Flotation-spectrophotometric method for the determination of mercury using iodide and brilliant green

This paper describes a sensitive spectrophotometric method for the determination of mercury, based on the flotation of a mercury iodide-brilliant green complex with cyclohexane at pH 5 and dissolution of the complex in acetone. The absorbance of the complex is measured at 625 nm. Beer's law is obeyed from 4–500 g/l of mercury. The method is highly sensitive ( = 5.96 × 10⁵ 1 mol^–1 cm^–1) and selective, as it is free from interferences by almost all cations and anions in the presence of EDTA. Furthermore, a two-level orthogonal array design as a chemometric method has been used for optimization of the various parameters involved. The method has been successfully applied to paper industry effluents.     

Flow-Injection Methods for Determining Europium in Mixtures of Lanthanides(III)

Possible approaches to the flow-injection determination of europium(III) in the presence of other lanthanides are studied. One of the approaches is based on the direct amperometric detection of europium(III) in a flow-injection system with a glassy-carbon electrode at a potential of –0.85 V (against a saturated calomel electrode). The linear calibration range is 5.0 × 10^–5–5.0 × 10^–4M of europium, and the limit of detection is 1.8 × 10^–5M (2.8 g/mL). The throughput capacity is 90 h^–1for a sample volume of 600 L. Another approach involves the online reduction of europium(III) to europium(II) in a flow Jones mini-reductor filled with amalgamated zinc, followed by the spectrophotometric detection of europium(II) using redox reactions between europium(II) and iron(III) in the presence of 1,10-phenanthroline, molybdophosphoric acid, or Methylene Blue. In the latter case, the calibration curve is linear in the range 0–5.0 × 10^–6M europium(III), the limit of detection is 9.0 × 10^–8M (0.014 g/mL). The throughput capacity is 180 h^–1for a sample volume of 200 L. The performance parameters of the proposed flow-injection methods are estimated using the analysis of artificial mixtures and dissolved samples of samarium(III) oxide and lanthanum(III) fluoride containing europium impurities as an example.     

Amplification method for the spectrophotometric determination of phosphorus

An amplification method with ion-exchange pre-separation for the spectrophotometric phosphorus determination in various alloys is described. Phosphorus is converted into molybdophosphoric acid, separated by selective extraction and back-extracted into the aqueous phase. The molybdenum is liberated by alkaline decomposition of the heteropoly acid. The spectrophotometric measurement is based on the reaction of molybdenum(VI) with 2,2-biquinoxalyl. High molar absorptivity of 1,1-dihydro-2,2-biquinoxalylene (=3.3 · 10⁴ L mol^–1 cm^–1) and the twelve-fold amplification guarantee high precision and makes the determination of phosphorus in the 1–6 g level possible.     

Extraction of99Mo-molybdophosphate and99mTc-pertechnetate from various acid solutions by bis/2-ethylhexyl/ phosphoric acid

The extractability of⁹⁹Mo-molybdophosphoric acid H₃/PMo₁₂O₄₀/ and^99mTc-pertechnetic acid /PTcO₄/ in 20% (v/v) bis /2-ethylhexyl/ phosphoric acid /HDEHP/ in benzene has been investigated at different concentrations of HCl, HBr and HNO₃ acids. The effect of extractant concentration and diluent on the extractability of molybdophosphoric and pertechnetic acids with 20% (v/v) HDEHP in benzene from different concentrations of HCl acid has also been studied.⁹⁹Mo-molybdo-phosphoric acid was found to be selectively extracted and separated from^99mTc-pertechnetic acid with 20% HDEHP in benzene at an acidity of about 0.49M HCl, HBr and HNO₃. The extractability of H₃/PMo₁₂O₄₀/ from these acids generally follows the decreasing order HBr>HCl>HNO₃. The separation factors /K_d molybdophosphoric/K_d pertechnetic/ were found to be 12.9×10⁵ and 7.6×10⁵ for HBr and HCl, respectively. The extractability of pertechnetic acid follows the order HCl>HBrHNO₃. Benzene is the diluent due to its radiation stability. A new procedure for the separation of^99mTc from⁹⁹Mo was suggested.     

Electrocatalytic Oxidation of Nitric Oxide on an Electrode Modified with Fullerene Films

Fullerene was immobilized on the surface of a glassy carbon electrode and reduced by an electrochemical method to form a partially reduced fullerene film. The films on the electrode showed stable electrocatalytic activity towards the oxidation of nitric oxide (NO). The catalytic current was proportional to the concentration of nitric oxide. Based on this property, a method for the detection of nitric oxide in aqueous solution is proposed. The detection conditions, such as supporting electrolyte, scan rate and thickness of the film were optimized. Under the optimized conditions, the catalytic currents increase linearly with the concentration of NO in the range of 3×10^–71.0×10^–4M, and the detection limit is 7.4×10^–8M. In addition, the modified electrode is very selective with respect to interferences including ascorbic acid, dopamine, and nitrite when further modified by a Nafion film on the surface of the electrode. The experimental results indicate that the partially reduced fullerene can act as an NO sensor featuring fast response and high stability.     

Superoxide sensor based on cytochrome c immobilized on mixed-thiol SAM with a new calibration method

Cytochrome c was immobilized on a mixed-thiol (mercaptoundecanoic acid/mercaptoundecanol) modified gold electrode (MUA:MU/cyt c electrode). Characterization of the cyt c electrode showed a quasi-reversible, electrochemical redox behavior with a formal potential of −13±5 mV (versus Ag/AgCl) for the surface adsorbed protein and 3±5 mV for covalently immobilized cyt c. The heterogeneous electron transfer rate constants were determined to be about 70 and 40 s⁻¹ for both states of the protein, respectively. They were found to be significantly higher than those of pure MUA-modified cyt c electrodes (MUA/cyt c electrodes). The interaction of superoxide radicals (O₂⁻) with the (MUA:MU)/cyt c electrode was characterized and used for an amperometric O₂⁻ detection. The influence of H₂O₂ and uric acid on the sensor signal was investigated. The sensitivity of the (MUA:MU)/cyt c electrode to O₂⁻ was significantly improved compared with that of the MUA/cyt c electrode. Based on a kinetic model for the superoxide detection system, a new calibration method was established. This simple and fast method used the spontaneous dismutation of KO₂ and was compared with the enzymatic superoxide generation system using xanthine oxidase.     

Sorption–Spectroscopic Determination of Phosphates in Waters as Molybdenum Heteropoly Acids

Conditions have been found for the preconcentration and reduction of molybdophosphoric acid from aqueous solutions on Amberlite XAD-7 and XAD-8 adsorbents in the batch and dynamic modes. A spectrophotometric method was developed for determining phosphate as molybdophosphoric acid after its extraction on Amberlite XAD-8 and desorption with acetonitrile. The concentration factor was 5.0 × 10³. A sorption–spectroscopic method was also proposed for determining phosphate in solutions as molybdovanadophosphoric acid, one which includes its adsorption on polyurethane foam, reduction in the sorbent phase, and the measurement of diffuse reflectance.     

Electroanalysis of dopamine at a gold electrode modified with N-acetylcysteine self-assembled monolayer

Voltammetric behavior of dopamine (DA) on a gold electrode modified with the self-assembled monolayer (SAM) of N-acetylcysteine has been investigated, and one pair of well-defined redox peaks of dopamine is obtained at the SAM modified gold electrode. The oxidation peak current increases linearly with the concentration of dopamine in the range of  mol l⁻¹. The detection limit is 8.0×10⁻⁷ mol l⁻¹. This method will be applicable to the determination of dopamine in injection of dopamine hydrochloride, and the good recovery of dopamine is obtained. Furthermore, The SAM modified gold electrode can resolve well the voltammetric responses of dopamine and ascorbic acid (AA), so it can also be applied to the determination of dopamine in the presence of ascorbic acid.     

A Novel Method for Separating the Anodic Voltammetric Peaks of Dopamine and Ascorbic Acid

This paper presents a simple and reliable method for separating the anodic peaks of dopamine (DA) and ascorbic acid (AA). Good electrochemical responses of DA and AA can be obtained at an electrochemically pretreated glassy carbon electrode (PGCE). The electrode reaction of DA is a quasi-reversible process, but the oxidation of AA is entirely irreversible. Consequently, the PGCE can be used to separate the anodic peaks of DA and AA. The separation can be higher than 290mV, and the detection limit (S/N=3) of DA is 6.0×10^–8M. This PGCE is very stable and can be used for as long as four weeks.     

Measurements of the viscosity of thin fluid films between two surfaces with and without adsorbed polymers

A new method has recently been introduced for making detailed measurements of hydrodynamic effects in very thin liquid films between two surfaces. The method is used to obtain viscosity profiles, i. e. the viscosity as a function of the separation, between surfaces with adsorbed polymer layers. Results are presented for pure cyclohexane between two mica surfaces and for polystyrene (M _w=9×10⁵) adsorbed onto mica from cyclohexane at 26°C (i. e. below the-temperature of 34.5°C). The range of distances over which the concept of an effective hydrodynamic layer is applicable is established, and a simple correspondence is found between the effective hydrodynamic thickness, the radius of gyration of the polymer and the range of the steric force between the two polymer-covered surfaces. Further experimental evidence is presented which suggests that, depending on the solvent conditions, similar correlations apply more generally to other polymer systems.     

Electrochemical behavior of a covalently modified glassy carbon electrode with aspartic acid and its use for voltammetric differentiation of dopamine and ascorbic acid

Aspartic acid was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopic (XPS) measurement and cyclic voltammetric experiments proved the aspartic acid was immobilized as a monolayer on the GCE. Electron transfer to Fe(CN)₆^4– in solution of different pH was studied by cyclic voltammetry. Changes in solution pH resulted in the variation of the charge state of the terminal group; surface pK_a values were estimated on the basis of these results. Because of electrostatic interactions between the negatively charged groups on the electrode surface and dopamine (DA) and ascorbic acid (AA), the modified electrode was used for electrochemical differentiation between DA and AA. The peak current for DA at the modified electrode was greatly enhanced and that for AA was significantly reduced, which enabled determination of DA in the presence of AA. The differential pulse voltammetric (DPV) peak current was linearly dependent on DA concentration over the range 1.8×10^–6–4.6×10^–4 mol L^–1 with slope (nA mol^–1 L) and intercept (nA) of 47.6 and 49.2, respectively. The detection limit (3) was 1.2×10^–6 mol L^–1. The high selectivity and sensitivity for dopamine was attributed to charge discrimination and analyte accumulation. The modified electrode has been used for determination of DA in samples, in the presence of AA, with satisfactory results.     

A novel salicylate-selective electrode based on a Sn(IV) complex of salicylal-imino acid Schiff base

A novel poly(vinyl chloride) membrane electrode with high selectivity toward salicylate (Sal^–), based on the use of the salicylal-imino acid Schiff base dibenyl complex of Sn(IV) [Sn(IV)-SIADBen] as ionophore is described. The influence of lipophilic charged additives on the performance of the electrode was studied. The results suggested that Sn(IV)-SIADBen according to a positively-charged carrier mechanism. The influence of several other variables was investigated in order to optimize the potentiometric response and selectivity of the electrode. The electrode based on Sn(IV)-SIADBen, with 30.44 wt% PVC, 64.55 wt% plasticizer [dioctyl phthalate (DOP)], 3.81 wt% ionophore, and 1.2 wt% anionic additive exhibited a linear response for the Sal^– ion over the concentration range 1.0×10^–1 to 2.5×10^–6 mol l^–1, and displayed an anti-Hofmeister selectivity sequence as follows: salicylate perchlorate > thiocyanate > benzoate > iodide > nitrate > chloride > nitrite acetate > citrate > sulfate. UV-Visible absorption spectra were used to examine the specific interaction of salicylate with the ionophore. The electrode was applied to clinical medical analysis, and the results obtained were consistent with those obtained by conventional methods.     

Sensitive flotation-spectrophotometric method for the determination of vanadium (IV) with 3,5-dinitrosalicylic acid and rhodamine B

Vanadium(IV) is determined by reaction with 3,5-dinitrosalicylic acid and rhodamine B in weakly acidic medium. The flotation of the ternary ion-association complex is carried out with cyclohexane followed by dissolution in acetone for subsequent spectrophotometric determination. The molar absorptivity is 5.91×10⁵ l mole^–1 cm^–1 at 555 nm. Beer's law is obeyed in the range 0.05–1.5 g vanadium(IV) in 25 ml. The method is selective for vanadium(IV) in the presence of sodium fluoride and has been applied to standard reference materials.     

Surface modification of amine-functionalised graphite for preparation of cobalt hexacyanoferrate (CoHCF)-modified electrode: an amperometric sensor for determination of butylated hydroxyanisole (BHA)

A cobalt hexacyanoferrate (CoHCF)-modified graphite paraffin wax composite electrode was prepared by a new approach. An amine-functionalised graphite powder was used for the fabrication of the electrode. A functionalised graphite paraffin wax composite electrode was prepared and the surface of the electrode was modified with a thin film of CoHCF. Various parameters that influence the electrochemical behaviour of the modified electrode were studied by varying the background electrolytes, scan rates and pH. The modified electrode showed good electrocatalytic activity towards the oxidation of butylated hydroxyanisole (BHA) under optimal conditions and showed a linear response over the range from 7.9 × 10⁻⁷ to 1.9 × 10⁻⁴ M of BHA with a correlation coefficient of 0.9988. The limit of detection was 1.9 × 10⁻⁷ M. Electrocatalytic oxidation of BHA was effective at the modified electrode at a significantly reduced potential and at a broader pH range. The utility of the modified electrode as an amperometric sensor for the determination of BHA in flow systems was evaluated by carrying out hydrodynamic and chronoamperometric experiments. The modified electrode showed very good stability and a longer shelf life. The modified electrode was applied for the determination of BHA in spiked samples of chewing gum and edible sunflower oil. The advantage of this method is the ease of electrode fabrication, good stability, longer shelf life, low cost and its diverse application for BHA determination. Figure Cyclic Voltammogram of () CoHCF modified electrode, () in presence of 1.9 x 10⁻⁵ M of BHA and () bare electrode, () in the presence of 1.9 x 10⁻⁵ M of BHA in 1.0 M NaCl, pH 7.0     

Adsorption of radioisotopes from their organic solutions on filter papers the Sb(III)-nonpolar solvents-system

The extraction of Sb(III) chloride by nonpolar solvents from 0.15M HCl was studied as a function of sulphuric acid concentrations in the aqueous phase. The distribution of Sb(III) chloride between the nonpolar solvents benzene, toluene, xylene, nitrobenzene, cyclohexane, chloroform and carbon tetrachloride and filter paper is reported. In case of benzene the Sb(III) activity (given in counts·s^–1·ml^–1) decreases from 1500 to 200 after 24 hours. The corresponding values are about 1200 and 540 for toluene, 1330 and 50 for xylene, 1050 and 700 for nitrobezene, 1080 and 22 for cyclohexane, 330 and 30 for chloroform and 130 and 40 for carbon tetrachloride. More than 95% of the adsorbed Sb(III) is desorbed by 1M HNO₃, 1M HCl or 0.5M H₂SO₄ by contacting the loaded filter paper with any of these acids for 27 hours.     

Potentiometric determination of mercury(II) and thiourea in strong acid solution using an ion-selective electrode with AgI-based membrane hydrophobised by PTFE

Summary The potentiometric determination of mercury(II) and thiourea (TU) in strong acid solution (pH 0–1) by using an all-solid-state ion-selective electrode with (Ag₂S 25%, AgI 25% and PTFE 50% m/m)-membrane is described. The linear response, 43 mV(pHg)^–1 and 80 mV(pTU)^–1, has been obtained in the concentration range from 10^–2 to under 10^–5 mol/l. By direct potentiometry at pH 0 mercury(II) can be determined in the presence of up to 10^–3 mol/l of iron(III). The change in potential in the tested concentration range of thiourea indicates the formation of Ag(TU) _1.4 ⁺ at the exposed surface of the membrane. This stoichiometry is in good agreement with that calculated from the average Ag/TU ratio in the potentiometric titration. The investigated electrode can be used as a good sensor for mercury(II) and thiourea in strong acid media and a wide variety of practical analytical systems.     

Carbon Paste Electrode Bulk-Modified with the Conducting Polymer Poly(1,8-Diaminonaphthalene): Application to Lead Determination

Carbon paste electrodes modified with conducting polymers for sensitive and selective determination of lead are presented. A novel method for generating a reproducible polymer-coated electrode surface is developed. We prove that 1,8-diaminonaphthalene (1,8-DAN) mixed with a carbon paste electrode leads to a conducting polymer in acidic medium while, in the same medium, this polymer is known to be non-conducting on platinum electrode. The electrode behaviour of poly(1,8-DAN), electropolymerised into carbon paste, was investigated by cyclic voltammetry in the presence of the ferri-ferrocyanide couple.A carbon paste electrode modified with poly(1,8-DAN) was used to determine Pb²⁺ in aqueous solutions. Pb²⁺ ions were firstly complexed and then electrochemically deposited by a potential step to –0.9V. The accumulated lead after reduction was anodically stripped by differential pulse voltammetry. Different parameters, such as pH of the solution, preconcentration time, and electropolymerisation procedures were studied.For a preconcentration time of ten minutes, the calibration graph was linear from 40 to 2070ngmL^–1 with r²=0.998. The detection limit was found to be 30ngmL^–1, and the relative standard deviation was 6%.     

Development of cysteine-modified screen-printed electrode for the chronopotentiometric stripping analysis of cadmium(II) in wastewater and soil extracts

Screen-printed carbon electrodes were fabricated with amino acid functionality by using in situ co-deposition of mercury and cysteine. The three-electrode configuration (graphite carbon working electrode, carbon counter electrode and silver/silver chloride reference electrode) incorporating a cysteine-modified working electrode exhibited good sensitivity towards cadmium(II). Several experimental variables affecting the sensor stripping response were characterised and optimised. These include cysteine and mercury concentrations, deposition time, deposition potential and stripping current. Surface analysis was also conducted using scanning electron microscopy (SEM) in order to characterize the electrode surface during cadmium analysis. The stripping chronopotentiometric response for cadmium(II) was linear in the concentration range 0.4–800 g L^–1 when a deposition time of 2 min was used. A detection limit of 0.4 g L^–1 was obtained using 0.025 M Tris–HCl buffer containing 0.1 M KCl (pH 7.4) as the supporting electrolyte. The analytical utility of the cysteine-modified sensor was demonstrated by applying it to cadmium analysis in various wastewater and soil samples collected from a contaminated site and extracted using acetic acid. The results obtained using the developed electrodes agreed satisfactorily with the values achieved using atomic absorption spectrometry and inductively coupled plasma mass spectrometry analysis. These results demonstrate the feasibility of using this type of sensor for cadmium analysis.     

Novel Coated-Wire Membrane Sensor Based on Bis(Acetylacetonato) Cadmium(II) for the Determination of Chromate Ions

A new electrode based on a complex of chromate ions with bis(acetylacetonato) cadmium(II) as a carrier was developed for detection of chromate in aqueous solution. The electrode exhibited linear response with Nernstian slopes of –28.8±0.5mVper decade for chromate within the concentration range of 2.5×10^–6–0.1M. The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plots was almost 1.0×10^–6M. The electrodes exhibited good selectivities for chromate. The response time of the electrode was <25s over the entire concentration range. The electrode can be used in the pH range 8.0–12.0 for chromate. It was used as an indicator electrode in titration with Pb(NO₃)₂ and for the determination of chromate ion in wastewater samples.