Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK _a,1 ^int=6.7±0.2 and logK _a,2 ^int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK ₁ = 3.7±0.1 and pK ₂ = 6.6±0.1 and the total acidity was 4.56 mmol g⁻¹. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10⁻⁵ mol dm⁻³) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm⁻³ NaNO₃. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm⁻³ salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed. Received: 20 July 1999 /Accepted in revised form: 20 October 1999     

Gelled Na<Subscript>2</Subscript>HPO<Subscript>4</Subscript> · 12H<Subscript>2</Subscript>O with amylose-g-sodium acrylate: heat storage performance,heat capacity and heat of fusion

A novel gelling method was studied to stabilize phase change material Na₂HPO₄ · 12H₂O with amylose grafted sodium acrylate. Gelled Na₂HPO₄ · 12H₂O shows stable heat storage performance prepared at optimized conditions: 2.7mass/mass% sodium acrylate, 0.4 mass/mass% amylose, 0.05–0.09 mass/mass% N, N′-methylenebisacrylamide, 0.05–0.09 mass/mass% K₂S₂O₈ and Na₂SO₃ (mass ratio 1:1), at 50 °C. Na₂HPO₄ · 12H₂O was dispersed in gel network as tiny crystals less than 0.1 mm. Melting points were in the range 35.4 ± 2 °C. Short-term thermal cycling proves the effectiveness of the novel method for eliminating phase separation in the gelled salt. Adiabatic calorimetric measurement of heat capacities shows two phase transitions, which correspond to melting of Na₂HPO₄ · 12H₂O and freezable bond water in gel, respectively. Heat of fusion of pure Na₂HPO₄ · 12H₂O was determined as 260.9 J g⁻¹. Distribution of extra water is: free water:freezable water:nonfreezing water = 0:0.85:0.15.     

First results on Fe solid-phase extraction from coastal seawater using anatase TiO2 nano-particles

This paper describes the application of TiO₂ nano-particles (anatase form) for the solid-phase extraction of iron from coastal seawater samples. We investigated the adsorption processes by infra-red spectroscopy. We compared in batch and on-(mini)column extraction approaches (0.1 and 0.05 g TiO₂ per sample, respectively), combined to external calibration and detection by inductively coupled plasma mass spectrometry at medium mass resolution. Globally, this titania phase was slightly more efficient with seawater than with ultra-pure water, although between pH 2 and pH 7, the Fe retention efficiency progressed more in ultra-pure water than in seawater (6.9 versus 4.8 times improvement). Different reaction schemes are proposed between Fe(III) species and the two main categories of titania sites at pH 2 (adsorption of [FeL _x ]^{(3 − x)+} via possibly the mediation of chlorides) and at pH 7 (adsorption of [Fe(OH)₂]⁺ and precipitation of [Fe(OH)₃]⁰). Under optimised conditions, the inlet system was pre-cleaned by pumping 6% HCl for ∼2 h, and the column was conditioned by aspirating ultra-pure water (1.7 g min⁻¹) and 0.05% ammonia (0.6 g min⁻¹) for 1 min. Then 3 g seawater sample was loaded at the same flow rate while being mixed on-line with 0.05% ammonia at 0.6 g min⁻¹ to adjust the pH to 7. The iron retained on the oxide powder was then eluted with 3 g 6% HCl (<0.002% residual salinity in the separated samples). The overall procedural blank was 220 ± 46 (2 s, n = 16) ng Fe kg⁻¹ (the titania was renewed in the column every 20 samples, with 2-min rinsing in between samples with 6% HCl at 1.5 g min⁻¹). The recovery estimated from the Canadian certified reference material CASS-2 was 69.5 ± 7.6% (2 s, n = 4). Typically, the relative combined uncertainty (k = 2) estimated for the measurement of ∼1 μg Fe kg⁻¹ (0.45 μm filtered and acidified to pH 1.5) of seawater was ∼12%. We applied our method to a similar sample, from the coastal region of the North Sea. The agreement well within stated uncertainties of our result with the value obtained independently by isotope dilution mass spectrometry further validated our method.     

Cloning,Expression, and Characterization of a Wide-pH-Range Stable Phosphite Dehydrogenase from <Emphasis Type="Italic">Pseudomonas</Emphasis> sp. K in <Emphasis Type="Italic">Escherichia coli</Emphasis>

A phosphite dehydrogenase gene (ptdhK) consisting of 1,011-bp nucleotides which encoding a peptide of 336 amino acid residues was cloned from Pseudomonas sp. K. gene ptdhK was expressed in Escherichia coli BL21 (DE3) and the corresponding recombinant enzyme was purified by metal affinity chromatography. The recombinant protein is a homodimer with a monomeric molecular mass of 37.2 kDa. The specific activity of PTDH-K was 3.49 U mg⁻¹ at 25 °C. The recombinant PTDH-K exhibited maximum activity at pH 3.0 and at 40 °C and displayed high stability within a wide range of pHs (5.0 to 10.5). PTDH-K had a high affinity to its natural substrates, with K _m values for sodium phosphite and NAD of 0.475 ± 0.073 and 0.022 ± 0.007 mM, respectively. The activity of PTDH-K was enhanced by Na⁺, NH₄⁺, Mg²⁺, Fe²⁺, Fe³⁺, Co²⁺, and EDTA, and PTDH-K exhibited different tolerance to various organic solvents.     

The kinetics of electroreduction of europium(III) cations at bismuth single-crystal electrode

Electroreduction of Eu(ClO₄)₃ and Eu₂(SO₄)₃ has been studied at electrochemically polished \textBi( 01[`1] ) {\text{Bi}}\left( {01\bar{1}} \right) single-crystal electrode in an acidic HClO₄ or H₂SO₄ (pH ~3) aqueous solution with LiClO₄ or Na₂SO₄ additions as a surface inactive electrolyte. The Eu³⁺ cations electroreduction rate depends on the electrode potential applied and the concentration of the supporting electrolyte as well as the concentration of the Eu³⁺ ions. At the more negative electrode potentials than the zero charge potential (zcp), the diffusion current plateaus were observed. The values of the rate constant for the heterogeneous reaction calculated at zcp are independent of the base electrolyte concentration studied. Analysis of the kinetic data corrected for the electrical double-layer effect shows that the coincidence of the corrected Tafel plots can be achieved, assuming that the effective charge of a reactant (+1.6) is significantly lower than it would be expected (z _A = +3).     

Bi5Nb3O15 as a photocatalyst: photocatalytic and photoelectrochemical studies

Bi₅Nb₃O₁₅ was prepared from a stoichiometric mixture of Bi₂O₃ and Nb₂O₅ at 300–500 °C. The prepared photocatalyst was characterized by diffuse reflection spectrum (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and particle size analysis. The band gap, crystal structure and average grain size were determined from the above methods to be 3.25 eV, distorted pyrochlore and 4–5 μm respectively. The photoelectrochemical behavior of hydrogen-reduced Bi₅Nb₃O₁₅ was investigated in 0.1 M Na₂SO₄ and using the Fe(CN)₆ ^3−/4− redox couple for measuring the current-voltage characteristics. The cyclic voltammetric studies revealed that the onset potential for photocurrent generation existed at −0.45 V, which is more negative to water reduction level at pH 7.0, and that of the photocurrent at 1.0 V was observed as 0.58 mA/cm². Photocatalytic hydrogen production has been achieved by using Bi₅Nb₃O₁₅ as a photocatalyst in presence of methyl viologen. The quantum yield for hydrogen production for this system was found to be 0.83. All the studies clearly indicated that Bi₅Nb₃O₁₅ has potential in solar energy conversion. Received: 22 May 1997 / Accepted: 18 September 1997     

Magnesium ion-conducting gel polymer electrolytes dispersed with fumed silica for rechargeable magnesium battery application

Effect of fumed silica dispersion on poly(vinylidene fluoride-co-hexafluoropropylene)-based magnesium ion-conducting gel polymer electrolyte has been studied using various physical and electrochemical techniques. The composite gel electrolytes are free-standing and flexible films with enough mechanical strength. The optimized composition with 3 wt.% filler offers a maximum ionic conductivity of ∼1.1 × 10⁻² S cm⁻¹ at ∼25 °C with good thermal and electrochemical stabilities. The Mg²⁺ ion conduction in the gel nanocomposite film is confirmed from the cyclic voltammetry, impedance spectroscopy, and transport number measurements. The space-charge layers formed between filler particles and gel electrolyte are responsible for the enhancement in ionic conductivity. The applicability of the gel nanocomposite to a rechargeable battery is examined by fabricating a prototype cell consisting of Mg [or Mg-multiwalled carbon nanotube (MWCNT) composite] and MoO₃ as negative and positive electrodes, respectively. The discharge capacity and the rechargeability of the cell have been improved when Mg metal is substituted by Mg-MWCNT composite. The discharge capacity of the optimized cell has found to be ∼175 mAh g⁻¹ of MoO₃ for an initial ten charge–discharge cycles.     

Texture of spherical aluminum phosphate particles

Spherical aluminum phosphate particles with a mean particle diameter of 477±16 nm, produced from aging of a solution containing Al(NO₃)₃, Na₂HPO₄ and HNO₃ at 100°C for 19 h in 20-cm³ Teflon-lined screw-capped Pyrex test tube without agitation, were characterized by various means. It was revealed from x-ray diffraction measurement and transmission electron microscope observation that amorphous particles are formed by agglomeration of small primary particles. The particles exhibited a high selective adsorption of H₂O though they adsorbed small amount of N₂ and CO₂. This characteristic phenomenon was explained by rehydration of Al³⁺ ions by H₂O molecules that were penetrated into the particles.     

The Cloud Point Behaviors and the Liquid–Liquid Equilibrium of L31—Inorganic Sodium Salt Aqueous Two-Phase Systems

The cloud point (CP) of triblock-copolymer L31 aqueous solution was determined with salting-out salts (Na₂SO₄/Na₂CO₃/NaF/NaCl/NaBr). The results show that all these salts can decrease the CP of L31 aqueous solution and form aqueous two-phase system (ATPS). With increasing concentrations of Na₂SO₄ and Na₂CO₃, an obvious phase inversion could be observed and phase inversion points were found. This was mainly due to the change in density, the salt-rich phase shifted from the top phase to the bottom phase. Meanwhile, the liquid-liquid equilibrium (LLE) data for L31-Na₂SO₄/Na₂CO₃/NaF/NaCl/NaBr ATPSs were measured at 288.15 K. The salt ability to decrease the CP and to induce the phase separation is as follows: Na₂SO₄?>?Na₂CO₃, NaF?>?NaCl?>?NaBr. Finally, the order of anions that reduced the CP and caused phase separation was obtained as follows: SO₄^2? >CO₃^2?, F^??>?Cl^??>?Br^?.     

Interfacial and aggregation properties of aqueous sodium <Emphasis Type="Italic">N</Emphasis>-dodecanoylsarcosinate solutions at different pH

The pH dependence of an anionic surfactant, sodium N-dodecanoylsarcosinate (SLAS), has been studied by measuring interfacial tension, fluorescence, dynamic light scattering, etc., in aqueous solutions with phosphate and borate buffers. The interfacial tension (γ) of SLAS decreases remarkably with a pH decrease and is constant at pH > 7.3. The observed values for the critical micelle concentration (cmc) and the surfactant concentration at which its γ value is reduced by 20 mN/m from that of pure water (C ₂₀) decrease with a pH decrease, while those also become constant at pH > 6.5 and >7.3, respectively. On the other hand, the interfacial excess of SLAS increases at pH < 7.3. These interfacial behaviors have been further investigated by the addition of Tl⁺ which replaces Na⁺ of SLAS. The observed γ values of LAS⁻ with the different counter cations are in the order of H⁺ < Tl⁺ < Na⁺. In order to reveal aggregation properties of SLAS, the aggregation number (N _agg), the micropolarity, the hydrodynamic radius (R _h) of micelle, and the fluorescence anisotropy of Rhodamine B (r) have been evaluated at various pHs. The N _agg value shows a decreasing tendency with a pH increase. The I ₁/I ₃ ratio and the R _h values do not strongly depend on pH. The r value decreases until pH 7 and remains constant at pH > 7.0. These interfacial and micelle properties have been discussed in detail considering the electrostatic interaction and the molecular structures of the hydrophilic headgroup.     

Phase behavior and solution properties of sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate in water

Sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate (SLGMS) is a conjugated anionic surfactant in which a glycerol residue connects with a hydrophilic sodium succinate and dodecanoate. Aqueous micellar phase (W_m), hexagonal (H₁), bicontinuous cubic (V₁), and lamellar (L_α) phases are successively formed with increasing the surfactant concentration in a binary SLGMS-water system. The Krafft point is below 0 °C. The effective cross sectional area per surfactant molecule, a _s, in the H₁ phase is almost constant, 0.5 nm², and the shape of cylindrical micelle is almost unchanged with surfactant concentration. The cmc value of SLGMS measured by means of surface tension, electrical conductivity, and fluorescence probe methods is in the range of 4∼9 × 10⁻⁵ mol/l that is much lower than that of sodium dodecanoate, 2 × 10⁻² mol/l, or SDS, 8 × 10⁻³ mol/l. Hence, it is considered that the polar glycerol part in the SLGMS acts as a hydrophobic part. The solubilization of oil in the SLGMS solution is much higher than that in the SDS solution and this also suggests that the glycerol and succinic units act as lipophilic moieties. Received: 15 June 2000/Accepted: 27 July 2000     

New electrochemiluminescent biosensors combining polyluminol and an enzymatic matrix

Performant reagentless electrochemiluminescent (ECL) (bio)sensors have been developed using polymeric luminol as the luminophore. The polyluminol film is obtained by cyclic voltammetry (CV) on a screen-printed electrode either in a commonly used H₂SO₄ medium or under more original near-neutral buffered conditions. ECL responses obtained after performing polymerization either at acidic pH or at pH 6 have been compared. It appears that polyluminol formed in near-neutral medium gives the best responses for hydrogen peroxide detection. Polymerization at pH 6 by cyclic voltammetry gives a linear range extending from 8 × 10⁻⁸ to 1.3 × 10⁻⁴ M H₂O₂ concentrations. Based on this performant sensor for hydrogen peroxide detection, an enzymatic biosensor has been developed by associating the polyluminol film with an H₂O₂-producing oxidase. Here, choline oxidase (ChOD) has been chosen as a model enzyme. To develop the biosensor, luminol has been polymerized at pH 6 by CV, and then an enzyme-entrapping matrix has been formed on the above modified working electrode. Different biological (chitosan, agarose, and alginate) and chemical (silica gels, photopolymers, or reticulated matrices) gels have been tested. Best performances have been obtained by associating a ChOD-immobilizing photopolymer with the polyluminol film. In this case, choline can be detected with a linear range extending from 8 × 10⁻⁸ to 1.3 × 10⁻⁴ M. This paper is based on the results presented in a poster that received a Poster Award on the occasion of XIII International Symposium on Luminescence Spectrometry in Bologna, Italy, on September, 7th-11th, 2008.     

Separation of 97Ru from niobium target using PEG based aqueous biphasic systems

Separation of no-carrier-added (NCA) ⁹⁷Ru from bulk niobium target has been carried out for the first time using green analytical technique, aqueous biphasic system. 50 % (w/v) polyethylene glycol (PEG)-4000, against 2 M solutions of various salts such as Na-citrate, Na-tartarate, Na-malonate, Na₂CO₃, NaHSO₃, Na₂SO₄, Na₂S₂O₃ K₂HPO₄, K₃PO₄, K₂CO₃ and 4 M KOH were employed at room temperature for the extraction of NCA ⁹⁷Ru from bulk niobium. Influence of molecular weight of PEG rich phase as well as pH of some salt rich phase (e.g., Na-tartarate) on the extraction behaviour of NCA ⁹⁷Ru into PEG rich phase was also observed. In the presence of sodium-tartarate salt solution, when volume of PEG-4000: Na-tartarate was 3:1, 91 % of NCA ⁹⁷Ru was extracted into the PEG rich phase without any contamination of niobium target. Dialysis of PEG rich phase containing NCA ⁹⁷Ru was carried out against deionised water to obtained pure NCA ⁹⁷Ru.     

Positional selectivity in electrophilic substitution reactions of π-excessive heterocycles (review)

Existing experimental data on positional selectivity in electrophilic substitution reactions of π-excessive heterocycles are classified. These data are discussed basing on the results of the authors' quantum-chemical calculations [RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d)] of the σ-complexes formed during attack of electrophiles such as H⁺, Me⁺, Me₃Si⁺, Br⁺, NO₂ ⁺, MeCO⁺, and SO₃ at the α- and β-positions of furan, thiophene, selenophene, pyrrole and its N-substituted derivatives, N-R-pyrroles (R = Me, t-Bu, SiMe₃, Si(i-Pr)₃, C₆H₄(p-NO₂), SO₂Ph, CHO, CO₂Me), and the corresponding α- and β-substituted electrophilic substitution products. The differences in energies of the α-and β-isomers of the σ-complexes characterize the preferred direction of electrophilic attack, while the differences in the energies of the isomeric products make it possible to assess the energy preference of one of them. Analysis of the obtained data demonstrates the effects of the studied heterocycles' structure, the nature of the electrophile, and the thermal and steric factors on the positional selectivity (α/β ratio) in electrophilic substitution reactions of π-excessive five-membered heteroaromatic compounds.     

Effective enhancement of selectivities and capacities for CO2 over CH4 and N2 of polymers of intrinsic microporosity via postsynthesis metalation

A series of metalized C-PIM-M (M = Na⁺, Mg²⁺, Al³⁺, PIMs = polymers of intrinsic microporosity) materials were prepared from a carboxyl-functionalized PIM (C-PIMs). The C-PIM-Na exhibited a high CO₂ adsorption capacity of 2.44 mmol/g and extreme low CH₄ uptake of 0.28 mmol/g at 273 K and 101 kPa among three metallated PIMs. It showed remarkably high CO₂/CH₄ and CO₂/N₂ selectivities at both 273 and 293 K due to an advantageous pore-blocking effect of Na⁺ cation.     

Self-consistent reaction field calculation of solvent reorganization energy in electron transfer: a dipole-reaction field interaction model

Based on the continuum dielectric model, this work has established the relationship between the solvent reorganization energy of electron transfer (ET) and the equilibrium solvation free energy. The dipole-reaction field interaction model has been proposed to describe the electrostatic solute-solvent interaction. The self-consistent reaction field (SCRF) approach has been applied to the calculation of the solvent reorganization energy in self-exchange reactions. A series of redox couples, O₂/O⁻ ₂, NO/NO⁺, O₃/O⁻ ₃, N₃/N⁻ ₃, NO₂/NO⁺ ₂, CO₂/CO⁻ ₂, SO₂/SO⁻ ₂, and ClO₂/ClO⁻ ₂, as well as (CH₂)₂C-(-CH₂-) _n -C(CH₂)₂ (n=1 ∼ 3) model systems have been investigated using ab initio calculation. For these ET systems, solvent reorganization energies have been estimated. Comparisons between our single-sphere approximation and the Marcus two-sphere model have also been made. For the inner reorganization energies of inorganic redox couples, errors are found not larger than 15% when comparing our SCRF results with those obtained from the experimental estimation. While for the (CH₂)₂C–(–CH₂–) _n –C(CH₂)₂ (n=1 ∼ 3) systems, the results reveal that the solvent reorganization energy strongly depends on the bridge length due to the variation of the dipole moment of the ionic solute, and that solvent reorganization energies for different systems lead to slightly different two-sphere radii. Received: 19 April 2000 / Accepted: 6 July 2000 / Published online: 27 September 2000     

Purification and Partial Characterization of an Exo-polygalacturonase from Paecilomyces variotii Liquid Cultures

An extracellular polygalacturonase (PG) produced from Paecilomyces variotii was purified to homogeneity through two chromatography steps using DEAE-Fractogel and Sephadex G-100. The molecular weight of P. variotii PG was 77,300 Da by gel filtration and SDS-PAGE. PG had isoelectric point of 4.37 and optimum pH 4.0. PG was very stable from pH 3.0 to 6.0. The extent of hydrolysis of different pectins by the purified enzyme was decreased with an increase in the degree of esterification. PG had no activity toward non-pectic polysaccharides. The apparent K _m and V _max values for hydrolyzing sodium polypectate were 1.84 mg/mL and 432 μmol/min/mg, respectively. PG was found to have temperature optimum at 65 °C and was totally stable at 45 °C for 90 min. Half-life at 55 °C was 50.6 min. Almost all the examined metal cations showed partial inhibitory effects under enzymatic activity, except for Na⁺¹, K⁺¹, and Co⁺² (1 mM) and Cu⁺² (1 and 10 mM).     

Viscosities of poly(N-vinylacetamide) solutions in the presence of salts, urea and guanidine hydrochloride

Summary  In the aqueous solutions of poly(N-vinylacetamide) in the presence of Na₂SO₄ or (NH₄)₂SO₄, the intrinsic viscosities were small and the Huggins constants were large compared with those in water. Similar but much less effects were observed for NaF. Opposite effects were found, however, for NaSCN. The poor solvent quality of aqueous sulfate solutions was significantly improved by the addition of guanidine hydrochloride to 3 M or higher. Urea was much less effective in improving the solvent quality. The different M28.8neffects observed between urea and GdnHCl may be explained by their different binding affinities to amide group. Solubilization of PNVA flocculates induced by the presence of 0.8 M Na₂SO₄ took place by adding urea to 1 M, although only small changes were detected in the viscosity behavior accompanying the addition of 1 M urea. Received: 12 June 1997 Accepted: 4 August 1997     

The effect of bias-temperature stress on Na<Superscript>+</Superscript> incorporation into thin insulating films

The action of Na⁺ incorporation into thin insulating films and transport therein under influence of a bias voltage and temperature (BT stress) is the subject of this work. Deposited onto highly n-doped Si wafers, the insulators get BT stressed and subsequently investigated by means of time-of-flight–secondary ion mass spectrometry (ToF-SIMS). A thin PMMA film, spin-coated onto the insulator, serves as host matrix for a defined amount of Na⁺, provided via sodium triflate. Combining BT stress and ToF-SIMS depth profiling enables the unambiguous detection of Na⁺, incorporated into the insulating material. The insulators of interest vary in their nitride content: SiO₂, SiO_xN_y, and Si₃N₄. For SiO₂, it is shown that once a threshold BT stress is exceeded, Na⁺ gets quantitatively incorporated from PMMA into the underlying insulator, finally accumulating at the SiO₂/Si interface. A quantitative assessment by combination of Butler–Volmer kinetics with hopping dynamics reveals activation energies of E _a = 1.55 − 2.04 eV for Na⁺ transport in SiO₂ with varying thickness. On the other hand, SiO_xN_y and Si₃N₄ films show a different Na⁺ incorporation characteristic in this type of experiment, which can be explained by the higher coordination of nitrogen and hence the reduced Na⁺ permeability within these insulators.     

The inhibition mechanism of pitting corrosion of pure aluminum by nitrate and sulfate ions in neutral chloride solution

The inhibition mechanism of pitting corrosion of natural oxide film-covered pure aluminum by NO⁻ ₃ and SO²⁻ ₄ ions has been examined in 0.1 M NaCl solution using potentiodynamic polarization experiments, a.c. impedance spectroscopy, Auger electron spectroscopy and a combination of the potentiostatic current transient method and optical microscopy. It was found that NO⁻ ₃ ions can be incorporated into the natural oxide film on pure aluminum at open-circuit potential, but the incorporation of SO²⁻ ₄ ions into the film hardly occurs. The incorporation of NO⁻ ₃ ions lowered the pitting susceptibility of pure aluminum in 0.1 M NaCl solution. Based upon the experimental results, it is suggested that the pitting corrosion inhibition mechanism by anions can be classified into two different groups: inhibition by competitive adsorption of anions (SO²⁻ ₄) with Cl⁻ ions and inhibition by the incorporation of anions (NO⁻ ₃) into the film rather than competitive adsorption. Both cases may impede the incorporation of Cl⁻ ions into the film, thus inhibiting pitting corrosion of natural oxide film-covered pure aluminum in chloride solutions. Received: 16 October 1998 / Accepted: 6 January 1999