the simultaneous determination of H+, Na+, NH4+ and K+ in acid rain. The results obtained showed a significant improvement in reproducibility when compared with those from a conventional pH-meter. Acid rain samples with a pH < 5 could be analyzed with this IC system.     

Coated wire silver-ion selective electrode based on a N,N'-bis(2-thienylmethylene)-1,2-diaminobenzene.

A novel membrane coated platinum-wire electrode (MCPWE) based on N,N'-bis(2-thienylmethylene)-1,2-diaminobenzene (BTMD) for highly selective determination of Ag+ ion has been developed. The influences of membrane composition and pH on the potentiometric responses of electrode were investigated. The potentiometric responses are independent of the pH of the test solution in the range of 5.0 - 9.0. The electrode shows a linear response for Ag+ ion over the concentration range of 1.0 x 10(-60 to 1.0 x 10(-1) M with a lower detection limit of 6.0 x 10(-7) M. The electrode possesses a Nernstian slope of 59.7 mV decade(-1) and a fast response time of < or = 17 s and can be used for at least 2 months without any observable deviation. The proposed electrode displayed very good selectivity for Ag+ ion with respect to NH4+ and alkali, alkaline earth and some common transition metal ions. The practical utility of the electrode has been demonstrated by its use as the indicator electrode in the potentiometric titration of an AgNO3 solution with a NaI solution and in determination of the silver content of a developed radiological film.     

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.     

Differential pulse voltammetric indirect determination of aluminium in drinking waters, blood, urine, hair, and medicament samples using L-dopa under alkaline conditions

The differential pulse voltammetric (DPV) indirect determination of aluminium using L-dopa under alkaline conditions on a glassy carbon working electrode was studied. The proposed method relies on the linear decrease of the DPV anodic peak current of L-dopa with increase in the concentration of aluminium added. Under the optimum experimental conditions (pH 8.5, 0.08 M NH4Cl-NH3.H2O buffer solution, and 4 x 10(-4) M L-dopa), the linear range is 2-18 x 10(-7) M AlIII. The detection limit is 7.6 x 10(-8) M and the relative standard deviation for 8 x 10(-7) M AlIII is 3.5% (n = 8). A number of foreign species were examined as potential interferents. The method was applied to the determination of aluminium in drinking waters, synthetic renal dialysate, sodium chloride injection, sucrafate, hydrothorax, blood, urine and hair samples. The physiological significance is discussed.     

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.     

Ion chromatographic determination of ammonia in air using a sampling tube of porous carbon.

The ion chromatographic determination of ammonia in air using a sampling tube of porous carbon carbonized at 500 degrees C was examined. When the mean recovery and the reproducibility for a series of five determinations were examined for 1 and 10 ppm ammonia gases, the mean recovery (n = 5) and the relative standard deviation were 97.0% and 3.5% for 1 ppm and 86.9% and 2.8% for 10 ppm, respectively. Furthermore, the recovery from 10 ppm ammonia gas increased with an increase in the extraction time, and a recovery of 99.7% was obtained for 180 min of extraction time. The mean collection efficiency for 1 and 10 ppm ammonia gas was 102.5% and 96.5%, respectively. The relation between the sampling volume and the peak area was linear, and the linearity was 0.999 of the correlation coefficient. The ammonia gas concentration in an actual goat shed could be determined by this sampling device for a sampling volume of 5 L at a flow rate of 1 L/min; 0.79 ppm of the determination value practically agreed as compare with 0.78 ppm from collection by a boric acid solution.     

Bacterial electrode for L-arginine

Streptococcus lactis bacterial cells are employed for the conversion of L-arginine into ammonia. An ammonia gas-electrode is used as a detector. The combination of bacterial action and the gas electrode responds linearly to L-arginine over the concentration range 8.0 x 10(-6) - 1.0 x 10(-3)M, with a slope of 59.0 mV/decade and is selective with respect to other l-amino acids.     

Anodic oxidation of etodolac and its square wave and differential pulse voltammetric determination in pharmaceuticals and human serum

A voltammetric study of the oxidation of etodolac has been carried out at the glassy carbon electrode. The electrochemical oxidation of etodolac was investigated by cyclic, linear sweep, differential pulse and square wave voltammetry using glassy carbon electrode. Different parameters were tested to optimize the conditions for the determination of etodolac. The dependence of intensities of currents and potentials on pH, concentration, scan rate, nature of the buffer was investigated. For analytical purposes, a very well resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 2.15 for differential pulse and square wave voltammetric techniques. The linear response was obtained in the ranges of 2.10(-6)-8.10(-5) M with a detection limit of 6.8x10(-7) and 6x10(-6)-8x10(-5) M with a detection limit of 1.1x10(-6) M for differential pulse and square wave voltammetric techniques, respectively. Based on this study, simple, rapid, selective and sensitive two voltammetric methods were developed for the determination of the etodolac in tablet dosage form and human serum.     

An enzymatic method for the kinetic measurement of L-asparaginase activity and L-asparagine with an ammonia gas-sensing electrode

A simple kinetic method to assay L-asparaginase and L-asparagine with an ammonia gas-sensing electrode is described. The method is based upon the de-amination of L-asparagine by L-asparaginase from Escherichia coli, resulting in the production of ammonia. The initial rate (mV/min) of ammonia release is proportional to the activity of L-asparaginase and also to the concentration of L-asparagine in the presence of a large amount of the enzyme. Optimal temperature, buffer composition and pH for the assays are specified. L-Asparaginase was determined in the range of 0.4-1.6 U in a 0.1 ml sample; the recovery was 98.1-103.8% for 16 determinations and sigma n was 1.59. L-Asparagine was determined in the concentration range of 1 x 10(-4)--1 x 10(-3) M with sigma n-1 1.92. The method was applied to the determination of 1-5 x 10(-4) M asparagine added to human serum with sigma n-1 1.96 for 5 determinations.     

Amperometric sensor for the determination of ascorbic acid based on Cobalt hexacyanoferrate modified electrode fabricated through a new route

A new approach was attempted to prepare a chemically modified electrode using Cobalt hexacyanoferrate (CoHCF) as the redox mediator and to study its stability and electrocatalytic activity for ascorbic acid (AA) oxidation. The basic principle underlying the electrode modification is the coordination of cobalt ion with the amino nitrogen of aniline adsorbed on the surface of a graphite rod. This surface was subsequently derivatized with ferrocyanide to get CoHCF film on the electrode surface. The CoHCF modified electrode as prepared above was characterized using cyclic voltammetry. The effect of scan rate, supporting electrolyte and pH of the medium on the performance of the modified electrode was investigated. The CoHCF modified electrode exhibited good electrocatalytic activity towards the oxidation of ascorbic acid and gave a linear response from 5.52 x 10(-5) M to 3.23 x 10(-2) M with a correlation coefficient of 0.9929. The detection limit was found to be 3.33 x 10(-5) M. Hydrodynamic voltammetry and chronoamperometry studies for the oxidation of ascorbic acid were also carried out. The electrode was highly stable and exhibited good reproducibility. This modified electrode was also applied for the determination of ascorbic acid in commercial samples.     

Differential pulse anodic stripping voltammetry of lead (lI) benzoylacetonate in chloroform: Application to the analysis of free-lead gasoline and gas oil samples

The voltammetric characteristics of lead(II) benzoylacetonate in chloroform at the mercury electrode are investigated. The conditions for nearly reversible reduction of lead(II) were optimized. Anodic stripping voltammetry for the determination of trace-lead was developed using differential pulse technique to strip amalgamed lead from hanging mercury drop electrode. The experimental conditions, such as scanning rate of electrode potential and deposition time of lead were optimized. The calibration graph was linear over concentration range 5x10(-8)-10(-6) M of lead(II). The detection limit was 2.5x10(-9) and the relative standard deviation for the determination of 4x10(-7) M Pb(II) was 2%. Preceded by decomposition of organolead compounds with concentrated nitric acid, then ashing at 300 degrees C and a solvent extraction of Pb(II) benzoylacetonate in chloroform, the suitability of the proposed method for the determination of lead in free-lead gasoline and gas oil was demonstrated as a typical example of application.     

Sensitivity improvement of ammonia determination based on flow-injection indophenol spectrophotometry with manganese(II) ion as a catalyst and analysis of exhaust gas of thermal power plant.

The sensitivity improvement of a flow-injection spectrophotometric method for the determination of ammonia was examined based on an indophenol blue coloration reaction with salicylate and hypochlorite in the presence of manganese(II) as a reaction promotion catalyst. The optimal conditions for achieving higher sensitivity of ammonia determination were examined using a three-line flow system. The limit of detection corresponding to a signal-to-noise ratio (S/N) of 3 was 0.005 mg l(-1) (approximately equal to 5 ppb) of NH4+. A calibration graph was linear in the range from 5 ppb to 1,000 ppb of ammonium ion. The relative standard deviations (n = 9) for 50 ppb and 100 ppb of ammonium ion were 6.4% and 2.2%, respectively. The proposed method was applied to the determination of ammonia in the exhaust gas of a thermal power plant. Prior to the FIA determination, ammonia in the exhaust gas was absorbed into a boric acid solution; the absorption solution was then analyzed by the proposed FIA.     

Cadmium ion-selective electrode based on tetrathia-12-crown-4

A new polyvinylchloride membrane sensor for Cd(2+) ions based on tetrathia-12-crown-4 as an ionophore was prepared. The sensor exhibits a Nernstian response for cadmium ions over a wide concentration range (4.0 x 10(-7) to 1.0 x 10(-1) M) with a slope of 29+/-1 mV decade(-1). The limit of detection is 1.0 x 10(-7) M (0.01 ppm). It has a fast response time of <10 s and can be used for at least 6 weeks without any divergence in potential. The electrode can be used in the pH range from 2.5 to 8.5. The proposed sensor shows fairly good discriminating ability towards Cd(2+) ion in comparison with some alkali, alkaline earth, transition and heavy metal ions. It was successfully applied for the direct determination of Cd(2+) in solution and, as an indicator electrode, in potentiometric titration of cadmium ions.     

Indirect determination of the pesticide dimethoxydithiophosphate in an FIA-AAS system with liquid-liquid back-extraction

A method has been developed for the determination of dimethoxydithiophosphate (DDTP) by liquid-liquid extraction in a flow-injection analysis (FIA) system with detection by atomic-absorption spectrometry (AAS). It is based on the formation of the Cu(DDTP)(2) complex and its extraction into chloroform, and back-extraction of the copper with an ammonia buffer (pH 10). The method uses small amounts of samples, avoids handling errors and is fast and highly reproducible. It features a detection limit of 0.39 ppm DDTP (2.45 x 10(-6)M in the organic phase) and a relative standard deviation of 1.6%. The method has been applied to the determination of the organophosphorus pesticide malathion in an agricultural formulation.     

Voltammetric determination of terbinafine in biological fluid at glassy carbon electrode modified by cysteic acid/carbon nanotubes composite film

The electrochemical oxidation of L-cysteine (CySH) in presence of carbon nanotubes (CNTs) formed a composite film at a glassy carbon electrode (GCE) as a novel modifier for directly electroanalytical determination of terbinafine without sample pretreatment in biological fluid. The determination of terbinafine at the modified electrode with strongly accumulation was studied by differential pulse voltammetry (DPV). The peak current obtained at +1.156 V (vs. SCE) from DPV was linearly dependent on the terbinafine concentration in the range of 8.0 x 10(-8)-5.0 x 10(-5 )M in a B-R buffer solution (0.04 M, pH 1.81) with a correlation coefficient of 0.998. The detection limit (S/N=3) was 2.5 x 10(-8 )M. The low-cost modified electrode showed good sensitivity, selectivity, and stability. This developed method had been applied to the direct determination of terbinafine in human serum samples with satisfactory results. It is hopeful that the modified electrode will be applied for the medically clinical test and the pharmacokinetics in future.     

Investigation of electrochemical behavior of lipid lowering agent atorvastatin calcium in aqueous media and its determination from pharmaceutical dosage forms and biological fluids using boron-doped diamond and glassy carbon electrodes

The electrochemical behavior of atorvastatin calcium at glassy carbon and boron-doped diamond electrodes has been studied using voltammetric techniques. The possible mechanism of oxidation was discussed with model compounds. The dependence of the peak current and potentials on pH, concentration, scan rate and nature of the buffer were investigated for both electrodes. The oxidation of atorvastatin was irreversible and exhibited a diffusion-controlled fashion on the diamond electrode. A linear response was obtained within the range of 9.65 x 10(-7) - 3.86 x 10(-5) M in 0.1 M H(2)SO(4) solution for both electrodes. The detection limits of a standard solution are estimated to be 2.11 x 10(-7) M with differential pulse voltammetry (DPV) and 2.05 x 10(-7)M with square wave voltammetry (SWV) for glassy carbon electrode, and 2.27 x 10(-7) M with DPV and 1.31 x 10(-7)M with SWV for diamond electrodes in 0.1 M H(2)SO(4) solution. The repeatability of the methods was found good for both electrodes. The methods were fully validated and successfully applied to the high-throughput determination of the drug in tablets, human serum and human urine with good recoveries.