Flow-injection analysis with a coated tubular solid-state copper(II)-selective electrode

The incorporation, behaviour and suitability of a home-made coated tubular solid-state copper(II)-selective electrode into the conduits of a flow-injection system is described. The compact tubular sensor (volume 7.8 μl) is constructed from a copper tube and Tygon tubing, treated with ammonium sulphide to give the copper/copper sulphide electrode and conditioned in an ascorbic acid medium. Interferences of foreign ions are similar to those found in batch analysis, but are less severe in the flow-injection system. Changes in carrier stream pH between 1 and 3 affected the electrode response, but sample pH has no or little influence in the range 1–7 depending on the copper(II) ion concentration in the samples. With 30-μl samples the flow-through electrode system covers a working range up to 5000 mg dm^?3 with a detection limit of 0.5 mg dm^?3. The system is suitable for the determination of copper(II) in effluent and tap water (relative standard deviation < 1.75% for 0.5–912 mg dm^?3 copper) and acidic copper sulphate plating solutions (relative standard deviation < 1.21% for 87–4135 mg dm^?3 copper) at a sample frequency of about 80 h^?1. The results obtained agree well with results by a standard atomic absorption spectrometric method.     

Copper(II)-selective electrode using 2,2'-dithiodianiline as neutral carrier

Poly(vinyl chloride) membrane electrode, that is highly selective and sensitive to Cu(II) ions, was developed by using 2,2'-dithiodianiline and dibutyl phthalate as carrier and plasticizer, respectively. The electrode exhibits good potentiometric response for Cu(II) over a wide concentration range (5.0x10(-2)-7.0x10(-7) mol l(-1)) with Nernstian slope of 30+/-1 mV per decade. The response time of the electrode is 10 s and it has been used for a period of one month and exhibits good selectivity towards Cu(2+) in comparison to alkali, alkaline earth, transition and heavy metal ions, with no interference caused by Pb(2+), Cd(2+) and Fe(+2) which are known to interfere with many other copper electrodes.     

Strontium(II)-selective electrode based on a macrocyclic tetraamide

A new PVC membrane electrode based on 5,7,12,14-dibenzo-2,3,9,10-tetraoxa-1,4,8,11-tetraazacyclotetradecane (I) as an ion carrier, o-nitrophenyloctyl ether (o-NPOE) as solvent mediator and sodium tetraphenylborate (NaTPB) as lipophilic additive was fabricated and investigated as Sr²⁺-selective electrode. The best performance was exhibited by the membrane having composition 8:200:4:120 (I:o-NPOE:NaTPB:PVC). The electrode exhibited a Nernstian response for strontium ion over a wide concentration range 3.98 × 10⁻⁶ to 1.0 × 10⁻¹ M with a slope of 29.0 ± 0.1 mV/decade of concentration and a detection limit of 2.82 × 10⁻⁶ M. It showed a response time of less than 10 s and could be used for at least 3 months without any divergence in potential. The proposed electrode showed a good discriminating ability towards strontium(II) ion over a wide variety of other metal ions including alkali, alkaline earth, transition, and heavy metal ions. The electrode can be used in the pH range of 2.5-10.5 and in mixtures containing up to 35% (v/v) non-aqueous content. It was used as an indicator electrode in potentiometric titration of strontium ion against EDTA.     

Copper(II)-selective electrode based on dithioacetal

A PVC membrane electrode for copper ion based on 1,3-dithiane,2-(4-methoxy phenyl) as ionophore and o-nitrophenyl octyl ether as a plasticizer is demonstrated. The electrode exhibits a Nernstian slope of 29.5±1 mV per decade in a linear range of 3.0×10⁻⁶ to 5.0×10⁻² M for Cu²⁺ ion. The detection limit of this electrode is 1.0×10⁻⁶ mol/l. This sensor has a very short response time of about 5 s and could be used in a pH range of 4.0-7.0. High selectivity was obtained over a wide variety of metal ions. The proposed electrode was successfully applied as an indicator electrode for the potentiometric titration of copper ion with EDTA and for the direct determination of copper in river water.     

Determination of nitrate in mineral water and sausage samples by using a renewable in situ copper modified electrode

A new approach for in situ electrodeposition of a renewable copper layer onto a copper electrode is reported. The active surface was obtained by anodic dissolution of a copper electrode at an appropriate potential and further redeposition of copper ions still remaining at the diffusion layer. Under optimal experimental conditions the peak current response increases linearly with nitrate concentration over a range of 0.1-2.5 mmol L⁻¹. The repeatability of measurements for nitrate was evaluated as 1.8% (N = 15) and the limit of detection of the method was found to be 11 μmol L⁻¹ (S/N = 3). Nitrate contents in two different samples (mineral water and sausages) compared well with those obtained from using the standard Griess protocol at a 95% of confidence level measured by the t-student test. The interference from chloride on the nitrate analysis and the possibility of simultaneous determination of nitrite were also examined.     

Determination of mercury(II) using a modified electrode with zeolite

Summary Chemically modified carbon paste electrodes are prepared by incorporating appropriate quantities of a zeolite directly into the paste mixture. Trace levels of mercury(II) can be preconcentrated on these electrodes. Cyclic voltammetry is used to quantify the accumulated ions. The response is characterized with respect to preconcentration, pH, and other variables. The procedure exhibits good linearity for 0.11 g/ml–2.2 g/ml mercury(II) ions and the peak current varies linearly with preconcentration time between 1 and 6 min for the conditions used.

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Bestimmung von Quecksilber(II) mit einer Zeolith-modifizierten Elektrode

     

Lead(II)-selective electrode based on phenyl disulfide.

A PVC membrane electrode for lead ions based on phenyl disulfide as the membrane carrier was developed. The electrode exhibits a good Nernstian slope of 29.3 +/- 0.7 mV/decade and a linear range of 2.0 x 10(-6)-1.0 x 10(-2) M for Pb(NO3)2. The limit of detection is 1.2 x 10(-6) M. It has a response time of 45 s and can be used for at least fifty days without any divergence in potential. The proposed membrane sensor revealed high selectivity for Pb2+ over a wide variety of other metal ions and could be used in the pH range of 3.5-6.3. The electrode was used as an indicator electrode in potentiometric titration of lead ions.     

Determination of radiocesium in environmental water samples using copper ferro(II)cyanide and sodium tetraphenylborate

Summary A procedure for the radiochemical separation and radiochemical purification of radiocesium (¹³⁴Cs and ¹³⁷Cs) in bulk environmental water samples is proposed. Radiocesium was removed from the water by cation-exchange with copper ferro(II)cyanide and was purified by precipitation with sodium tetraphenylborate. The influence of the concentration of potassium in the water sample on the chemical yield was investigated. The validation of the proposed method was carried out by analyzing reference materials. The application of the method was demonstrated with the determination of the concentration of radiocesium in water samples from rivers around NPP “Kozloduy”, Bulgaria, Danube and Ogosta.     

Determination of stability constant of copper(II) with thiosemicarbazide by use of a cupric ion-selective electrode

The stability constant of copper(II)—thiosemicarbazide complex has been determined potentiometrically by means of a cupric ion-selective electrode. Value of logK_CuL2 obtained in 0. 1 M KNO₃ at 25 °C corresponded well with the values given in the literature measured spectrophotometrically.     

The behaviour of copper(II)-selective electrodes in chloride-containing solutions

Calibration curves for copper(II) in solutions containing various chloride ion concentrations are examined in detail. The mechanism of the interferences on the behaviour of the copper(II)-selective electrode is discussed. The anomalous slopes observed can be explained by considering the sensitivity of the electrode membrane to the silver ions released from the electrode surface by chemical interaction with the solution. The effects of the formation of silver-chloride complexes and of the changes in the membrane are described.     

Simultaneous determination of cadmium(II), lead(II) and copper(II) by using a screen-printed electrode modified with mercury nano-droplets

We are presenting a strategy for the fabrication of disposable screen-printed electrodes modified with mercury nano-droplets and capable of sensing heavy metal ions. They were prepared by coating electrodes with a mixture of multi-walled carbon nanotubes and chitosan, this followed by adsorption of mercury. The resulting sensor was characterized by cyclic voltammetry and impedance spectroscopy. Also the effects caused by adsorption of mercury were investigated. It is shown that square wave anodic stripping voltammetry enables simultaneous determination of cadmium(II), lead(II) and copper(II), for which detection limits of 12, 23 and 20 nM, respectively, are found. Relative standard deviations for ten determinations at 0.6 µM concentrations of these ions are in the range of 3.0 to 5.7%. The applicability was tested by analyzing river water and showed recoveries between 94.1 and 104.6%, thus demonstrating its utility for in-field monitoring of these heavy metal ions.     

Determination of tantalum with hexafluorotantalate(V)-selective coated graphite electrode

A hexafluorotantalate(V)-selective coated-graphite electrode was prepared by coating a graphite rod with brilliant green-hexafluorotantalate(V) extract in 1-chloronaphthalene in a PVC matrix. Potential measurements were made against an HF-resistant plastic sleeve (Ag/AgCl) external reference electrode. The concentrations of sulfuric and hydrofluoric acids, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1M each in the test solutions. The electrode responded to hexafluorotantalate(V) in the linear range 5.0 × 10^–6-5.0 × 10^–3 M, with a slope of -58 mV per decade and detection limit of 8.0 × 10^–7 M within 5–15s. The relative standard deviation for six determinations of 1.0 × 10^–4 M tantalum(V) was 2%. The life-time of the electrode was 60 days. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) were studied and the electrode was found to be highly selective to hexafluorotantalate(V). Niobium, the element that commonly occurs with tantalum ores, showed a very low level of interference. The newly developed coated-graphite electrode has been applied to the determination of tantalum in tantalite-columbite ores and several synthetic matrices by direct, sample addition, standard addition, and Gran's plot potentiometric techniques with reasonable precision (2–4%) and accuracy.     

Determination of potassium ions in biodiesel using a nickel(II) hexacyanoferrate-modified electrode

In this work, nickel hexacyanoferrate-modified electrode was developed to determine potassium ions in biodiesel by potentiometry. The modified electrodes exhibit a linear response to potassium ions in the concentration range of 4.0 × 10⁻⁵ to 1.0 × 10⁻² mol L⁻¹, with a detection limit of 1.9 × 10⁻⁵ mol L⁻¹, and a near-Nernstian slope (53–55 mV per decade) at 25 °C. The method developed in this work was compared with flame photometry and the potassium concentration found in biodiesel showed that the modified electrode method gives results similar to those obtained by flame photometry.     

New cadmium(II)-selective electrode based on a tetraazacyclohexadeca macrocyclic ionophore.

A poly(vinyl chloride)-based membrane of 3,4:11,12-dibenzo-1,6,9,14-tetraazacyclohexadecane with sodium tetraphenyl borate (NaTPB) as an anion excluder and dibutylphthalate (DBP), dibutyl(butyl)phosphonate (DBBP), tris(2-ethylhexyl)phosphate (TEHP) and tributyl phosphate (TBP) as a plasticizing solvent mediator was prepared and investigated as a Cd2+-selective electrode. The best performance was observed with the membrane having a ligand-PVC-DBP-NaTPB composition of 2:25:60:1, which worked well over a wide concentration range (1.6 x 10(-6) - 1.0 x 10(-1) M) with a Nernstian slope of 29.5 mV per decade of activity between pH 2.0 - 6.0. This electrode showed a fast response time of 13 s and was used over a period of 4 months with good reproducibility (S = 0.4 mV). The selectivity coefficient for mono-, di- and trivalent cations indicated excellent selectivity for Cd2+ ions over a large number of cations. Anions such as NO3- and SO4(2-) , did not interfere. The membrane sensor has been successfully used to determine Cd2+ in real samples.     

Chloride interference with non-stoichiometric copper sulphide copper(II)-selective electrodes: Part 3. New equation for the electrode response

A general equation for the electrode potential response under metastable equilibrium conditions is derived for the non-stoichiometric copper sulphide electrodes immersed in solutions containing Cu(II) and Cu(I) and the interfering chloride ions. The theoretically predicted “anomalous” slopes (S_II = δ E/δ) log C_Cu(II) of the calibration curves, greater than 29.6 mV at 25° C, were confirmed experimentally in concentrated chloride solutions. The inapplicability of the Nicolsky equation for the electrodes at equilibrium is discussed on the basis of the theory presented. The new relationship (Eqn. 13) can be adapted for other ion-selective electrodes for which the exchange reactions with the solution species are fast enough and mass transport is not inhibited.     

Biosorption of Ni(II) from electroplating wastewater by modified (Eriobotrya japonica) loquat bark

Biosorption of nickel ions from aqueous solutions by modified loquat bark waste (MLB) has been investigated in a batch biosorption process. The biosorbent MLB was characterized by FTIR analysis. The extent of biosorption of Ni(II) ions was found to be dependent on solution pH, initial nickel ions concentration, biosorbent dose, contact time, and temperature. The experimental equilibrium biosorption data were analyzed by three widely used two-parameters Langmuir, Temkin and Freundlich isotherm models. Langmuir and Temkin isotherm models provided a better fit with the experimental data than Freundlich isotherm model by high correlation coefficients R². The maximum adsorption capacity was 27.548 mg/g of Ni(II) ions onto MLB. The thermodynamic analysis indicated that the biosorption behavior of nickel ions onto MLB biosorbent was an endothermic process, resulting in higher biosorption capacities at higher temperatures. The negative values of ΔG° (−5.84 kJ/mol) and positive values of ΔH° (13.33 kJ/mol) revealed that the biosorption process was spontaneous and endothermic. Kinetic studies showed that pseudo-second order described well the biosorption experimental data. The modified loquat bark (MLB) was successfully used for the biosorption of nickel ions from synthetic and industrial electroplating effluents.     

Determination of selenium(IV) by cathodic stripping voltammetry using a copper-modified mercury-film electrode modified with copper

The behavior of selenium(IV) at a mercury-film electrode previously modified with copper was studied by cathodic stripping voltammetry using an automatic system for replacing solutions without opening the circuit. The effect of chemical (the composition and concentration of the supporting electrolyte, the concentrations of mercury (II) and copper(II)) and electrochemical parameters (the potential and time of electrolysis) on the analytical signal of selenium was studied at each stage of the stripping cycle (the formation of the mercury-film electrode modified with copper, the electrochemical preconcentration of selenium, and the subsequent stripping of the concentrate). The detection limit for selenium(IV) was found to be 40 ng/L (5.0 × 10^?10 M) at an electrolysis time of 10 min.     

Kinetic influences on studies of copper(II) hydrolysis by copper ion-selective electrode

The hydrolysis of micromolar solutions of copper(II) is dominated by the formation of a copper—hydroxy colloid, both in the presence and absence of atmospheric carbon dioxide. Presence of the colloid was inferred from the response of the copper ion-selective electrode and confirmed by light scattering measurements. The colloid is only slowly converted to thermodynamically more stable species, with the result that copper—carbonate complexes are not formed to any significant extent in solutions exposed to atmospheric carbon dioxide. However, copper—carbonate complexes are formed in solutions containing a constant amount of total carbonate. Speciation and complexation capacity measurements should be interpreted with caution because thermodynamic equilibrium may not be attained in solution.     

The formation of mixed copper(II) sulfide—silver(I) sulfide membranes for copper(II)-selective electrodes

Several methods for the preparation of mixed copper(II) sulfide—silver(I) sulfide precipitates have been investigated. Pellets of these materials have been tested for their suitability as copper(II)-selective membranes. Homogeneous membranes with satisfactory electrochemical properties can be prepared only from precipitates obtained by addition of the metal salts to sodium sulfide Membrane leakages, limits of detection, calibration curves and titration curves with different types of precipitate are discussed.