Ion-selective electrodes for anionic surfactants using a cyclam derivative as ionophore

The cyclam derivative 1,4,8,11-tetra(n-octyl)-1,4,8,11-tetraazacyclotetradecane (L) has been used as carrier for the preparation of PVC-based membrane ion-selective electrodes for anionic surfactants. Different membranes were prepared using L as ionophore, tetra-n-octylammonium bromide (TOAB) as cationic additive and dibutyl phthalate (DBP) or o-nitrophenyl octyl ether (NPOE) as plasticizers. The final used electrode contained a membrane of the following composition: 56% DBP, 3.4% ionophore, 3.8% TOAB and 36.8% PVC. This electrode displays a Nernstian slope of -60.0+/-0.9 mV/decade in a 2.0 x 10(-3) to 7.9 x 10(-6) mol dm(-3) concentration range and a limit of detection of 4.0 x 10(-6) mol dm(-3). The electrode can be used for 144 days without showing significant changes in the value of slope or working range. The electrode shows a selective response to dodecyl sulfate (DS-) and a poor response to common inorganic cations and anions. The selective sequence found was DS->ClO4->HCO3->SCN->NO3- approximately CH3COO- approximately I->Cl->Br->IO3- approximately NO2- approximately SO3(2-)>HPO4(2-)>C2O4(2-)>SO4(2-), i.e. basically following the Hoffmeister series except for the hydrophilic anion bicarbonate. Most of the potentiometric coefficients determined are relatively low indicating that common anions would not interfere in the DS- determination. A complete study of the response of the electrode to a family of surfactant was also carried out. The electrode showed a clear anionic response to DS- and to Na-LAS and a much poorer response to other anionic surfactants and to non-ionic surfactants. Also the electrode shows certain non-linear cationic response in the presence of cationic and zwitterionic surfactants. The electrode was used for the determination of anionic surfactants in several mixtures, and the results obtained were compared to those found using a commercially available sensor.     

Simultaneous determination of trace amounts of cadmium, nickel and cobalt in water samples by adsorptive voltammetry using ammonium 2-amino-cyclopentene dithiocarboxylate as a chelating agent

This paper reports the use of an adsorptive voltammetric technique for the simultaneous detection of Cd(II), Ni(II) and Co(II) using ammonium 2-amino-cyclopente dithiocarboxylate as a selective complexing agent. Scans containing three resolved peaks corresponding to these metals were obtained in synthetic and real samples. The reduction current peaks of the metals that were distinctly separated by 200 mV or more, allowing their determination over a wide range of concentrations. These metals can be quantified at concentrations above 1.33x10(-8) mol dm(-3) Cd(II), 8.51x10(-9) mol dm(-3) Ni(II) and 3.39x10(-10) mol dm(-3) Co(II). The influence of pH, ligand concentration, scan rate, accumulations time and applied potential was investigated. The R.S.D. at a concentration level of 1.78x10(-7) mol dm(-3) of Cd(II), 3.40x10(-7) mol dm(-3) and Ni(II) and 1.7x10(-9) mol dm(-3) of Co(II) was 2.5% for Cd(II), 2.7% for Ni(II) and 3.3% for Co(II). The method was applied to various water samples.     

Spectrophotometric determination of sulfide in the presence of sulfite and thiosulfate via the precipitation of bismuth(III) sulfide.

A photometric method has been developed for the determination of sulfide at 10(-5) mol dm(-3) levels, which is based on the reaction of sulfide with a given excess amount of bismuth(III) to form a precipitate of bismuth(III) sulfide and on the spectrophotometric measurement of the residual bismuth(III) at 335 nm after extracting with bismuthiol II reagent from an aqueous solution containing acetate buffer into benzene. The presence of sulfite and thiosulfate up to 0.002 mol dm(-3) did not cause any interference in the determination of sulfide, because both sulfite and thiosulfate do not produce any precipitate with bismuth(III). A linear calibration plot with a negative slope was obtained for sulfide over the range of 5.00 x 10(-7) - 3.00 x 10(-5) mol dm(-3) (16.0 - 960 ppb). An experimental calibration plot was in accord with the theoretical plot, taking into account the known excess of bismuth(III), showing that the reaction of sulfide with bismuth(III) proceeded to completion. The relative standard deviation of results from 10 replicate determinations of standard sulfide (2.00 x 10(-5) mol dm(-3)) was 0.44%. The proposed method was successfully applied to the determination of sulfide in hotspring water samples without any pretreatment.     

Protonation Equilibria of Mononuclear Vanadate: Thermodynamic Evidence for the Expansion of the Coordination Number in VO(2)(+)

A spectrophotometric investigation of the protonation of HVO(4)(2-) has been conducted at vanadium(V) concentrations low enough (5 x 10(-5) mol dm(-3)) to prevent the formation of polynuclear ions. Equilibrium constants as well as enthalpy and entropy changes for the formation of H(2)VO(4)(-) and VO(2)(+) have been determined in ionic medium: 1.0 mol dm(-3) NaCl and NaClO(4), respectively. The percentage concentration of the neutral species, H(3)VO(4), is so low (apparently <1%) that the protonation of H(2)VO(4)(-) to form VO(2)(+) occurs virtually in a single step. The disproportionately great stability of VO(2)(+) relative to H(3)VO(4) is explained in terms of an increase in the coordination number of vanadium when the cationic species is formed. This explanation is based on a comparison of the thermodynamic quantities of the vanadium(V) species with those of various other oxyacids reported in the literature.     

Synergistic effect of Ni(II) and Co(II) ions on the sulfite induced autoxidation of Cu(II)/tetraglycine complex

The synergistic effect of Ni(II) and Co(II) on the sulfite induced autoxidation of Cu(II)/tetraglycine was investigated spectrophotometrically at 25.0 degrees C, pH = 9.0, 1 x 10(-5) mol dm(-3) < or = [S(IV)] < or = 8 x 10(-5) mol dm(-3), [Cu(II)]= 1 x 10(-3) mol dm(-3), 1 x 10(-6) mol dm(-3) < or = [Ni(II)] or [Co(II)] < or = 1 x 10(-4) mol dm(-3), [O2] approximately 2.5 x 10(-4) mol dm(-3), and 0.1 mol dm(-3) ionic strength. In the absence of added nickel(II) or cobalt(II), the kinetic traces of Cu(III)G4 formation show a large induction period (about 3 h). The addition of trace amounts of Ni(II) or Co(II) increases the reaction rate significantly and the induction period drastically decreases (less than 0.5 s). The effectiveness of Cu(III)G4 formation becomes much higher. The metal ion in the trivalent oxidation state rapidly oxidizes SO3(2-) to SO3*-, which reacts with oxygen to produce SO5*-. The strongly generated oxidants oxidize Cu(II)G4 to Cu(III).     

Potentiometric PVC membrane sensor for the determination of scopolamine in some pharmaceutical formulations.

A novel PVC membrane electrode for the determination of scopolamine ion based on the formation of an ion-association complex of scopolamine with the phosphotungstate counter anion as an electroactive material dispersed in a PVC matrix is described. The sensor shows a fast, stable, near-Nenstian response for 1 x 10(-2) mol dm(-3) to 1 x 10(-6) mol dm(-3) scopolamine at 25 degrees C over the pH range of 3 - 7 with a cationic slope of 54.5 +/- 0.5 mV/decade. The lower detection limit is 8 x 10(-7) mol dm(-3) and the response time is 15 -45 s. The selectivity coefficients for scopolamine relative to the number of interfering substances were investigated. There was negligible interference from the studied cations, anions, and pharmaceutical excipients. The determination of scopolamine in aqueous solution shows an average recovery of 100.0% and a mean relative standard deviation (RSD) of 1.5% at 500 microg/cm3. The direct determination of scopolamine in some formulations (scopolamine injection and eye drops) gave results that compare favorably with those obtained by the United State of Pharmacopoeia method. Potentiometric titration of scopolamine with sodium tetraphenylborate and phosphotungstic acid as a titrant was monitored with the developed scopolamine electrode as an end point indicator electrode.     

Malonic acid concentration as a control parameter in the kinetic analysis of the Belousov-Zhabotinsky reaction under batch conditions

The influence of the initial malonic acid concentration [MA]0 (8.00 x 10(-3) < or = [MA]0 < or = 4.30 x 10(-2) mol dm(-3)) in the presence of bromate (6.20 x 10(-2) mol dm(-3)), bromide (1.50 x 10(-5) mol dm(-3)), sulfuric acid (1.00 mol dm(-3)) and cerium sulfate (2.50 x 10(-3) mol dm(-3)) on the dynamics and the kinetics of the Belousov-Zhabotinsky (BZ) reactions was examined under batch conditions at 30.0 degrees C. The kinetics of the BZ reaction was analyzed by the earlier proposed method convenient for the examinations of the oscillatory reactions. In the defined region of parameters where oscillograms with only large-amplitude relaxation oscillations appeared, the pseudo-first order of the overall malonic acid decomposition with a corresponding rate constant of 2.14 x 10(-2) min(-1) was established. The numerical results on the dynamics and kinetics of the BZ reaction, carried out by the known skeleton model including the Br2O species, were in good agreement with the experimental ones. The already found saddle node infinite period (SNIPER) bifurcation point in transition from a stable quasi-steady state to periodic orbits and vice versa is confirmed by both experimental and numerical investigations of the system under consideration. Namely, the large-amplitude relaxation oscillations with increasing periods between oscillations in approaching the bifurcation points at the beginning and the end of the oscillatory domain, together with excitability of the stable quasi-steady states in their vicinity are obtained.     

Polarographic study of acrolein and its determination by flow injection with amperometric detection at a mercury electrode

A study of the electrochemical behavior of acrolein at a dropping mercury electrode using different polarographic techniques is described. Theoretical studies of the reversibility of the wave of acrolein were carried out using two different polarographic techniques: direct current tast and differential pulse. Differential pulse polarography may be used to determine acrolein concentration in a Britton-Robinson buffer solution of pH 10 in the ranges 2 x 10(-7)10(-8) and 5 x 10(-8)-10(-4) mol dm(-3) and a coefficient of variation of 1.7% for a concentration of 10(-5)mol dm(-3). A flow injection method with amperometric detection at a potential of -1.4V using a mercury electrode is also described. Before each injection, any drop hanging from the tip of the capillary needs to be dislodged and a new electrode drop dispensed; three different drop sizes were tested. A linear relationship between peak intensity and acrolein concentration was obtained in the range 10(-5)-10(-7) mol dm(-3), with a detection limit of 9.8 x 10(-8) mol dm(-) 3 and a coefficient of variation of 2.9% for a 2 x 10(-7) mol dm(-3) concentration. Several organic and inorganic species were tested in order to ascertain whether they interfered with the signal for acrolein. The proposed methods were applied to the determination of acrolein in seawater samples.     

Cathodic stripping voltammetry of the anticancer agent 5-fluorouracil and determination in urine

Cathodic stripping voltammetry was used to determine 5-fluorouracil (5-FU) in the presence of traces of Cu(II). It was found that the addition of 5 x 10(-9) mol dm(-3) Cu(II) to the measurement cell greatly enhanced the peak current of the adsorbed molecule. Different parameters were tested to optimize the conditions for the determination of 5-FU. The adsorbed form is reduced irreversibly. It was observed that by controlling the deposition potential, the technique could be directed to the determination of Cu(II) or the drug. The linear range was from 5 x 10(-9) to 6 x 10(-8) mol dm(-3) for 5-FU and from 6 x 10(-9) to 5 x 10(-8) mol dm(-3) for Cu(II). Detection limits of 4.6 x 10(-10) and 5 x 10(-10) mol dm(-3) were obtained for 5-FU and Cu(II), respectively. The method was applied to urine and molecules or ions which may interfere were studied.     

Binding of sodium dodecyl sulfate to linear and star homopolymers of the nonionic poly(methoxyhexa(ethylene glycol) methacrylate) and the polycation poly(2-(dimethylamino)ethyl methacrylate): electromotive force, isothermal titration calorimetry, surface tension, and small-angle neutron scattering measurements

We investigated the binding of sodium dodecyl sulfate (SDS) to various linear and star polymers of the nonionic methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and the ionic 2-(dimethylamino)ethyl methacrylate (PDMAEMA), the latter being a polycation at low pH. The dodecyl sulfate ion selective electrode (EMF), isothermal titration calorimetry (ITC), and surface tension (ST) were applied to gain detailed information about interactions. In all cases there is evidence of significant binding of SDS over an extensive SDS concentration range spanning from ca. 10(-6) to 0.1 mol dm(-3). At pH 3, the polymer PDMAEMA is a strong polycation and here the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At their natural pH of 8.6, PMHEGMA and PDMAEMA polymers are essentially nonionic and bind SDS in the form of polymer-bound aggregates in the concentration range of ca. 1 x 10(-3) to 3 x 10(-2) mol dm(-3). All the polymers also bind SDS to a lesser extent at concentrations below 1 x 10(-3) mol dm(-3) reaching as low as 10(-7) mol dm(-3). This low concentration binding process involves the polymer and nonassociated SDS monomers. As far as we are aware, this is the first example that such a low concentration noncooperative binding process could be observed in SDS/neutral polymer systems by EMF and ST. We also showed that the nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) and the cationic cetyltrimethylammonium bromide (C16TAB) interact with star PDMAEMA. We believe that the interaction of C12EO6 and CTAB is of similar noncooperative type as the first SDS binding process in the range from ca. 10(-5) to 0.3 x 10(-3) mol dm(-3). At the high concentration binding limit Csat of SDS, the above polymers become fully saturated with bound SDS micelles. We applied small angle neutron scattering (SANS) to determine the structure and aggregation numbers of the star polymer/bound SDS micelles and calculated the stoichiometry of such supramolecular complexes. The SANS data on PDMAEMA star polymers in the presence of C12EO6 showed only a limited monomer binding in contrast to linear PDMAEMA, which showed monomer C12EO6 binding at low concentrations but micellar aggregates at 6 x 10(-3) mol dm(-3).     

Non-extraction batchwise and FIA determination of cationic and nonionic surfactants using Cu(II)-chromazurol S-surfactant complexes

Batchwise and FIA determinations have been developed for cationic and nonionic surfactants, based on the formation of ternary Cu(II)-chromazurol S-surfactant compounds. Optimum reaction conditions have been found (pH 8.0, lambda=590 and 630 nm, respectively). For the batchwise measurement, the molar absorption coefficient values epsilon(590)=5.1-5.7x10(4) l mol(-1) cm(-1) for cationic surfactants, epsilon(630)=0.7-1.5x10(4) l mol(-1) cm(-1) for nonionic surfactants. A factorial design has been carried out to determine the optimum flow conditions. Calibration curves were constructed and statistically evaluated for both the batchwise and FIA determination. For example, the linear concentration ranges for batch determination of the cationic surfactant cetyltrimethylammonium bromide and nonionic surfactant Marlophen NP 10 are 0-15 mug ml(-1) (R=0.9996, R.S.D.=6.62-0.64%) and 13-53 mug ml(-1) (R=0.9993, R.S.D.=4.48-1.40%), respectively; the respective detection limits are 0.02 and 4.0 mug ml(-1). For FIA determination of the same surfactants, the linear concentration ranges are 0-13 mug ml(-1) (R=0.9995, R.S.D.=4.44-0.49%) and 66-397 mug ml(-1) (R=0.9994, R.S.D.=8.92-1.12%), respectively, detection limits are 0.08 and 38 mug ml(-1), respectively.     

Determination of cationic surfactants in water samples by their enhanced resonance light scattering with azoviolet

A simple assay of cationic surfactants in water samples was developed based on the measurements of enhanced resonance light scattering (RLS). At pH 6.09 and ionic strength 0.03 M, the interactions of azoviolet (AV) with cationic surfactants, including zephiramine (Zeph) and cetyl trimethyl ammonium bromide (CTMAB), result in enhanced RLS signals characterized by the peaks of 470.0, 485.0 and 495.0 nm. The enhanced RLS intensity is proportional to the concentration of cationic surfactant of Zeph in the range of 0.2~6.0x10(-6) M, and to that of CTMAB in the range of 0.4~4.8x10(-6 )M. The limit of determination (3 sigma) is 2.1x10(-8) M and 3.8x10(-8) M for the two surfactants, respectively. Determinations of cationic surfactants in synthetic and tap water samples were successfully made with a recovery of 90.5~108.6%.     

Determination of manganese in some medicinal plants and their infusions by a catalytic spectrophotometric method

The catalytic effect of manganese (II) on the oxidation of the azo dye 3-Methyl-6-(2-hydroxyethoxy)-2-[2-methoxy-4-N (N, N diethylamino) phenylazo] benzothiazolium methylsulphate, with potassium periodate in the presence of 1,10-phenanthroline in weakly acidic media was studied. The reaction was followed spectrophotometrically by measuring the decrease in the absorbance of the dye at 560 nm. Under the optimum conditions (4 x 10(-5) mol dm(-3) azo dye, 4 x 10(-4) mol dm(-3) potassium periodate, 1 x 10(-4) mol dm(-3) 1,10-phenanthroline, 0.1 mol dm(-3) buffer--pH 3.0, 70 degrees C, 8 min) manganese (II) in the range 0.1-5 ng cm(-3) could be determined by the fixed-time method with a detection limit of 0.035 ng cm(-3). The developed method is highly sensitive, selective, and simple. The method was applied successfully to the determination of total manganese in some medicinal plants and to analyse their infusions for trace amounts of total manganese and free manganese (II) ions without separation.     

Flow-injection determination of trace amounts of dopamine by chemiluminescence detection

A flow-injection analysis (FIA) for the determination of dopamine has been developed. The method is based on the inhibition effect of dopamine on the iron(II)-induced chemiluminescence (CL) of 10,10'-dimethyl-9,9'-biacridinium dinitrate (lucigenin). The presence of a non-ionic surfactant, polyoxyethylene (23) lauryl ether (Brij 35), caused an increase in the inhibition effect. The present method allows the determination of dopamine over the range 1x10(-8)-2x10(-7) mol dm(-3). The relative standard deviation was 0.7% for eight determinations of 6x10(-8) mol dm(-3) dopamine. The detection limit (S/N=3) was 2x10(-9) mol dm(-3) with the sampling rate of 40 samples h(-1). The effect of other catecholamines and compounds of similar structure on the lucigenin CL reaction was studied: quinone, hydroquinone, norepinephrine, pyrocatechol and l-dopa suppressed the CL intensity.     

Effect of high salt concentrations on the stabilization of bubbles by silica particles

The stabilization of air bubbles by hydrophobically modified silica particles has been investigated in detail. The silica particles used had a nominal primary particle size of 20 nm and were made hydrophobic by treatment with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains ("% SiOR"). Contact-angle (theta) measurements of pure water droplets on flats made from compressed samples of the particles showed a steep increase in theta above ca. 20% SiOR. Other measurements also showed a significant increase in theta when the salt concentration was raised to 1-3 mol dm(-3). Bubbles were formed in a sonicated dispersion of particles by suddenly lowering the pressure. Maximum stability was obtained with 33% SiOR particles and 2-3 mol dm(-3) NaCl. Under these conditions, theta was around 40 degrees. Above a threshold size of around 70 microm, bubbles were extremely stable to disproportionation and coalescence and bubble stability increased significantly with an increase in the NaCl concentration from 0.5 to 3 mol dm(-3). Furthermore, rheological measurements showed that at NaCl concentrations in this range weak particle gels were formed with a finite yield stress. The strength of these gels increased with an increasing NaCl concentration between 0.5 and 3 mol dm(-3) and with an increasing time of aging the dispersions, implicating this as part of the mechanism leading to an increased bubble stability in these systems. Dispersions in the absence of NaCl showed little or no foamability at all. Use of CaCl2 and Al(NO)3 at similar ionic strengths showed that equivalent stability could not be obtained with these salts. Atomic force microscopy (AFM) measurements of the adhesion between a pure (0% SiOR) silica sphere and flat showed a significant increase in the adhesion between 0.5 and 3 mol dm(-3) NaCl, even though in this concentration range no significant change in the electrostatic repulsion might be expected. It is concluded that the increased particle-particle adhesion, effective hydrophobicity, and bubble-stabilization properties of the particles at high NaCl concentrations are probably due to the collapse of protruding polysilicic acid chains on the surface of the silica.     

Equilibrium constant of the gamma-sultone and extraction constant of sodium and potassium ions from water into benzene in presence of 18-crown-6 and bromocresol green

An extraction procedure for the spectrophotometric determination of sodium or potassium involving a 1:1:1 metal ion: crown ether: sulfonephthalein dye monoanion ion associate has been described in the literature. In a recent paper from this laboratory the total dye content of the organic phase resulting from partitioning bromocresol green (HBCG) alone between water and benzene was determined from back extraction into an aqueous phosphate buffer. In benzene, bromocresol green is present to a large extent as a colorless lactone form, or gamma-sultone. The sultone equilibrium constant, K(dist) (sultone)=[sultone](o)/[BCG(-)](w)gamma(BCG(-))(w)a(H(+)) was found to be 373+/-42 mol(-1) dm(3). In the present study, sodium was extracted from water into benzene with 18-crown-6 and BCG(-), yielding K(dist) (sultone)=232 mol(-1) dm(3) and the conventional extraction constant, K(ex), of (5.49+/-0.49)x10(3) mol(-2) dm(6). Corresponding values from potassium extraction data are 256 mol(-1) dm(3) and (4.80+/-0.51)x10(6) mol(-2) dm(6), respectively. In the absence of ethanol, the calibration plot of sodium closely followed Beer's law, but that of potassium curved downward, approaching linearity in 15% ethanol. High 18-crown-6, low bromocresol green concentrations are analytically favorable for the extraction of potassium from the 15% aqueous ethanol mixture. All plots from the back-extractions (a measure of the total dye content in the benzene phase) were linear. Ethanol markedly reduced the reagent blank, when C(MCl)=0, as a result of the sultone being partitioned less favorably into benzene.     

Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4'-oxyazobenzene: UV-visible, 1H NMR and ab initio studies

alpha-Cyclodextrin, beta-cyclodextrin, N-(6(A)-deoxy-alpha-cyclodextrin-6(A)-yl)-N'6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea and N,N-bis(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea (alphaCD, betaCD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4'-oxyazobenzene, E-3(-). In aqueous solution at pH 10.0, 298.2 K and I = 0.10 mol dm(-3)(NaClO(4)) spectrophotometric UV-visible studies yield the sequential formation constants: K(11) = (2.83 +/- 0.28) x 10(5) dm(3) mol(-1) for alphaCD.E-(-), K(21) = (6.93 +/- 0.06) x 10(3) dm(3) mol(-1) for (alphaCD)(2).E-3(-), K(11) = (1.24 +/- 0.12) x 10(5) dm(3) mol(-1) for betaCD.E-(-), K(21) = (1.22 +/- 0.06) x 10(4) dm(3) mol(-1) for (betaCD)(2).E-(-), K(11) = (3.08 +/- 0.03) x 10(5) dm(3) mol(-1) for .E-3(-), K(11) = (8.05 +/- 0.63) x 10(4) dm(3) mol(-1) for .E-3(-) and K(12) = (2.42 +/- 0.53) x 10(4) dm(3) mol(-1) for .(E-3(-))(2). (1)H ROESY NMR studies show that complexation of E-3(-) in the annuli of alphaCD, betaCD, 1 and 2 occurs. A variable-temperature (1)H NMR study yields k(298 K)= 6.7 +/- 0.5 and 5.7 +/- 0.5 s(-1), DeltaH = 61.7 +/- 2.7 and 88.1 +/- 4.2 kJ mol(-1) and DeltaS = -22.2 +/- 8.7 and 65 +/- 13 J K(-1) mol(-1) for the interconversion of the dominant includomers (complexes with different orientations of alphaCD) of alphaCD.E-3(-) and (alphaCD)(2).E-3(-), respectively. The existence of E-3(-) as the sole isomer was investigated through an ab initio study.     

From alcohols to sugars-how does the reactivity of alpha-hydroxyalkyl radicals change with structure? A quantitative examination by pulse radiolysis

Rate constants are reported for the 1-electron reduction of the azo dye Orange II in water (pH 7.0) by 10 different alpha-hydroxy radicals. The radicals were created by pulse radiolysis of aqueous solutions of the corresponding alcohol/sugar. The rate constants varied from 1 x 10(8) to 2.7 x 10(9) mol(-1) dm(3) s(-1) and radicals with beta-hydroxy groups had the lowest rate constant. The reaction was found to be controlled by the reduction potentials of the radicals, with steric influences having little effects. Good fits of the data were obtained using the Marcus equation with lambda =140 kJ/mol.     

Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 1. Foam films

Using the interferometric method of Scheludko-Exerowa for investigation of foam films, we have obtained results using a hydrophobically modified inulin polymeric surfactant (INUTEC SP1). Measurements were carried out at constant INUTEC SP1 concentration of 2 x 10(-)(5) mol.dm(-)(3) and at various NaCl concentrations (in the range 1 x 10(-)(4) to 2 mol.dm(-)(3)). At constant capillary pressure of 50 Pa, the film thickness decreased gradually with an increase in NaCl concentration up to 10(-)(2) mol.dm(-)(3) NaCl above which the film thickness remains virtually constant at about 16 nm. This reduction in film thickness with an increase in NaCl concentration is due to the compression of the double layer and at the critical electrolyte concentration (C(el,cr) = 10(-)(2) mol.dm(-)(3)) the electrostatic component of the disjoining pressure is completely screened and the remaining pressure is due to the steric interaction between the adsorbed polymer layers. Disjoining pressure-thickness (Pi-h) isotherms were obtained at C(el) < C(el,cr) (10(-)(4) - 10(-)(3) mol.dm(-)(3)) and C(el) > C(el,cr) (0.5, 1, and 2 mol.dm(-)(3)). In the first case, the disjoining pressure isotherms could be fitted using the classical DLVO theory, Pi = Pi(el) + Pi(vw), and using the constant charge model. At C(el) > C(el,cr), the main repulsion is due to the steric interaction between the polyfructose loops that exist at the air-water interface, i.e., Pi = Pi(st) + Pi(vw). Under these conditions, there is a sharp transition from DLVO to non-DLVO forces. In the latter case, the interaction could be described using the de Gennes' scaling theory. This gave an adsorbed layer thickness of 6.5 nm which is in reasonable agreement with the values obtained at the solid-solution interface. The Pi-h isotherms showed that these foam films are not very stable and they tend to collapse above a critical capillary pressure (of about 1 x 10(3) Pa), and these results could be used to predict the foam stability.     

Amperometric detection with microelectrodes in flow injection analysis: theoretical aspects and application in the determination of nitrite in saliva

The construction of a wall-jet cell with amperometric detection using a set of disc electrodes whose radii ranged from 5 to 750 mum has been proposed. The influence of some experimental parameters like flow rate and electrode radius on hydrodynamic voltammograms recorded for a 0.5 mmol dm(-3) potassium ferrocyanide solution also containing 0.1 mol dm(-3) KCl has been discussed. Some considerations regarding the current signals obtained from flow injection experiments using both a 5- and a 750-mum radius platinum electrode were carried out in order to achieve the lowest limit of detection, a value of 0.03 mumol dm(-3) ferrocyanide being calculated by using the 5-mum radius microelectrode as amperometric detector. The wall-jet cell has been used in the determination of nitrite in saliva by quantifying the triiodide formed in the reaction of the analyte with excess iodide in acidic medium. A 12.5-mum platinum disc microelectrode maintained at +0.2 V vs. Ag/AgCl was used as amperometric detector. Peaks obtained in fiagrams after injection of diluted saliva to the carrier stream containing 0.1 mol dm(-3) sulphuric acid and 20 mmol dm(-3) potassium iodide were compared to an analytical curve obtained in the same conditions (r(2)=0.997) for a nitrite concentration in the range 1-10 mumol dm(-3). The concentration of nitrite in the saliva sample after the appropriate correction for dilution was found to be 2.3 ppm (0.05 mmol dm(-3)), in a good agreement with results obtained by using a standard spectrophotometric procedure (2.5 ppm). The limit of detection of the method was calculated as 0.2 mumol dm(-3), and the reproducibility was checked by measuring the peak current for 19 injections of 10 muM nitrite, the standard deviation being 3.7%.