Determination of hardness in tapwater and upland soil extracts using a long-term stable divalent cation selective electrode based on a lipophilic acrylate resin as a membrane matrix

A divalent cation-selective electrode, which utilizes a lipophilic resin as a matrix for the sensing membrane, and which has long-term stability has been developed. The sensing membrane is a lipophilic acrylate resin which is impregnated with a solution of 1-decylalcohol and the calcium salt of bis[4-(1,1,3,3-tetramethylbutyl) phenyl] phosphate at concentrations of 0.08 g ml(-1) each. The electrode exhibited nearly equal selectivity to Ca(2+) and Mg(2+) ions and could be used as a water hardness sensor. The electrode shows a Nernstian response with a slope of 29 mV decade(-1) to both Ca(2+) and Mg(2+) ions in the concentration range from 10(-5) M to 10(-1) M and could be used in the pH range from 3 to 10 for the determination of 10(-3) M Ca(2+) and Mg(2+) solutions. The initial performance of the electrode could be maintained for 1 year, since the lifetime test of the electrode was conducted in tapwater at a continuous flow rate of 4 ml min(-1). The hardnesses of tapwater and upland soil extracts were determined using the developed electrode and the analytical results were in good agreement with those obtained by chelatometric titration using an EDTA solution as the titrant. A coefficient factor of correlation 0.998 was obtained between the electrode method and titrimetry. The long-term stability of the electrode was found to be due to strong affinity of 1-decylalcohol to the lipophilic acrylate resin.     

Development of pH measurement system for legal traceability of pH standard solutions

The best reproducible technology of pH measurement for precise pH buffer solutions regulated by Japanese Industrial Standards (JIS) was studied. A pH meter was devised with a high resolution of +/- 0.0001 pH. An 18-bit analog-to-digital converter is used, one-bit resolution corresponding to 0.0019 mV (ca. 0.000032 pH) against an input electrode potential +/- 500 mV. Digital data were treated smoothly for some types of noise, a reproducibility of +/- 0.0002 pH being obtained with a potentiometer. A flow cell was devised to attain temperature control within +/- 0.03 degrees C and air-tight measurement prevented contamination with carbon dioxide. Also, the flow cell has a structure such that potassium chloride (KCl) inner solution effused from a ceramic junction of the reference electrode designed so as not to touch the glass membrane. A combination pH electrode (a glass electrode and a reference electrode) was assembled to minimize the dead volume of sample solution. This highly sensitive pH measuring system, consisting of a pH meter, a flow cell, a combination pH electrode, a circulating water thermostat and a peristaltic pump, was used for the certification of pH standard solutions in Japanese metrological law. The performance of this system was within +/- 0.0006 pH reproducibility and 20-30 min response time (5 min within +/- 0.0002 pH) at a sample flow rate of 3 ml min (-1).     

Co-immobilization of polymeric luminol,iron(II) tris(5-aminophenanthroline) and glucose oxidase at an electrode surface,and its application as a glucose optrode

The anodic polymerization of 3-aminophthalhydrazide (luminol) and iron(II) tris 5-aminophenanthroline (Fe(phen-NH2)3(2+)) has been reported in this paper. A bilayer electrode was developed based on these polymers and the ITO conductive glass (denoted ITO[Fe(phen-NH2)3(2+)]luminol electrode). This electrode emitted light (lambdaem: 430 nm) as it was brought into contact with H2O2. At pH 10, the resulting electrochemiluminescence (ECL) showed a linear relationship with the concentration of H2O2 in the range of 10 microM(-1) mM. This bilayer electrode also showed an application potential for the detection of glucose after being further modified with glucose oxidase (denoted ITO[Fe(phen-NH2)3(2+)]luminol]GOx electrode). Although the resulting ECL decayed more rapidly in concentrated glucose solutions (e.g., I M) because of the consumption of luminol during use, the decay became less severe in diluted glucose solutions (e.g., 10 mM). According to the flow injection analysis, a linear relationship existed between the ECL and the concentration of glucose from 10(-5)-10(-3) M at pH 9. The detection limit could reach a level of 5 x 10(-5) M at this pH.     

Construction of a new Hg(2+)-selective liquid-state electrode

The construction and basic characteristics of a new liquid-state Hg(2+)-selective membrane electrode are discussed. The membrane consists of the PAN chelate of Hg(II), dissolved in CHCl(3). The linear response range of the electrode is 10(-1) -10(-5)M Hg(2+) with a slope of 28.5 mV/decade of concentration. The response time of the electrode in dilute solutions is less than 3 min, and in concentrated solutions it is only a few sec. The electrode has been used in precipitation or complexation titrations in which Hg(2+) is involved (e.g., in the determination of some organic compounds in drug synthesis).     

Multi-analyte sensing: a chemometrics approach to understanding the merits of electrode arrays versus single electrodes

A peptide-modified electrode array with a different peptide on each electrode is compared with a single electrode modified with many peptides for the voltammetric measurement of concentrations of Cu(2+), Cd(2+) and Pb(2+) in solution. The single gold electrode was modified simultaneously with peptides Gly-Gly-His, glutathione and angiotensin I each coupled to thioctic acid, and thioctic acid itself, and the calibration of mixtures of ions was compared with previously published data from an array of four sensors each with an individual modification. Calibration at the multi-peptide single-electrode sensor was by two-way partial least squares (voltammetric current measured with variables 'sample' x 'potential') and for the electrode array by three-way NPLS1 ('sample' x 'potential' x 'electrode'). The advantage of designing experiments to yield multi-way data is demonstrated and discussed.     

Measuring rapid kinetics by a potentiometric method in droplet-based microfluidic devices

Droplets containing RNA and Mg(2+) were generated in microfluidic channels. By integrating a group of pneumatic valves and phase separation channels in the microfluidic system, the rapid RNA-Mg(2+) binding kinetics was studied by measuring the Mg(2+) ion concentration using an ion-selective electrode.     

Strontium(II) sensor based on a modified calix[6]arene in PVC matrix.

5,11,17,23,29,35-Hexakis(1,1,3,3-tetramethylbutyl)-37,38,39,40,41,42-hexakis(carboxy methoxy)calix[6]arene (I) has been evaluated as an ionophore for the analysis of Sr2+. The influences of the nature of the plasticizers (DBA, CN, DOP, NPOE) and of the anion excluder (NaTPB) on the characteristics of the electrode were discussed. The best electrode was fabricated with a membrane having composition 6:150:170:3 (I:PVC:DBA:NaTPB). The response to Sr2+ was Nernstian in the range 1.9 x 10(-5) to 1.0 x 10(-1) M of Sr2+. The influence of pH has also been studied. The electrode exhibited better potential stability and had an operational lifetime of 4 months. The K(A,B)(Pot) values showed that other alkaline earth metal ions are well discriminated. The sensor has also been used as an indicator electrode in the potentiometric titration of sodium carbonate with strontium(II) ions.     

Determination of the total dissolved sulphide in the pH range 3-11.4 with sulphide selective ISE and Ag/A92S electrodes

An electrochemical method for the determination of the total sulphide concentration of sewage water samples has been studied using a potentiometric cell containing either a sulphide ion selective ISE-glass electrode pair or a Ag/Ag(2)S electrode-glass electrode system. The performance of the two sulphide ion sensors was investigated and compared in both acidic and basic pH ranges. It was proved that the cell potential can be made directly proportional to the logarithm of the total sulphide concentration when both the pH < lg K(2) - 1.5 condition (acidic range) and also when the lg K(2) + 1.5 < pH < lg K(1) - 1.5 condition prevails in the system (alkaline range); where K(1) and K(2) are the first and second protonation constants of the sulphide ion, respectively. A suitable calibration method for a wide range of sulphide concentration is also presented for both ranges of the pH scale. The overall performance of the measuring system was tested using model solutions and real waste water samples.     

Electrochemical behavior of pyrroloquinoline quinone immobilized on silica gel modified with zirconium oxide

Pyrroloquinoline quinone (PQQ) was immobilized on the silica gel surface modified with zirconium oxide, designated as Si:Zr, by the carboxylic groups of the PQQ molecule and the zirconium oxide on the silica surface. The electrochemistry of PQQ immobilized on the Si:Zr matrix, incorporated in a carbon paste electrode, was evaluated using the cyclic voltammetry technique. The Si:Zr:PQQ-modified electrode showed a redox couple at E(m)=(E(pa1)+E(pc))/2=-0.150 V vs SCE at pH 7, close to that observed in aqueous solution, and another oxidation peak, E(pa2)=-0.100 V vs SCE. Studies in different pH solutions in the range of 3-7 showed that the first oxidation peak, E(pa1), is highly dependent on the solution pH shifting from to -0.175 to 0.100 V vs SCE, while E(pa2) remains practically constant at 0.100 V as the pH decreases from 7 to 3. The immobilized PQQ electrode presented the property to electrocatalyze the NADH at 150 mV vs SCE. The effect of addition of Ca(2+) ions on the electrode electroactivity for the NADH oxidation was also verified. Different from that observed for the PQQ immobilized on other electrode materials, the Ca(2+) ions did not influence the electrocatalytical response; however, the electrode stability was considerably improved in the presence of Ca(2+) ions, indicating that the matrix surface has a great influence on the electrochemical behavior of PQQ.     

Anodic stripping voltammetric determination of mercury by use of a sodium montmorillonite-modified carbon-paste electrode

A sodium montmorillonite (SWy-2)-modified carbon-paste electrode has been examined for determination of trace levels of mercury. Because of its strong cation-exchange and adsorptive characteristics, SWy-2 greatly improves the sensitivity of determination of Hg(2+). Hg(2+) is preconcentrated and reduced on the modified electrode surface at -0.40 V and then stripped from the electrode surface during the positive potential sweep. The conditions used for determination, e.g. supporting electrolyte, pH, amount of SWy-2, accumulation potential, and accumulation time, were optimized. The peak current was linearly dependent on the concentration of mercury from 1 x 10(-9) to 5 x 10(-7) mol L(-1). The detection limit (signal-to-noise ratio=3) was 1 x 10(-10) mol L(-1) after accumulation for 6 min. When the SWy-2-modified carbon-paste electrode was used to detect mercury in water samples the average recovery was 101.11%.     

The behavior of a poly(aniline) solid contact pH selective electrode based on N,N,N',N'-tetrabenzylethanediamine ionophore.

A hydrogen ion-selective solid-contact electrode based on N,N,N',N'-tetrabenzylethanediamine has shown the best Nernstian slope and selectivity and the widest response range in a Tris buffered pH sample solution. Its linear dynamic range was pH 3.50-11.94, and the Nernstian slope showed 52.1 mV/pH (at 20 +/- 0.2 degrees C). When it was directly applied to human whole blood (in pH range 6.0-8.5) we could obtain the same satisfying results. This electrode continuously contacted a Tris 7.47 buffered solution, human whole blood and a hydrofluoric acid solution for one month without any loss of performance. Also, hydrofluoric acid did not influence the surface of this electrode, and thus it was maintained without showing any changes in potentials after being used in a hydrofluoric acid solution. The standard deviation in the determined e.m.f. differences was 1.5 mV (N = 5) for Tris buffer solutions of pH 6.5 and 1.1 mV at a Tris buffer solutions of pH 8.5. The 90% response time of the electrodes obtained by injecting of hydrochloric acid into the Tris buffer sample solution was less than 10 s. Especially, in the this paper, with these potential response characteristics of hydrogen ion selective poly(aniline) solid contact electrode, we have also presented the pH response mechanism of this electrode and the role of poly(aniline) and a doped anion in a poly(aniline) layer.     

Development of ultra-high sensitive and selective electrochemiluminescent sensor for copper(II) ions: a novel strategy for modification of gold electrode using click chemistry

A promising and highly sensitive electrochemiluminescence (ECL) sensor for the detection of Cu(2+) based on Cu(+)-catalyzed click reaction is described in this paper. Firstly, 1-azidoundecan-11-thiol was assembled on the Au electrode surface via a simple thiol-Au reaction, then the propargyl-functionalized Ru(bpy)(3)(2+)-doped SiO(2) nanoparticles (Ru-SNPs) ECL probe was covalently coupled on the electrode surfaces via click chemistry. Cu(+), the catalyst for click chemistry, is derived from the electrolytic reduction of Cu(2+)via the Bulk Electrolysis with coulometry (BE) technique and without any reductants. It is found that the ECL intensity detected from the electrode surface has a linear relationship with the logarithm of Cu(2+) concentration in the range of 1.0 × 10(-15) to 1.0 × 10(-11) M with a detection limit of 1.0 × 10(-16) M. Also, the method is highly specific even in the presence of high concentrations of other metal cations. It has been applied to detect trace Cu(2+) in complex samples (hepatoma cell) without sample treatment.     

Determination of cadmium and copper with ET-AAS after electrochemical deposition on a graphite electrode

The electrodeposition of cadmium and copper on a special graphite disk electrode has been performed at controlled potential. The electrode with the deposit has been inserted into the graphite atomizer HGA-400 by an adapted automatic sampler for the final determination by ET-ASS. The sensitivity of determination has been 0.371 (microg l(-1))(-1) for cadmium and 0.025 (microg l(-1))(-1) for copper for 2 min electrodeposition and increased linearly with the time of deposition. The limit of detection (3s(bl)) has been 7.9 ng l(-1) Cd(2+) and 0.11 microg l(-1) Cu(2+) for 2 min deposition and it has been improved with increased time of electrodeposition. The technique has been applied to the determination of both metals in seawater and to speciation in the presence of EDTA complexing agent.     

Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the simultaneous determination of Pb(II), Cu(II) and Hg(II) ions in natural water and sugar cane spirit (cacha?a) is described. Pb(II), Cu(II) and Hg(II) were pre-concentrated on the surface of the modified electrode by complexing with 2-benzothiazolethiol and reduced at a negative potential (-0.80 V). Then the reduced products were oxidised by DPASV procedure. The fact that three stripping peaks appeared on the voltammograms at the potentials of -0.48 V (Pb2+), -0.03 V (Cu2+) and +0.36 V (Hg2+) in relation to the SCE, demonstrates the possibility of simultaneous determination of Pb2+, Cu2+ and Hg2+. The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 3 min accumulation time, 25 mV s(-1) scan rate in phosphate solution pH 3.0. Using such parameters, calibration graphs were linear in the concentration ranges of 3.00-70.0 x 10(-7) mol L(-1) (Pb2+), 8.00-100.0 x 10(-7) mol L(-1) (Cu2+) and 2.00-10.0 x 10(-6) mol L(-1) (Hg2+). Detection limits of 4.0 x 10(-8) mol L(-1) (Pb2+), 2.0 x 10(-7) mol L(-1) (Cu2+) and 4.0 x 10(-7) mol L(-1) (Hg2+) were obtained at the signal noise ratio (SNR) of 3. The results indicate that this electrode is sensitive and effective for simultaneous determination of Pb2+, Cu2+ and Hg2+ in the analysed samples.     

Highly sensitive determination of cadmium and lead using a low-cost electrochemical flow-through cell based on a carbon paste electrode

A low-cost thin-layer electrochemical flow-through cell based on a carbon paste electrode (CPE), was constructed for the highly sensitive determination of cadmium(II) (Cd(2+)) and lead(II) (Pb(2+)) ions. The sensitivity of the proposed cell for Cd(2+) and Pb(2+) ion detection was improved by using the smallest channel height without the need for any complicated electrode modification. Under the optimum conditions, the detection limits of Cd(2+) and Pb(2+) ions (0.08 and 0.07 μg dm(-3), respectively) were 13.8- and 11.4-fold lower than that of a commercial flow cell (1.1 and 0.8 μg dm(-3), respectively). Moreover, the percentage recoveries of Cd(2+) and Pb(2+) for the in-house designed thin-layer flow cell were higher than those for the commercially available cell in all tested water samples, and within the acceptable range. The proposed flow cell is promising as an inexpensive and alternative one for the highly sensitive monitoring of heavy metal ions.     

Assignment of standard pH values [pH*(s)] to buffers in 50 mass % methanol + water from 288.15 to 308.15 K

The secondary dissociation constants of o-phthalic and phosphoric acids have been determined in methanol + water (50 mass %) from reversible e.m.f. measurements of the cell of the type: Pt, H(2)(1 atm)|M(2)A(m), MHA(m), MCl|AgCl; Ag at different temperatures (288.15-308.15 K) and at different ionic strengths. To minimize the unsteadiness in potential measurements palladium coated platinum electrodes have been used. The large set of such e.m.f. values has been analyzed in terms of a multi-linear regression method recommended in recent IUPAC documents. The thermodynamic values DeltaG degrees , DeltaH degrees and DeltaS degrees , for the respective equilibria, were estimated. Standard pH values [pH*(s)] have been assigned to buffers in methanol + water (50 mass %) at temperatures between 288.15 and 308.15 K.     

Flow-injection determination of phosphate with a cadmium ion-selective electrode

A flow-injection method is described, in which phosphate standards are introduced into a reagent stream containing Cd(2+) ,resulting in the formation of Cd(3)(PO(4))(2). The associated reduction in free metal concentration is sensed by a cadmium-selective electrode. With the exception of major interference from iodide and moderate interference from bromide and thiocyanate, the system exhibits excellent response to phosphate and selectivity over several common anions in solutions buffered at pH 8.4. A maximum sampling rate of 160/hr is possible for phosphate standards in the concentration range 10(-1)-10(-1)M with a 10(-4)M Cd(2+) reagent stream at a total flow-rate (carrier and reagent stream combined) of 8.4 ml/min.     

Determination of copper by electrothermal AAS after electrodeposition on a graphite disk electrode

The electrodeposition of copper on a graphite electrode at a constant potential with subsequent atomization in the graphite atomizer HGA-400 has been studied. A special graphite disk electrode is suitable for electrochemical enrichment at E = -0.7 V vs. SCE and the determination of copper by electrothermal-atomic absorption spectrometry (ET-AAS) if atomized at 2300 degrees C. In this way copper was determined in potable water and free Cu(2+) could be distinguished from that bound in chelate speciations after using a suitable deposition potential of the working electrode. This approach seems to be an alternative to the commonly used anodic stripping voltammetry (ASV) for the preconcentration and determination of free metal ions.     

Multi-competitive interaction of As(III) and As(V) oxyanions with Ca(2+), Mg(2+), PO(3-)(4), and CO(2-)(3) ions on goethite

Complex systems, simulating natural conditions like in groundwater, have rarely been studied, since measuring and in particular, modeling of such systems is very challenging. In this paper, the adsorption of the oxyanions of As(III) and As(V) on goethite has been studied in presence of various inorganic macro-elements (Mg(2+), Ca(2+), PO(3-)(4), CO(2-)(3)). We have used 'single-,' 'dual-,' and 'triple-ion' systems. The presence of Ca(2+) and Mg(2+) has no significant effect on As(III) oxyanion (arsenite) adsorption in the pH range relevant for natural groundwater (pH 5-9). In contrast, both Ca(2+) and Mg(2+) promote the adsorption of PO(3-)(4). A similar (electrostatic) effect is expected for the Ca(2+) and Mg(2+) interaction with As(V) oxyanions (arsenate). Phosphate is a major competitor for arsenate as well as arsenite. Although carbonate may act as competitor for both types of As oxyanions, the presence of significant concentrations of phosphate makes the interaction of (bi)carbonate insignificant. The data have been modeled with the charge distribution (CD) model in combination with the extended Stern model option. In the modeling, independently calculated CD values were used for the oxyanions. The CD values for these complexes have been obtained from a bond valence interpretation of MO/DFT (molecular orbital/density functional theory) optimized geometries. The affinity constants (logK) have been found by calibrating the model on data from 'single-ion' systems. The parameters are used to predict the ion adsorption behavior in the multi-component systems. The thus calibrated model is able to predict successfully the ion concentrations in the mixed 2- and 3-component systems as a function of pH and loading. From a practical perspective, data as well as calculations show the dominance of phosphate in regulating the As concentrations. Arsenite (As(OH)(3)) is often less strongly bound than arsenate (AsO(3-)(4)) but arsenite responses less strongly to changes in the phosphate concentration compared to arsenate, i.e., deltalogc(As(III))/deltalogc(PO(4)) approximately 0.4 and deltalogc(As(V))/deltalogc(PO(4)) approximately 0.9 at pH 7. Therefore, the response of As in a sediment on a change in redox conditions will be variable and will depend on the phosphate concentration level.     

Electroreduction of aromatic oximes: diprotonation, adsorption, imine formation, and substituent effects

Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.