Effect of model organic compounds on square-wave voltammetric stripping analysis at the Nafion/chelating agent mercury film electrodes

The effect of various organic compounds on the Nafion/chelating agent mercury film electrodes (NCAMFEs) in square-wave anodic-stripping voltammetry (SWASV) is explored. Two chelating agents used to prepare the NCAMFEs are dimethylglyoxime and 2,2'-bipyridyl. Triton X-100, sodium dodecyl sulfate, albumin, gelatin, starch, camphor, and humic acid are used as model organic compounds, while cadmium, lead, and copper are used as test metal ions. The NCAMFEs are considerably more resistant to organic interferences than the Nafion-coated mercury film electrode. The implications of these interferences for the reliability and feasibility of stripping measurements using the NCAMFEs in real samples are discussed. Results presented for untreated urine and natural water samples demonstrate the analytical utility of the NCAMFEs in SWASV.     

Voltammetry of lead cations on a new type of silver composite electrode in the presence of other cations

A new type of silver composite electrode was examined, prepared from silver, graphite powder, and methacrylate resin. The effects of the presence of various cations (cadmium, copper, bismuth, thallium), anions (chlorides), surface-active substances (Triton X-100), and oxygen on the anodic-stripping voltammetric determination of lead were studied. It was found that the effect of underpotential deposition at the composite electrode differs from that produced at a metallic silver electrode, mainly at low concentrations of the deposited metal. The use of this type of silver composite electrode in differential pulse anodic-stripping voltammetry enables direct determination of lead in natural water samples without elimination of surface-active substances (LOD about 3 g L^–1).     

Removal of humic acid and surfactant interferences in trace metal determinations by differential-pulse anodic stripping voltammetry with use of adsorption and chelate ion-exchanger columns in a flow-injection system

A flow-injection differential-pulse anodic stripping voltammetry (d.p.a.s.v.) method is modified so that interferences from humic acids or surfactants are eliminated. The injected, slightly acidic sample is passed through a silica anion-exchanger column to remove compoundswith a strong tendency to adsorb to the electrode. The sample then passes to a chelate ion-exchange column containing immobilized 8-quinolinol. The metal ions are retained and later eluted with acid into the voltammetric cell. The results show that the interferences from up to 500 mg 1^–1 humic acid or at least 50 mg 1^-1 Triton X-100 can be removed and that the metal ion can be determined in a range similar to that for normal d.p.a.s.v. methods. The complete cycle time for a determination was 12 min.     

Vergleich von Potentiometrischer Stripping-Analyse und Anodischer Stripping-Voltammetrie in der Flie?injektionsanalyse. St?rungen bei der Bleibestimmung

Summary The interferences caused by inorganic ions, organic anions and Triton X-100 in the determination of lead by flow injection potentiometric and voltammetric stripping analysis at a mercury film electrode were investigated. The experiments were performed in the concentration range of 1–10 mg/l Pb²⁺. For both methods linear calibration plots were obtained in the presence of an excess of nitrate, chloride, perchlorate and sulphate. Iodide strongly interferes due to mercury complexation.Metal ion interference caused by formation of irreversible amalgams (as in the case of codeposition of Ni, Co, Fe and Cr) is avoided by proper choice of deposition potential. No effect of water soluble reduced species, i.e. Fe(II) and Cr(II), on the stripping signals was observed. The presence of acetate, citrate and tartrate does not limit the determination of lead by potentiometric stripping analysis (PSA) but has a marked effect on anodic stripping voltammetric (ASV) signals. No influence of Triton X-100 on the determination of lead by PSA was found. In ASV 10^–3% Triton X-100 diminished the peak current by 15%.

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Herrn Prof. Dr. Rolf Neeb aus Anlaß seines 60. Geburtstags gewidmet     

Reversible and irreversible adsorption of surfactants at a hanging mercury drop electrode.

The adsorption-desorption phenomena of surfactants were studied by measuring differential capacity-potential curves in a static solution and differential capacity-time curves in a flowing solution. The surfactants investigated were Aerosol OT, cetylpyridinium chloride, Hyamin 1622, tetrabutylammonium bromide, Triton X-100 and trioctylphosphineoxide. The differential capacity-potential and differential capacity-time curves for these surfactants showed different shapes, with and without peaks. The differential capacity-time curves were used to study the adsorption reversibility of the surfactants at a mercury electrode. The adsorptions of Hyamin 1622 and Triton X-100 were irreversible at all the potentials investigated. The adsorptions of Aerosol OT and trioctylphosphineoxide were irreversible except at the potential more positive than -0.2 V. The adsorption of tetrabutylammonium bromide was almost reversible at any potential investigated. The adsorption of cetylpyridinium chloride was complicated, indicating different orientations of adsorption.     

Estimation of non-ionic, surface-active substances in aqueous solutions by means of the Controlled Growth Mercury Electrode

In this paper, a quick and simple tensammetric method of estimation of non-ionic surfactants (NS) in aqueous solutions is proposed. The method makes use of the variation in the differential capacity of double layer in relation to the time of accumulation (C(d)-t(acc)) of non-ionic surfactants at the hanging mercury drop electrode, generated by a single, very quick opening of the valve. Under such conditions, the capacity current measured at the potential of maximum adsorption diminishes with accumulation time of non-ionic surfactants. The proposed method, which was verified for model surfactant (Triton X-100), may also be applied in the determination of other NS. Modifications in construction of the CGME electrode and its improved metrological parameters played an important role in the presented procedure. In addition, other measurements were performed using standard electrochemical techniques, whereby current-time and differential capacity-potential curves were recorded. Satisfactory results were obtained in the determination of Triton X-100 in the range from 0.05 to 20 mg L(-1) (R.S.D.=6%, recovery=94-103%, r=0.999, DL=0.15 mg L(-1)). Applicability of the method was presented using the water samples from Bia?ka and Dunajec rivers, from which NS were removed by addition of fumed silica.     

Spectrophotometric determination of traces of lead with bromopyrogallol red and cetyltrimethylammonium or cetylpyridinium bromide

A method is described based on the sensitizing effect of cetyltrimethylammonium or cetylpyridinium bromide on the lead-Bromopyrogallol Red colour reaction. The reaction is instantaneous and the colour remains stable for over 120 hr in the presence of Triton X-100. Both colour systems obey Beer's law up to 5.5 ppm of lead. Methods are described for dealing with interferences.     

Effect of surface-active compounds on voltammetric stripping analysis at the mercury film electrode

The effects of various organic compounds on the differential-pulse anodic-stripping voltammetric response at the in-situ plated mercury film electrode are explored. These effects vary from metal to metal and from one organic compound to another. The most pronounced effects are observed in measurements of copper. The main effect of the organic compound is to depress the peak current rather than change the peak shape or potential. The differences between the organic interferences observed at the mercury film electrode and those reported at the hanging mercury drop electrode are explained by the different morphology and geometry of the two electrodes. The implications of these interferences for the reliability and feasibility of stripping measurements in natural waters are discussed. Gelatin, camphor, humic acid, starch, agar, sodium dodecyl sulphate and albumin were used as representative organic compounds, and cadmium, lead, and copper as test metal ions.     

Determination of surface-active compounds in precipitation studies by ac polarography

The simple procedure is based on measurement of the capacity current of the mercury electrode by a.c. polarography after adsorption of the surfactant. Nonionic surfactants of the Triton-X series were studied in connection with precipitation of colloidal silver iodide. Useful results were achieved in the 10^?4–10^-7 mol dm^?3 range for Triton X-100, X-305 and X-705 by adjusting the sorption time, the type of molecular transport to the electrode and the dilution ratio.     

Donnan dialysis matrix normalization for the voltammetric determination of metal ions

The efficacy of Donnan dialysis for removal of interferences by surfactants, complexing agents, and electroactive organic compounds on anodic stripping voltammetry and differential pulse polarography is demonstrated. Up to at least 0.05% gelatin and 0.005% Triton X-100 do not alter the rate of dialysis of Cd(II), Pb(II), Zn(II), and Cu(II). As the ion-exchange membrane is not permeable to the surfactants, subsequent voltammetric determinations can be performed by a working curve method. Recoveries of metals from a variety of real samples were generally above 90%. The presence of humic acid (50 mg l^-1) does not alter the transport of Cu(II), Pb(II), or Cd(II) from pH 3 solution but does at pH 8. The transport is related to the free and labile-complexed ion concentration which suggests that Donnan dialysis can be used for speciation as well as for enrichment and matrix normalization.     

Determination of the Micellar Properties of Triton X-100 by Voltammetry Method

ABSTRACT

The diffusion coefficient of the micelle, the first CMC and the second CMC of Triton X-100 are determined by cyclic voltammetry without any probe. The first CMC and the second CMC of Triton X-100 are 3.1x lO^?1 and 1.3× 10^?1 respectively. The viscosity of the micelle solution, the micellar aggregation number and the micellar size increase but the diffusion coefficient decreases with Triton X-100 concentration increasing. The mechanism of the electrochemical reaction of Triton X-100 at platinum electrode is deduced by measurements of conductivity, pH and cyclic voltammetry.     

Simultaneous voltammetric determination of cobalt, nickel and labile zinc, using 2-quinolinethiol in the presence of surfactants without prior digestion

2-Quinolinethiol forms very stable cobalt and nickel complexes, which are strongly chemisorbed on the hanging mercury drop electrode (HMDE). This allows the simultaneous determination of cobalt, nickel and labile zinc traces; Co and Ni by differential pulse adsorptive stripping voltammetry (DPAdSV) and Zn by differential pulse sweep voltammetry (DPSV), even in the presence of a large amount of surfactant like Triton X-100. This is advantageous for determining these metals directly in natural waters, without previous UV-irradiation or acid digestion. The surfactant does not only not interfere, but shows a beneficial effect with respect to the resolution between copper and nickel peaks. The method has been tested in estuary waters. The determination limits are 1 nmol/l Ni, 10 nmol/l Co and 1 mol/l Zn, with RSD less than 6.3%.     

Interaction of nonionic surfactants with copolymer microgel particles of NIPAM and acrylic acid

Binding of the nonionic surfactants Triton X-100 and Triton X-405 onto linear copolymers of N-isopropylacrylamide (NIPAM) and acrylic acid and to cross-linked microgel particles of similar composition but differing in their cross-link densities has been studied. The binding capacities vary for each of these polymeric systems, being smallest for the linear copolymer. The binding is also significantly less in all cases for the more hydrophilic surfactant, namely, Triton X-405. By comparing estimates of the pore or "cage" size within the microgel particles with the dimensions of the free micelles in solution, it is concluded that micelles of Triton X-100 form within the microgel particles more readily for the lower cross-linked microgel particles. However, micelles do not form as easily inside either microgel for Triton X-405. The swelling/deswelling behavior of each of the two microgels, in the presence of the surfactants, has been explained in terms of their relative binding behavior and how this contributes to the osmotic pressure difference inside and outside the microgel particles and also in terms of micelle "bridging" of the polymer network, causing shrinkage.     

Square-wave anodic stripping voltammetric determination of thallium(I) at a Nafion/mercury film modified electrode

A Nafion/mercury film electrode (NMFE) was used for the determination of trace thallium(I) in aqueous solutions. Thallium(I) was preconcentrated onto the NMFE from the sample solution containing 0.01 M ethylenediaminetetraacetate (EDTA), and determined by square-wave anodic stripping voltammetry (SWASV). Various factors influencing the determination of thallium(I) were thoroughly investigated. This modified electrode exhibits good resistance to interferences from surface-active compounds. The presence of EDTA effectively eliminated the interferences from metal ions, such as lead(II) and cadmium(II), which are generally considered as the major interferents in the determination of thallium at a mercury electrode. With 2-min preconcentration, linear calibration graphs were obtained over the range 0.05-100 ppb of thallium(I). An even lower detection limit, 0.01 ppb, were achieved with 5-min accumulation. The electrode is easy to prepare and can be readily renewed after each stripping experiment. Applicability of this procedure to various water samples is illustrated.     

Dynamic Interfacial Tension of Surfactant Mixtures at Liquid-Liquid Interfaces

Dynamic interfacial tensions for surfactant mixtures at liquid-liquid interfaces were obtained with a drop volume tensiometer. The surfactants tested were Triton X-100, palmitic acid, and Span 80 at both the water-hexadecane and water-mineral oil interfaces. Two-surfactant mixtures were examined with the surfactants initially dissolved in different phases to minimize bulk-phase interactions. For concentrations below the CMC, it was found that the adsorption kinetics of palmitic acid and Triton X-100 mixtures were dominated by the latter surfactant. Apparent diffusion coefficients were obtained for Triton X-100 both in the absence and in the presence of palmitic acid. These values were largely insensitive to the presence of palmitic acid. For mixtures of Span 80 and Triton X-100, the adsorption kinetics were found to be influenced significantly by both surfactants. In this case, relative changes in surfactant concentrations affected the dynamic interfacial tension of the mixed system. A previously proposed multicomponent adsorption model described the dynamic interfacial tension adequately at low concentrations of Triton X-100, when desorption could be neglected. At higher concentrations, modifications were needed to account for solubilization into the oil phase. These corrections allowed the model to describe the long time adsorption quite well. However, predicted values of short time interfacial tensions were overestimated, likely due to a synergistic interaction of the two surfactants. Copyright 1999 Academic Press.     

Determination of trace metals by anodic-stripping voltammetry with a new kind of mercury electrode: The long-lasting sessile-drop electrode

A new kind of semi-stationary mercury drop electrode is described, which can be used for the determination of trace metals in natural waters by differential-pulse anodic-stripping voltammetry. Results are reported for the determination of zinc in potassium chloride and in samples of sea-water. The reproducibility of the electrode and of the results obtained with it for zinc at the 10(-8)M level are very satisfactory.     

Suppression of surfactant interferences in anodic stripping voltammetry by sodium dodecyl sulfate

It was investigated whether interferences from surfactants in anodic stripping voltammetry (ASV) could be remedied by the anionic surfactant sodium dodecyl sulfate (SDS) which causes little or no interference in itself. Cadmium and lead were used as test analytes, and measurements were performed in acetate buffer as well as in 0.1 M HNO₃. One hundred parts per million of the interfering surfactant was added. SDS eliminated severe interference from the non-ionic surfactants Triton^© X-100 and dodecyl octaethylene glycol ether as well as from the polymer polyethylene glycol 6000 and from the cationic surfactant cetyl trimethyl ammonium bromide. SDS could not remedy the extraordinarily severe interference from the cationic surfactant cetyl pyridinium chloride. Two anionic surfactants were also tested as interferents but they had little detrimental effect on the ASV signals. The effect of SDS was explained by the formation of mixed micelles which scavenge the interferent in the bulk solution and by competitive displacement of the interferent at the electrode surface.     

Spectrophotometric determination of mercury(II) and silver(I) with copper(II) and diethyldithiocarbamate in the presence of triton X-100

Procedures for the determination of mercury and silver by displacement of diethyldithiocarbamate (DDTC) from its copper complex in the presence of 1% Triton X-100, and measurement of the decrease in the Cu(DDTC)(2) absorbance, are described. The use of the surfactant avoids the need for an extraction step. Reproducibility within 1% and detection limits of 0.25 ppm Hg(II) and 0.45 ppm Ag(I) have been obtained, and linear calibration ranges up to 13 ppm Hg(II) and 15 ppm Ag(I). In the presence of 0.1M EDTA very good selectivity is achieved.     

A pyrolytic carbon film electrode for voltammetry-III Application to anodic-stripping voltammetry

The potential applicability of a pyrolytic carbon film electrode in the differential-pulse anodic-stripping voltammetric determination of cadmium and lead in sea-water is demonstrated. The performance at the 10⁻¹⁰M level is compared with that of a satisfactory glassy-carbon electrode. The two types of electrode display comparable behaviour in anodic-stripping voltammetry, but the pyrolytic carbon film electrode needs less pretreatment.     

Influences of surfactant and nanoparticle assembly on effective interfacial tensions

We have studied assembly at air-water and liquid-liquid interfaces with an emphasis on systems containing both surfactants and nanoparticles. Anionic surfactants, sodium dodecyl sulfate (SDS) and non-ionic surfactants, Triton X-100 and tetraethylene glycol alkyl ethers (C(8)E(4), C(12)E(4) and C(14)E(4)), effectively decrease the surface tension of air-water interfaces. The inclusion of negatively charged hydrophilic silica nanoparticles (diameters of approximately 13 nm) increases the efficiency of the SDS molecules but does not alter the performance of the non-ionic surfactants. The former is likely due to the repulsive Coulomb interactions between the SDS molecules and nanoparticles which promote the surfactant adsorption at air-water interfaces. For systems involving trichloroethylene (TCE)-water interfaces, the SDS and Triton X-100 surfactants effectively decrease the interfacial tensions and the nanoparticle effects are similar compared to those involving air-water interfaces. Interestingly, the C(12)E(4) and C(14)E(4) molecules, with or without the presence of nanoparticles, fail to decrease the TCE-water interfacial tensions. Our molecular dynamics simulations have suggested that the tetraethylene glycol alkyl ether molecules tend to disperse in the TCE phase rather than adsorb at the TCE-water interfaces.