Sulfur speciation by capillary zone electrophoresis: conditions for sulfite stabilization and determination in the presence of sulfate, thiosulfate and peroxodisulfate

Capillary zone electrophoresis (CZE) was used for the separation of the sulfur species SO3(2-), SO4(2-), S2O(3-) and S2O8(2-). Using an electrolyte system with 9.5 mmol L(-1) potassium chromate as UV-absorbing probe and 1 mmol L(-1) diethylenetriamine (DETA) as electroosmotic flow modifier, various possibilities for the stabilization of sulfite and electrophoretic separation of the sulfur anions were investigated. By adding 5% propanol as a stabilizer to both the working electrolyte and the sample solution, a good stabilization for sulfite and a separation of the sulfur anions in a short analysis time (4 min) was achieved. The advantages by using propanol instead of other stabilizers often used in analytical techniques are discussed. The electrophoretic separation of the sulfur anions was optimized with respect to the pH of the working electrolyte and concentration of the electroosmotic flow modifier (DETA). The detection limits achieved for SO3(2-), SO4(2-), S2O3(2-) and S2O8(2-) were 0.35, 0.25, 0.78 and 0.80 mg L(-1), respectively.     

Determination of sulfur contents of SO3, S2O3 and S based on the electrocatalytic interaction with homogeneous mediator tris(2,2'-bipyridyl)Ru(II)

A sensitive voltammetric method has been developed for the determination of total or single species of sulfur anions containing sulfide, sulfite and thiosulfate. The method is based on the catalytic effect of tris(2,2'-bipyridyl)Ruthenium(II) (Ru(bpy)₂^+ 2) as a homogeneous mediator on the oxidation of those anions at the surface of a glassy carbon electrode. A reversible redox couple of Ru(II)/Ru(III) were observed as a solute in aqueous solution. Cyclic voltammetry study showed that the catalytic current of the system depends on the concentration of the anions. Optimum pH values for voltammetric determination of sulfite, thiosulfate and sulfide has been found to be 5.6, 10.0 and 10.0, respectively. Under the optimized conditions the calibration curves have been obtained linear in the concentration ranges of 0.8–500.0, 0.4–1000.0 and 0.5–5000.0 µmol L^− 1 of SO₃²⁻, S₂O₃²⁻ and S²⁻, respectively. The detection limits have been calculated to be 0.40, 0.17 and 0.33 µmol L^− 1 for SO₃²⁻, S₂O₃²⁻ and S²⁻, respectively. The diffusion coefficients of sulfite and thiosulfate have been estimated using chronoamperometry. The chronoamperometric method also has been used to determine the catalytic rate constant for catalytic reaction of the Ru(bpy)₂^+ 2 with sulfite and thiosulfate. Finally the proposed method has been used for the determination of total sulfur contents in real samples of water and wastewater. Moreover the sulfite content in sugar and sulfur dioxide in air has been determined with satisfactory results.     

Effects of the composition of acid solutions and the presence of organic acids on oxygen electroreduction to hydrogen peroxide in a carbon black gas-diffusion electrode

This paper reports on the effects of the K₂SO₄, H₂SO₄, NaCl, HCl, and tetrabutylammonium bromide concentrations (0.01–0.0002 M) and the presence of formic, acetic, and butyric acids in the electrolyte on the kinetic characteristics of oxygen reduction to H₂O₂ in a carbon black gas-diffusion electrode (GDE) and on the H₂O₂ accumulation kinetics in electrolyte at current densities of 30–100 mA/cm². The introduction of K₂SO₄ and tetrabutylammonium bromide in the electrolyte led to an increase in the transfer coefficient α and a decrease in the coefficients in the Tafel equation. The concentration and the current efficiency of H₂O₂ decreased with the salt to acid concentration ratio. The organic acids reduced the current efficiency of H₂O₂ and increased the electrode polarization. Peracids with a current efficiency of up to 0.27% and concentration of up to 7.5 mM were obtained. Solutions of H₂O₂ with concentrations of 0.6–3.3 M and current efficiencies of 17–75% were obtained at current densities of 30–100 mA/cm² in electrolytes with salt and inorganic acid concentrations of 0.9–40 g/l and in the presence of organic acids.     

Application of co-electroosmotic capillary electrophoresis for the determination of inorganic anions and carboxylic acids in soil and plant extract with direct UV detection

Summary Indirect UV detection in capillary zone electrophoresis (CZE) is frequently used for the determination of inorganic anions and carboxylic acids. However, there are few reports on direct UV detection of these solutes in real samples. This paper describes the use of direct UV detection of inorganic anions and organic acids in environmental samples using co-electroosmotic capillary zone electrophoresis (co-CZE) at 185 nm. The best separation and detection of the solutes was achieved using a fused silica capillary with an electrolyte containing 25 mM phosphate, 0.5 mM tetradecyltrimethylammonium bromide (TTAB) and 15% acetonitrile (v/v) at pH 6.0. Four common inorganic anions (Cl⁻, NO₂ ⁻, NO₃ ⁻ and SO₄ ²⁻) and 11 organic acids (oxalic, formic, fumaric, tartaric, malonic, malic, citric, succinic, maleic, acetic, and lactic acid), were determined simultaneously in 15 min. Linear calibration plots for the test solutes were obtained in the range 0.02–0.5 mM with detection limits ranging from 1–9 μM depending on the analyte. The proposed method was successfully used to determine inorganic anions and carboxylic acids in soil and plant tissue extracts with direct injection of the sample.     

Chemical analysis of sulfur species in geothermal waters

Analytical methods have been developed to determine sulfur species concentrations in natural geothermal waters using Reagent-Free™ Ion Chromatography (RF™-IC), titrations and spectrophotometry. The sulfur species include SO₄²⁻, S₂O₃²⁻, and ∑S²⁻ with additional determination of SO₃²⁻ and S_xO₆²⁻ that remains somewhat semiquantitative. The observed workable limits of detections were ≤0.5 μM depending on sample matrix and the analytical detection limits were 0.1 μM. Due to changes in sulfur species concentrations upon storage, on-site analyses of natural water samples were preferred. Alternatively, the samples may be stabilized on resin for later elution and analysis in the laboratory. The analytical method further allowed simultaneous determination of other anions including F⁻, Cl⁻, dissolved inorganic carbon (DIC) and NO₃⁻ without sample preservation or stabilization. The power of the newly developed methods relies in routine analysis of sulfur speciation of importance in natural waters using techniques and facilities available in most laboratories doing water sample analysis. The new methods were successfully applied for the determination of sulfur species concentrations in samples of natural and synthetic waters.     

Detection of transferrin isoforms in human serum: comparison of UV and ICP–MS detection after CZE and HPLC separations

Two methods for separation of transferrin (Tf) sialoforms, capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) with conventional UV absorbance detection, have been investigated and compared. First, conditions affecting the separation of the Tf isoforms by capillary zone electrophoresis and HPLC were carefully optimized. The use of 15 mmol L⁻¹ borate buffer (pH 8.4) containing 3 mmol L⁻¹ diaminobutane (DAB) as additive enabled good separation of the Tf isoforms by CE (75 cm×50 μm i.d. fused silica capillary) at 25 kV. In HPLC, a gradient of ammonium acetate (from 0 to 250 mmol L⁻¹ in 45 min) buffered at pH 6 (Tris-HCl) proved suitable for separation of Tf isoforms on a Mono-Q HR 5/5 anion-exchange column. On-line specific detection of the iron associated with the different Tf isoforms, after Fe saturation, by inductively coupled plasma mass spectrometry (ICP–MS) was studied in detail to compare its analytical performance with UV detection. For both CE and HPLC an octapole reaction system (ORS) ICP–MS instrument was used to minimize polyatomic interferences on the ⁵⁶Fe major isotope. Limits of detection of the different isoforms were in the range of 0.02–0.04 μmol L⁻¹ Tf for HPLC–ICP (ORS)–MS. This hybrid technique proved more selective and reliable detection of transferrin isoforms with 2, 3, 4, 5, and 6 sialic acid residues (S₂, S₃, S₄, S₅, and S₆) in real serum samples. Interesting results from iron speciation of Tf in serum from healthy individuals and from pregnant women are given.     

Dye-coated stationary-phases: A retention model for anions in ion-interaction chromatography

Summary The separation of inorganic anions (NO₃ ⁻, NO₂ ⁻, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, S₂O₃ ²⁻) by ion-interaction chromatography mediated with a specific dye has been investigated. Chromatography was performed on a LiChrospher RP-18 colum dynamically coated with crystal violet, using acetonitrile-water buffered with phthalate as the mobile phase. The presence of the dye in the eluent enabled indirect spectrophotometric detection of the analytes, which have no significant UV absorption. Retention data were collected for the different anions by varying the composition of the mobile phase according to a full factorial experimental design. A theoretical model for the retention of singly- and doubly-charged analytes, on the basis of the two main processes of ion-exchange and ion-pair formation, has been proposed and validated with the experimental data.     

Influence of support acidity on electronic state of platinum in oxide systems promoted by SO4 2− anions

The electronic state of platinum supported on SO₄/ZrO₂, SO₄/TiO₂, SO₄/Al₂O₃, and SO₄/SiO₂ systems and on systems unpromoted by sulfur was investigated by diffuse-reflectance IR spectroscopy using CO as the probe molecule. The introduction of SO₄ ²⁻ anions increases the electron deficit on platinum particles. This suppresses the formation of bridging CO complexes with the metal, leads to the high-frequency shift of absorption maxima of CO adsorbed in the linear form, and stabilizes positively charged metal species (Pt^δ+ and Pt⁺) during the reduction process. The formation of the positively charged species includes the interaction between the acidic protons and the metal particles yielding [Pt−H]^δ+ adducts. The extent of the influence of the support on the electronic state of the metal increases in the series SO₄/SiO₂<SO₄/Al₂O₃<SO₄/TiO₂<SO₄/ZrO₂ in parallel with an increase in the strength of the acid sites in the system. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1094–1099, June, 1998.     

Theoretical studies on anionic clusters of sulfate anions and carbon dioxide, SO 4?1/?2 (CO2)n, n?=?1?4

Geometry optimizations were performed on monoanionic and dianionic clusters of sulfate anions with carbon dioxide, SO₄^−1/−2(CO₂) _n , for n = 1–4, using the B3PW91 density functional method with the 6-311 + G(3df) basis set. Limited calculations were carried out with the CCSD(T) and MP2 methods. Binding energies, as well as adiabatic and vertical electron detachment energies, were calculated. No covalent bonding is seen for monoanionic clusters, with O₃SO–CO₂ bond distances between 2.8 and 3.0 ?. Dianionic clusters show covalent bonding of type [O₃S–O–CO₂]⁻², [O₃S–O–C(O)O–CO₂]⁻², and [O₂C–O–S(O₂)–O–CO₂]⁻², where one or two oxygens of SO₄⁻² are shared with CO₂. Starting with n = 2, the dianionic clusters become adiabatically more stable than the corresponding monoanionic ones. Comparison with SO₄^−1/−2(SO₂) _n and CO₃^−1/−2(SO₂) _n clusters, the binding energies are smaller for the present SO₄^−1/−2(CO₂) _n systems, while stabilization of the dianion occurs at n = 2 for both SO₄⁻²(CO₂) _n and SO₄⁻²(SO₂) _n , but only at n = 3 for CO₃⁻²(SO₂) _n .     

Coordination chemistry of copper(II) complexes with N4, N4S2, and N4O2 donor macrocyclic ligands: Biological aspects—antifungal, synthesis, spectral studies, and magnetic moments

Three macrocyclic ligands and their complexes with copper(II) salts (with anions Cl⁻, NO ₃ ⁻ , and NCS⁻) were prepared and investigated using a combination of microanalytical analysis, melting point, molar conductance measurement, magnetic susceptibility measurement, and electronic, IR and ESR spectral studies. Ligands L¹, L², and L³ having N₄, N₄O₂, and N₄S₂ core, respectively, and all the donor atoms of these ligands are bonded with Cu, which is confirmed by a seven-line pattern observed at half-field in the frozen (H₂O: MeOH = 10: 1 at pH 10) solution ESR spectrum. The polycrystalline ESR data (g _∥ = 2.20–2.27, g _⊥ = 2.01–2.05, and A _∥ = 120–270) of all the complexes together with the high asymmetry geometry suggest that all complexes appear to be near the static distortion (CuN₄O₂ and CuN₄S₂ chromophore geometry). The electronic spectra of the complexes involve two bands at the same intensity corresponding to a cis-distorted octahedral geometry. A common structural feature of both ligand L² and ligand L³ is that two different donor atoms at five-membered heterocyclic aromatic ring due to this N₄O₂ and N₄S₂ chromophore form stable six-membered chelate rings with metals via these two, Cu-O and Cu-S, new interactions comparatively to the first macrocyclic ligand, which has four-membered N,N′-chelate rings. The cyclic voltammetric studies point to a two-step electron transfer indicating the reduction of the two copper atoms to copper(I), i.e., Cu(III)Cu(II) ⇄ Cu(II)Cu(I) ⇄ Cu(I)Cu(0). The molar conductance for the complexes corresponds to 1: 2 and is nonelectrolyte in nature. The magnetic moment (μ_eff) of the complexes lie in the range between 1.80–1.96 μ_B. Finally, these complexes were screened for their antimicrobial activity against Aspergillus-niger of fungal strains. The text was submitted by the authors in English.     

The Decomposition of Aqueous Dithionite and its reactions with polythionates SnO2−6 (n = 3–5) studied by ion-pair chromatography

Aqueous dithionite decomposes at 20°C and pH values not far from 7.0 to give thiosulfate and sulfite from which trithionate may form. Addition of thiosulfate accelerates this reaction only at pH < 6. The pH dependence is explained by formation of HS₂O₃^? ions which are reduced by dithionite to HS^? and SO^2?₃. Sulfide destroys dithionite by nucleophilic cleavage, probably with formation of sulfoxylate and thiosulfite ions. The polythionates S_nO^2?₆ (n = 3–5) are reduced by dithionite at pH = 7.0 and 20°C according to S_nO^2?₆ + S₂O^2?₄ + 2 H₂O→S₂O₃^2? + S_n–3SO₃^2? + 4H¹ + 2SO₃^2? The reaction rate rapidly increases with the number n of sulfur atoms. In secondary reactions sulfite attacks S_nO₆^2? ions with thiosulfate formation.     

Negative Ion Formation by Thermal Surface Ionization of Sulfur Dioxide

The generation of negative ions from SO₂ in the gas phase was studied using the thermal surface ionization method. Six anion types were measured: O⁻, S⁻, SO⁻, and SO₂⁻ and anions with m/z=96 and m/z=128. The most abundant anion formed was S⁻ and the formation routes are discussed for each of the six anions. O⁻, S⁻, and SO⁻ are formed via dissociative electron attachment to the molecule, whereas the generation of SO₂⁻ and anions with m/z=96 and m/z=128 are probably associated with the formation of H₂SO₄ in the gas inlet system and the ion source. Using statistical thermodynamics the dissociation temperatures of SO₂ and SO in the gas phase are calculated and values of above 1800 °C are obtained for both molecules. We also estimated the optimal filament temperatures for the formation of all anions measured, indicating that for SO₂ the optimal temperature is related to the electron affinity of the molecules: the optimal temperature increases with decreasing value of the electron affinity for the molecule corresponding to the respective anion.     

Determination of Sulfur Compounds in Water Samples by Ion Chromatography Dynamic Reaction Cell Inductively Coupled Plasma Mass Spectrometry

An ion chromatography‐inductively coupled plasma mass spectrometric (IC‐ICP‐MS) method for the speciation of sulfur compounds, namely sulfite [SO₃^2?], sulfate [SO₄^2?] and thiosulfate [S₂O₃^2?], was described. Ionic sulfur compounds were well separated in about 3 min by ion chromatography with a Hamilton PRP‐X100 column as the stationary phase and 60 mmol L^?1 NH₄NO₃ and 0.1% v/v formaldehyde (HCHO) solution (pH = 7) as the mobile phase. The analyses were carried out using dynamic reaction cell (DRC) ICP‐MS. The sulfur‐selective chromatogram was determined at m/z 48 as ³²S¹⁶O⁺ by using its reaction with O₂ in the reaction cell. The method avoided the effect of polyatomic isobaric interferences at m/z 32 caused by ¹⁶O¹⁶O⁺ and ¹⁴N¹⁸O⁺ on ³²S⁺ by detecting ³²S⁺ as the oxide ion ³²S¹⁶O⁺ at m/z 48, which is less interfered. The detection limit of various species studied was in the range of 3.6–4.6 ng S mL^?1. The accuracy of the method has been verified by comparing the sum of the concentrations of individual sulfur compounds obtained by the present procedure with the total concentration of sulfur in several natural water samples. The recovery was in the range of 97–102% for various compounds studied.     

Kinetics of Sulfur Oxide, Sulfur Fluoride, and Sulfur Oxyfluoride Anions with Ozone

Rate constants and product branching ratios were measured for eleven sulfur oxide, sulfur fluoride, and sulfur oxyfluoride anions reacting with O₃. The SO^– ₂ ion reacts rapidly to form –O^– ₃, SO^– ₃, and e^–. The temperature dependence of the branching ratio shows more reactive detachment and less SO^– ₃ formation at higher temperature. SO^– ₃ reacts with O₃, forming SO^– ₄ at 1/3 to 1/4 of the collisional rate from 200 to 500 K, respectively. At 300 K, SF^– ₆ charge transfers to O₃ at 20% of the collisional rate. F₂SO^– ₂ reacts with O₃ at a few percent of the collision rate, forming both O^– ₃ and FSO^– ₃; The ion F₃SO^– reacts slowly with O₃ to form F₃SO^– ₂. The ions SO^– ₄, SF^– ₅, FSO^– ₂, FSO^– ₃, F₃SO^–, and F₅SO^– are unreactive with O₃. A trend is noted relating the ion reactivity with the coordination of the central sulfur atom, i.e., the number of S–F bonds plus two times the number of S=O bonds. Only ions with a sulfur coordination of 4 or 6 are reactive, although the reaction rate constants are generally small. The reactivity trends appear to be partially explained by spin conservation. These reactions are all sufficiently slow, so O₃ reactions should not play a major role in SF₆/O₂ discharges. All ions studied have been found to be unreactive with O₂.     

A tr esr study of the reactions of excited organic triplet molecules with inorganic sulfur oxoanions

The reactions of excited triplet ketones including benzophenone, 4,4′-dihydroxybenzophenone, and acetone with inorganic sulfur oxoanions: sulfite (SO₃ ⁼), metabisulfite (S₂O₅ ⁼), and dithionite (S₂O₄ ⁼) in solution were studied by TRESR. Observation of radical anion intermediates polarized by the Triplet Mechanism demonstrated the reactions of electron transfer from the sulfur dianions to the organic triplet states. Thus, electron transfer from SO₃ ⁼ to the triplet organic carbonyl compounds gave rise to the SO₃ ^? radical; reactions with metabisulfite and dithionite were more complex and may be interpreted as occurring via electron transfer to the carbonyl system followed by dissociation of the S-S bond in the resulting sulfur oxoanion radical. The initial radical ion pairs thus formed consist of pairs of anion radicals, one organic and one inorganic.     

Investigation on the formation and decay of the N,N,N′,N′-tetramethylbenzidine radical cation using various oxidants and reductants by stopped-flow spectrophotometry

N,N,N′,N′-Tetramethylbenzidine (TMB) is an aromatic amine that undergoes oxidation by various oxidizing agents such as Ce⁴⁺, MnO⁻₄, Cr₂O²⁻₇; HSO⁻₅, S₂O⁻₈, H₂O₂, Cl₂, Br₂ and I₂, thereby serving as a reducing substrate. One-electron oxidation of TMB results in a radical cation (TMB^˙+), and on further oxidation leads to the product dication (TMB⁺⁺) were monitored by stopped-flow spectrophotometer at the absorption wavelength of TMB^˙+(λ_max; 460 nm). ESR data was also provided to confirm the formation of radical cation. The rates of both the formation and decay of TMB^˙+ have been followed by a total second-order kinetics, a first-order dependence each on [TMB] (or) [TMB^˙+] and [oxidant]. The kinetic and transition state parameters have been evaluated for the effects of pH and temperature on the formation and decay of TMB^˙+ and discussed with suitable reaction mechanisms. Also, the rate constants for the reactions of the radical cation with various reducing agents such as sulfite (SO²⁻₃), thiosulfate (S₂O²⁻₃), dithionite (S₂O²⁻₄) and disulfite (S₂O²⁻₅) and ascorbic acid (vitamin C, AH₂ were determined. Besides these, this article also explains how TMB acts as a better electron relay than unsubstituted benzidine, even though both of them undergo one-electron oxidation and are used in the chemical routes to solar energy conversions. The observed rate constants for electron transfer were correlated theoretically using Marcus theory. The observed and calculated rate constants have good correlation.     

Chemical analysis of rain and snow samples from Chernogolovk/Russia by IC, TXRF and ICP–MS

 Data are reported for a first measurement campaign for a great number of cations (the range of concentrations for main components is given in ng/mL: Na:<70–880; Mg: 12–160; Al:<7–120; K: 200–1600; NH₄ ⁺: 2500–9000; Ca: 300–2500; Fe:<140–1200; Zn: 20–320) and anions (HCOO^-: 100–1800; CH₃COO^-: 400–14500; C₂O^2- ₄: <100–460; NO^- ₂: <20–55; NO^- ₃: 660–9900; SO^2- ₃:<100–1150; SO₄ ^2-: 450–19700; Cl^-: 170–3200) in rain and snow samples of the Chernogolovka region 80 km northeast of the centre of Moscow. These data serve as a first assessment of the situation of the atmosphere in this region. For the determination of fluoride, formate, acetate, nitrite, bromide, nitrate, sulfite, and sulfate by IC, a relevant development of the method was carried out. For some metallic cations, a critical quality control was achieved by parallel measurements with ICP-MS, TXRF, and IC. Furthermore, time resolved measurements were performed for a rain event. Filtration experiments were undertaken to differentiate between the presence of a series of elements and species in liquid or solid state in the collected aqueous samples. Received: 25 July 1996 / Revised: 26 September 1996 / Accepted: 13 October 1996     

Simple spectrophotometric method for the determination of sulfur dioxide by its decolorizing effect on the peroxovanadate complex

A simple, rapid, and economical spectrophotometric method is developed for the determination of sulfur dioxide in sugar and air samples. The developed method is based on a red-brown peroxovanadate complex (λ_max = 470 nm) produced in 2 M sulfuric acid when ammonium metavanadate is treated with hydrogen peroxide. Under fixed concentrations of hydrogen peroxide and ammonium metavanadate, when sodium metabisulfite (Na₂S₂O₅ = 2SO₂) is added, it preferentially reacts with hydrogen peroxide producing sulfuric acid, and the unreacted hydrogen peroxide then reacts with ammonium metavanadate; therefore, the concentration of sulfur dioxide is directly proportional to a decrease in the concentration of the peroxovanadate complex. The stoichiometric ratio between hydrogen peroxide and ammonium metavanadate as well as the stability constant of the complex are determined by the modified Job’s method and the respective values are found to be 1: 1 and 2.5 × 10⁴ mol⁻¹ L, respectively. The system obeys Lambert-Beer’s law in the concentration range 3.57–64.26 ppm of sulfur dioxide. The molar absorptivity, correlation coefficient, and Sandell’s sensitivity values are found to be 0.649 × 10³ L mol⁻¹ cm⁻¹, 0.9908, and 0.1972 μg cm⁻², respectively. The method is applied to the determination of sulfur dioxide present in commercial sugars and air samples. The results obtained are reproducible with a standard deviation of 0.02–0.05. For method validation, sulfur dioxide is also determined separately following the AOAC method for an air sample and the ICUMSA method for commercial sugars. The results obtained by the developed and official methods are in good agreement. The text was submitted by the authors in English.     

Chemical analysis of rain and snow samples from Chernogolovk/Russia by IC, TXRF and ICP–MS

 Data are reported for a first measurement campaign for a great number of cations (the range of concentrations for main components is given in ng/mL: Na:<70–880; Mg: 12–160; Al:<7–120; K: 200–1600; NH₄ ⁺: 2500–9000; Ca: 300–2500; Fe:<140–1200; Zn: 20–320) and anions (HCOO^-: 100–1800; CH₃COO^-: 400–14500; C₂O^2- ₄: <100–460; NO^- ₂: <20–55; NO^- ₃: 660–9900; SO^2- ₃:<100–1150; SO₄ ^2-: 450–19700; Cl^-: 170–3200) in rain and snow samples of the Chernogolovka region 80 km northeast of the centre of Moscow. These data serve as a first assessment of the situation of the atmosphere in this region. For the determination of fluoride, formate, acetate, nitrite, bromide, nitrate, sulfite, and sulfate by IC, a relevant development of the method was carried out. For some metallic cations, a critical quality control was achieved by parallel measurements with ICP-MS, TXRF, and IC. Furthermore, time resolved measurements were performed for a rain event. Filtration experiments were undertaken to differentiate between the presence of a series of elements and species in liquid or solid state in the collected aqueous samples. Received: 25 July 1996 / Revised: 26 September 1996 / Accepted: 13 October 1996     

A study of the aerobic reaction between dopamine and Fe(III) in the presence of S<Subscript>2</Subscript>O<Subscript>3</Subscript><Superscript>2−</Superscript>

The reaction between Fe(III) and dopamine in aqueous solution in the presence of Na₂S₂O₃ was followed through UV–Vis spectroscopy, pH and oxy-reduction potential (Eh) measurements. The formation and quick disappearing of the complex [Fe(III)HL¹⁻]²⁺, HL¹⁻ = monoprotonated dopamine was observed with or without S₂O₃ ²⁻ at pH 3. An unexpected reaction occurs in presence of thiosulfate forming the stable anion complex [Fe(III)(L²⁻)₂]¹⁻, L²⁻ = dopacatecholate (λ = 580 nm) and the auto-increasing of the pH, from 3 to 7. It was proposed that H⁺ and molecular oxygen are consumed by free radical thiosulfate formed during the reaction.