A new ion-selective electrode for anionic surfactants

The new ligand 7-methyl-7,13-di-octyl-1,4,10-trioxa-13-aza-7-azonia-cyclopentadecane (L¹) has been designed, synthesised and used as ionophore in the development ion-selective electrodes for anionic surfactants. Different PVC-membrane anionic-surfactants-selective electrodes were prepared by using L¹ as ionophore and bis(2-ethylhexyl)sebacate (BEHS), dibutyl phthalate (DBP) and nitrophenyl octyl ether (NPOE) as plasticizers. The PVC-membrane electrode containing L¹ and NPOE (electrode E1) showed a Nernstian response to lauryl sulfate with a slope of −59.5 mV per decade in a range of concentrations from 1.3 × 10⁻⁶ to 6.8 × 10⁻³ M and a detection limit of 6.0 × 10⁻⁷ M. The electrode E1 also showed a reasonable response to other alkyl sulfates and alkylbenzene sulfonates, whereas it does not respond to carboxylates and to cationic and non-ionic surfactants. A similar electrode to E1 but additionally containing the cationic additive n-octylammonium bromide was also prepared (electrode E2) and compared with the response of E1. Selectivity coefficients for different anions with respect to lauryl sulfate were determined by means of the fixed interference method considering lauryl sulfate as the principal anion and using a concentration of 1.0 × 10⁻² mol dm⁻³ for the corresponding interfering anion. The selectivity sequence found for the electrode E1 was: LS⁻ > SCN⁻ > ClO₄⁻ > CH₃COO⁻ > I⁻ > HCO₃⁻ > Br⁻ > NO₃⁻ > NO₂⁻ > Cl⁻ > IO₃⁻ > phosphate > SO₃²⁻ > C₂O₄²⁻ > SO₄²⁻. Electrode E1 showed remarkably better selectivity coefficients than electrode E2.     

Simultaneous electrochemical determination of nitrate and nitrite in aqueous solution using Ag-doped zeolite-expanded graphite-epoxy electrode

In this work a new electrochemical sensor based on an Ag-doped zeolite-expanded graphite-epoxy composite electrode (AgZEGE) was evaluated as a novel alternative for the simultaneous quantitative determination of nitrate and nitrite in aqueous solutions. Cyclic voltammetry was used to characterize the electrochemical behavior of the electrode in the presence of individual or mixtures of nitrate and nitrite anions in 0.1 M Na₂SO₄ supporting electrolyte. Linear dependences of current versus nitrate and nitrite concentrations were obtained for the concentration ranges of 1-10 mM for nitrate and 0.1-1 mM for nitrite using cyclic voltammetry (CV), chronoamperometry (CA), and multiple-pulsed amperometry (MPA) procedures. The comparative assessment of the electrochemical behavior of the individual anions and mixtures of anions on this modified electrode allowed determining the working conditions for the simultaneous detection of the nitrite and nitrate anions. Applying MPA allowed enhancement of the sensitivity for direct and indirect nitrate detection and also for nitrite detection. The proposed sensor was applied in tap water samples spiked with known nitrate and nitrite concentrations and the results were in agreement with those obtained by a comparative spectrophotometric method. This work demonstrates that using multiple-pulse amperometry with the Ag-doped zeolite-expanded graphite-epoxy composite electrode provides a real opportunity for the simultaneous detection of nitrite and nitrate in aqueous solutions.     

Coated wire ion-selective electrode for palladium(II) and its application to catalyst analysis

The construction, performance and application of polymeric membrane (PME) and coated wire (CWE) palladium(II)-selective electrodes based on the ion pair between tetra bromopalladate(II) and hexadecylpyridinium cation (HDP⁺) in a poly(vinyl chloride) matrix, plasticized with o-NPOE are described. The influence of membrane composition, bromide ion concentration and pH on the potentiometric responses of electrodes were investigated. Nernstian responses were obtained for the two type of electrodes with low limits of detection (1.0×10⁻⁶ M for PME and 5.0×10⁻⁸ M for CWE). The potentiometric responses are independent of the pH of test solution in the range 3–8. The response time of electrodes are fast (30 s for PME and 10 s for CWE), and they can be used for at least 3 months without any considerable divergence in potentials. The proposed electrodes revealed good selectivity for palladium(II) respect to different cations and anions. They were used to the direct potentiometric determination of palladium(II) in silicon-alumina catalysts.     

Sequential injection analysis of chloride and nitrate in waters with improved accuracy using potentiometric detection

Accurate simultaneous analysis of different anionic species using ion-selective electrodes (ISEs) can be achieved even for non-specific sensors by resorting to an ordinary least squares multiple regression in the vicinity of the predicted concentrations. In this work the potentialities of this approach are evidenced by the determination of nitrate and chloride in synthetic and real water samples in which chloride concentration was significantly higher than nitrate. An AgCl/Ag₂S electrode based on a homogeneous crystalline membrane together with a PVC electrode based on tert-octylammonium bromide dissolved in dibutylphthalate were used as potentiometric detectors for chloride and nitrate, respectively. For the implementation of the procedure, an automatic system based on sequential injection analysis was used. The results obtained by the standard addition method were biased for low concentrations of nitrate and were dependant on the relative proportion of NO₃⁻/Cl⁻. The results obtained by the proposed methodology for chloride determination were slightly better when compared to those obtained by the standard addition method. In relation to nitrate determination, the proposed methodology yielded values with a relative root mean square error of prediction (RRMSEP) of 2.8%, while for standard addition calibration, mean error values were approximately 12.1%.     

Flow injection analysis for calcium in serum,water and waste waters by spectrophotometry and by ion-selective electrode

The Flow Injection technique is shown to provide fast, reliable and sensitive methods for the determination of calcium in various aqueous as well as serum samples; spectrophotometric or potentiometric detection can be used. At sampling rates of 100–110 samples per hour, with 30-μl sample injections, high reproducibility of measurement and low reagent consumption are achieved in both methods. In the spectrophotometric method, the analytical readout is available within 12 s after sample injection at a total reagent consumption of 0.75 ml per analysis. The potentiometric measurement of the calcium activity in serum is placed on a reliable basis by alternating measurements of serum samples and aqueous standards without incurring any non-reproducible changes in potential between aqueous and serum solutions. This permits the simultaneous determination of pH and pCa, the analytical readout being available within XXX s of sample injection. The good agreement between the results obtained with the Flow Injection method and those attained by atomic absorption and EDTA titrations as well as pCa stat-measurements show that the new methods are potentially suitable for routine analysis.     

Experimental evidence for the surface change of solid state copper ion-selective electrodes in chloride medium

The surface changes and potentiometric response of three copper ion-selective membranes (CuS precipitate-based, CuS/Ag₂S precipitate based and CuS mineral (covellite)-based) were studied after pretreatment in chloride ion containing solutions. The composition change of the membranes was identified by X-ray photoelectron spectroscopy (XPS). A correlation between surface change and response function was proved.     

Simplified version of the sodium salicylate method for analysis of nitrate in drinking waters

The salicylate method for determination of nitrate in drinking waters has been simplified by using the calibration solutions for 54 days, and therefore, reducing the work time. The results obtained can be considered satisfactory with regard to limit of determination (0.10 mg l⁻¹ nitrate as nitrogen (NO₃⁻-N)), precision (relative standard deviations <4% for five determinations were found) and accuracy (recoveries from 88 to 106% for nitrate in spiked drinking water were obtained).     

Determination of water,ammonium nitrate and sodium nitrate content in ‘water-in-oil’ emulsions using TG and DSC

The main component of an emulsion explosive is a water-in-oil emulsion consisting of a supersaturated ammonium nitrate (AN) water phase, finely dispersed in an oil phase. Quantitative determination of nearly all the components in a W/O emulsion is possible using thermogravimetry (TG) and differential scanning calorimetry (DSC). Isothermal TG measurements enable determination of water content, while cycled DSC measurements allow the amount of ammonium nitrate to be determined. In the case that sodium nitrate (SN) is also added to AN as an oxidizing agent, it is necessary to quantitatively separate both salts from organic matter with diethyl ether. On the basis of the TG curve of the precipitated salts, the amount of AN can then be calculated, and that of SN is obtained from TG measurement of the original sample.     

Interactions of ionic surfactants with gelatin in fluid solutions and the gel state studied by fluorescence techniques,potentiometry, and measurements of viscosity and gel strength

The photobehavior of pyrene (fine structure of the monomeric fluorescence band; excited state lifetimes; excimer/monomeric fluorescence intensity ratios and excited state quenching rates by oxygen) in fluid solutions of gelatin and in the gel state, both in the absence and presence of ionic surfactants, has been examined. Surfactants considered were sodium dodecylsulfate and dodecyltrimethylammonium bromide. Potentiometric measurements performed by using surfactant selective electrodes allowed for the determination of the binding capacity of the surfactants onto gelatin in the gel state. When complemented with viscosity and gel strength measurements, the sesults obtained allow for a discussion of the effects of the surfactant-gelatin interactions on microscopic properties of the solutions or gels and their relation with macroscopic properties.     

A modified spectrophotometric method for nitrate in plants,soils and water by nitration of 3,4-dimethylphenol

A spectrophotometric method suitable for rapid and precise routine analysis of plant tissue, soils and water samples with a wide range of nitrate nitrogen values is described. Ethanol replaces the acetone sometimes used as solvent for the 3,4-dimethylphenol reagent, so that spectral interference caused by acetone is eliminated. The time required for complete nitration of the reagent is reduced from 25 min to that required for mixing of reactants. Chloride ion levels of up to 6.0% in plant tissue, 1940 μg g^-1 and 970 μg ml^-1 in soil and water, respectively, do not interfere. The probable reaction mechanism of acetone interference is discussed.     

Simultaneous analysis of nitrate and nitrite in a microfluidic device with a Cu-complex-modified electrode

A CE microsystem coupled with a microchip and a copper-(3-mercaptopropyl) trimethoxysilane (Cu-MPS) complex-modified carbon paste electrode (CPE) was developed for the simultaneous analysis of nitrite and nitrate. The method is based on the electrocatalytic reduction of both analytes with the modified electrode. The Cu-MPS complex was characterized by voltammetric, XPS, and FT-IR analyses. Experimental parameters affecting the sensitivity of the modified electrode were assessed and optimized. The best separation was achieved in a 60 mm separation channel filled with a 20 mM acetate buffer of pH 5.0 containing 3.0 mM CTAB at separation field strength of -250 V/cm within 90 s. The detection potential for the simultaneous analysis of nitrite and nitrate was found to be -225 mV versus Ag/AgCl. A reproducible response (RSD of 3.2% (nitrite) and 2.8% (nitrate), n = 8) for repetitive sample injections reflected the negligible electrode fouling at the modified CPE. The interference effect was examined for other inorganic ions and biological compounds. A wide hydrodynamic range between 0.25 and 120 microM was observed for analyzing nitrite and nitrate with the sensitivities of 0.069 +/- 0.003 and 0.065 +/- 0.002 nA/microM, and the detection limits, based on S/N = 3, were found to be 0.09 +/- 0.007 and 0.08 +/- 0.009 microM, respectively. The applicability of the method to water and urine samples analyses was demonstrated.     

Development of a simple method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry

In this work, it was developed a method for the determination of nitrite and nitrate in groundwater by high-resolution continuum source electrothermal molecular absorption spectrometry of NO produced by thermal decomposition of nitrate in a graphite furnace. The NO line at 215.360 nm was used for all analytical measurements and the signal obtained by integrated absorbance of three pixels. A volume of 20 μL of standard solution or groundwater sample was injected into graphite furnace and 5 μL of a 1% (m/v) Ca solution was co-injected as chemical modifier. The pyrolisis and vaporization temperatures established were of 150 and 1300 °C, respectively. Under these conditions, it was observed a difference of thermal stability among the two nitrogen species in the presence of hydrochloric acid co-injected. While that the nitrite signal was totally suppressed, nitrate signal remained nearly stable. This way, nitrogen can be quantified only as nitrate. The addition of hydrogen peroxide provided the oxidation of nitrite to nitrate, which allowed the total quantification of the species and nitrite obtained by difference. A volume of 5 μL of 0.3% (v/v) hydrochloric acid was co-injected for the elimination of nitrite, whereas that hydrogen peroxide in the concentration of 0.75% (v/v) was added to samples or standards for the oxidation of nitrite to nitrate. Analytical curve was established using standard solution of nitrate. The method described has limits of detection and quantification of 0.10 and 0.33 μg mL⁻¹ of nitrogen, respectively. The precision, estimated as relative standard deviation (RSD), was of 7.5 and 3.8% (n = 10) for groundwater samples containing nitrate–N concentrations of 1.9 and 15.2 μg mL⁻¹, respectively. The proposed method was applied to the analysis of 10 groundwater samples and the results were compared with those obtained by ion chromatography method. In all samples analyzed, the concentration of nitrite–N was always below of the limit of quantification of both the methods. The concentrations of nitrate–N varied from 0.58 to 15.5 μg mL⁻¹. No significant difference it was observed between the results obtained by both methods for nitrate–N, at the 95% confidence level.     

Nondestructive and rapid determination of nitrate in soil, dry deposits and aerosol samples using KBr-matrix with diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS)

This paper presents the development of a new, rapid and precise analytical method for submicrogram levels of nitrate (NO₃⁻) in environmental samples like soil, dry deposit samples, and coarse and fine aerosol particles. The determination of submicrogram levels of nitrate is based on the selection of a quantitative analytical peak at 1385 cm⁻¹ among the three observed vibrational peaks and preparing calibration curves using different known concentrations of nitrate by diffuse reflectance Fourier transform infra red spectrometric (DRIFTS) technique. Pre-weighed and ground infrared (IR) grade KBr was used as substrate over which remarkably wide range of known concentration of nitrate was sprayed and dried. The dried sample was analyzed by DRIFTS and absorbance was measured. Eight calibration curves for four different concentration ranges of nitrate for absorbance as well as peak area were prepared for samples containing lower and relatively higher values of nitrate. The relative standard deviation (n = 8) for the nitrate concentration ranges, 0.05-40, 0.05-1.5, 1.5-25, 5-40 μg/0.1 g KBr were in the range 1.6-2.3% for the above calibration curves. The limit of detection (LOD) of the method is 0.07 μg g⁻¹ NO₃⁻. The F- and t-tests were performed to check the analytical quality assurance test. The noteworthy feature of the reported method is the noninterference of any of the associated cations. The results were compared with that of ion-chromatographic method with high degree of acceptability. The method can be applied in wide concentration ranges. The method is reagent less, nondestructive, very fast, repeatable, and accurate and has high sample throughput value.     

A tutorial on the application of ion-selective electrode potentiometry: An analytical method with unique qualities,unexplored opportunities and potential pitfalls; Tutorial

Ion-selective potentiometry enjoys practical utility as a simple analytical technique to measure ionic constituents in complex samples. Advances in the field have improved the selectivity and decreased the detection limit of ion-selective electrodes (ISEs) by orders of magnitude such that trace analysis in micro and nanomolar concentrations is now possible with potentiometric sensors. This tutorial reviews the fundamental principles of ion-selective potentiometry, describes the practical considerations involved in the use of these sensors to measure real samples, and discusses the statistical evaluation of experimental results compared with alternative analytical techniques.     

Simultaneous electrochemical determination of arsenic, copper, lead and mercury in unpolluted fresh waters using a vibrating gold microwire electrode

In this work, a simple, rapid, reliable and low cost method for simultaneous electrochemical determination of As, Cu, Hg and Pb ions, on a vibrating gold microwire electrode combined with stripping voltammetry, is described for the first time.The multi-element detection was performed in the presence of oxygen by differential pulse anodic stripping voltammetry (DPASV) in HCl 0.1 M with NaCl 0.5 M. This media was found optimum in terms of peak resolution, peak shape and sensitivities, and has a composition similar to seawater to which the method could potentially be applied. The gold microwire electrode presented well defined, undistorted, sharp and reproducible peaks for trace concentrations of Cu, Hg and Pb and As presented a reproducible peak with a small shoulder. Using a gold vibrating microwire electrode of 25 μm diameter and 30 s deposition time, the detection limits of As, Cu, Hg and Pb were 0.07, 0.4, 0.07 and 0.2 μg L⁻¹, respectively. Possible effects of Al, Cd, Cr, Fe, Mn, Ni, Sb and Zn were investigated but did not cause any significant interferences.Finally, the method was applied for the simultaneous determination of these four metals in unpolluted river water samples and the results were validated by Atomic Absorption Spectroscopy with Electrothermal Atomization (AAS-EA) or by Inductively Coupled Plasma Mass Spectrometry (ICP-MS).     

Kinetic-potentiometric assay of formaldehyde in pharmaceutical and industrial samples, monitored by copper solid ion selective electrode, after its reaction with the copper(II)-bis-(ethylenediamine) complex

A kinetic-potentiometric method is described for the quantitative assay of formaldehyde (HCHO) in pharmaceutical and industrial preparations. It is based on the reaction of HCHO with (ethylenediamine)-Cu(II)-sulfate [Cu(CH₂NH₂)₂(H₂O)₂] · SO₄. The changes in potential, resulting from the release of the Cu(II) cations, are monitored with a Cu(II)-ion selective electrode. The calibration curve for the HCHO is linear in the concentration range 50–250 mg L⁻¹, with a limit of detection of 8.5 mg L⁻¹. The method shows very good reproducibility with an RSD of 2.6% for successive injections (n = 5) of 150 mg L⁻¹ HCHO primary solution, while it is interference free. The method was successfully tested in various industrial and pharmaceutical preparations.     

Simultaneous trace-levels determination of Hg(II) and Pb(II) ions in various samples using a modified carbon paste electrode based on multi-walled carbon nanotubes and a new synthesized Schiff base

A modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTs) and 3-(4-methoxybenzylideneamino)-2-thioxothiazolodin-4-one as a new synthesized Schiff base was constructed for the simultaneous determination of trace amounts of Hg(II) and Pb(II) by square wave anodic stripping voltammetry. The modified electrode showed an excellent selectivity and stability for Hg(II) and Pb(II) determinations and for accelerated electron transfer between the electrode and the analytes. The electrochemical properties and applications of the modified electrode were studied. Operational parameters such as pH, deposition potential and deposition time were optimized for the purpose of determination of traces of metal ions at pH 3.0. Under optimal conditions the limits of detection, based on three times the background noise, were 9.0 × 10⁻⁴ and 6.0 × 10⁻⁴ μmol L⁻¹ for Hg(II) and Pb(II) with a 90 s preconcentration, respectively. In addition, the modified electrode displayed a good reproducibility and selectivity, making it suitable for the simultaneous determination of Hg(II) and Pb(II) in real samples such as sea water, waste water, tobacco, marine and human teeth samples.     

Electrochemistry of solutions in liquid ammonia. Part VII. The use of glass electrodes for ammonium and sodium cations as ion-selective sensors at −40 °C

The behavior of Li/La glass electrodes as specific ion sensors for NH ₄ ⁺ and Na⁺ cations in liquid ammonia solutions at –40°C has been assessed using concentration cells with transference. The measurement of emf's of cells with very high resistances due to glass at –40°C has been overcome partly by the use of thinly blown Li/La glass and mainly through the design and use of a floating shield emf measuring system. For solutions of NH₄NO₃, NH₄I, NH₄BF₄, and NH₄Cl almost linear pNH₄ vs. emf responses were observed between pNH₄ 0 and 5; for NH₄NO₃ solutions the slope (40±1 mV/pNH₄) was invariant for substantial increases in Na⁺ concentrations. Solutions of NaNO₃, NaCN, NaClO₄, NaNCS, NaN₃, and NaNO₂ gave almost linear pNa vs. emf responses but the slopes were markedly dependent upon the NH ₄ ⁺ concentration. Estimates of the mean molal ion activity coefficients for nitrate solutions were obtained from earlier transference data: ±(NaNO₃)=0.14±0.02 and ±(NH₄NO₃)=0.30±0.03 at 10^– molal concentration in fair agreement with earlier data.     

Design of a field flow system for the on-line spectrophotometric determination of phosphate, nitrite and nitrate in natural water and wastewater

This study describes the design and optimisation of a field flow system for the in-situ collection and on-line determination of phosphate, nitrate and nitrite by flow injection analysis-spectrophotometry. The method is based on the initial determination of phosphate as its phosphoantimonylmolybdenum blue complex which is then oxidized on-line by nitrite and the decrease in absorbance is monitored at 880 nm. Nitrate is determined as the difference between total and initial nitrite content in a separate flow after reduction to nitrite in a cadmium reductive column. The calibration curves were linear in the range 0–2.00 mg L⁻¹ P-phosphate, 0–10.00 mg L⁻¹ nitrite and 0–7.00 mg L⁻¹ nitrate with correlation coefficients of 0.9979, 0.9993 and 0.9995, respectively. The detection limits, calculated as 3S/N, were 0.15 mg L⁻¹ for P-phosphate, 0.17 mg L⁻¹ for nitrite and 0.09 mg L⁻¹ for nitrate. The reproducibility was below 3.0% (n = 7). Method validation in the analysis of natural water and wastewater samples revealed that it can efficiently be applied to the determination of the target analytes, with recoveries in the range of 92–108%. Correspondence: Athanasios G. Vlessidis, Laboratory of Analytical Chemistry, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece     

A New Neutral Carrier for Silver Ions Based on a Bis(Thiothiazole) Derivative and its Evaluation in Membrane Electrodes

Membrane electrodes based on 2,2-dithiobis(benzothiazole), DTBBT, as a neutral carrier for silver ions are described. Silver-selective membrane electrodes formulated with 2wt% DTBBT ionophore and 50mol% TFPB in an FPNPE plasticized poly (vinyl chloride) exhibited near-Nernstian responses towards silver ions (60.3±0.5mVdecade^–1) over a wide silver ion activity range of 0.83µM to 94mM. Increasing the amount of anionic sites, TFPB, to 100 or 150mol% (relative to the DTBBT weight) resulted in super-Nernstian responses toward silver ions. Membrane electrodes prepared using a low dielectric constant plasticizer, however, exhibited sub-Nernstian responses. Polymer membrane electrodes with optimal composition (i.e., 2wt% DTBBT, 50mol% TFPB in FPNPE plasticized poly(vinyl chloride)) exhibited high potentiometric selectivity towards silver ions over alkali, alkaline earth, transition metal ions, as well as heavy metal ions such as Hg²⁺ and Pb²⁺. A good correlation was found between the potentiometric selectivity coefficients and the change in the UV-visible spectra of the ionophore upon exposure to different metal ions. The overall performance of the silver-selective membrane electrodes based on DTBBT ionophore, which is available at low cost, was found to be comparable to the performance of silver electrodes prepared with Fluka silver ionophore-IV. A DTBBT-based silver electrode was used as an indicator electrode for titrations of silver ions using standard sodium chloride solutions. Sharp inflections occur at the end point, and the data obtained showed 99.4% recovery with a standard deviation of 0.7% (n=3). In addition, the applicability of the DTBBT-based silver-selective electrode is illustrated by measuring the silver concentrations in natural water spiked with silver nitrate and by analyzing the silver in electroplating wastewater samples. The results obtained utilizing a DTBBT-based silver electrode showed very good agreement with the standard methods of analysis.