Lars-Olof Öhman

Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution : 3. Potentiometric Study of Aluminium(III) Hydrolysis and Aluminium(III) Hydroxo Carbonates in 0.6 M Na(Cl)

Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution : 3. Potentiometric Study of Aluminium(III) Hydrolysis and Aluminium(III) Hydroxo Carbonates in 0.6 M Na(Cl)

Coordination of acetate to Al(III) in aqueous solution and at the water-aluminum hydroxide interface: a potentiometric and attenuated total reflectance FTIR study

Attenuated total reflectance FTIR spectroscopy was used to characterize Al(III)-acetate complexation at 25 degrees C, in homogeneous aqueous solution and at the water-aluminum hydroxide interface. The data collected in aqueous solution, at [Al](tot) = 0.080 M, [OAc](tot) = 0.040 M, and pH < 4.3, indicate the presence of only one dominating complex which, by spectral analysis, is shown not to involve monodentate acetate coordination but rather a syn-syn bridging geometry. In quantitative terms, the data strongly support the appearance of a binuclear mixed-hydroxo species [Al-2(OH)(2)OAc](3+) which, in view of the above, probably consists of a dihydroxo-bridged Al-2(mu-OH)(2)-unit to which the acetate ion bridges via the apices of the two Al(III) octahedra. Also at the water-aluminum hydroxide interface, only one dominating surface complex is indicated. Zn contrast to the aqueous species this complex is a weak mononuclear outer-sphere complex, and it is speculated that, due to a lack of structural flexibility of the surface Al(III) octahedra, the formation of a bridging inner-sphere complex is prohibited. Based on the concept of surface complexation, and utilizing the extended constant capacitance model to account for contributions from electrostatic forces, an equilibrium model, which quantitatively describes these interactions in an ionic medium of 0.1 M NaCl, is presented. Copyright (C) 1998 Elsevier Science Ltd. (Less)

Polysilicate equilibria in concentrated sodium silicate solutions

Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and 29Si n.m.r. methods. Experimental data cover the ranges: 11.36 ⩽ pH ⩽ 14.09, 2.4 ⩽[Na+]tot/mol dm–3⩽ 9.6, 2.1 ⩽[Si]tot/mol dm–3⩽ 9.3 with a variation of SiO2/Na2O between 3.3 and 1.0. From available pH and n.m.r. data an equilibrium model is derived describing polymerization and protonation of silicate species in these water-glass solutions. With SiO2/Na2O = 1.0, i.e. in the most alkaline solutions (pH ≳ 13), dimeric as well as cyclic forms of tri- and tetra-meric silicate species predominate. At higher SiO2/Na2O, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6–8 dominating. With SiO2/Na2O ≳ 3.3, i.e. close to the precipitation boundary of amorphous silica, cage-like polysilicate species dominate.The average charge per Si atom for the different polysilicate species varied between –2 in the most alkaline solutions down to –0.5 close to the precipitation boundary. The buffering capacity of the different water-glass solutions was found to decrease with increasing SiO2/Na2O.The change in the different formation constants (log βpq) due to variation in total Si and SiO2/Na2O, was found to be linear with respect to the Na+ concentration, i.e. log βpq= log β°pq+k[Na+].Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO2/Na2O and total Si concentrations are derived using the computer program SOLGASWATER.

Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. Part 13. A potentiometric and 27Al nuclear magnetic resonance study of speciation and equilibria in the aluminium(III)-oxalic acid-hydroxide system

Equilibria between Al3+, oxalic acid (H2L), and OH– were studied in 0.6 mol dm–3 NaCl medium at 25 °C. Potentiometric titrations (glass electrode) and 27Al n.m.r. measurements were performed to study speciation and equilibria within the concentration ranges 0.2 ⩽–log [H+]⩽ 7.2, 0.0005 ⩽b⩽ 0.02 mol dm–3, 0.0005 ⩽c⩽ 0.025 mol dm–3, and 0.5 ⩽c/b⩽ 16 [b and c are the total concentrations of aluminium(III) and oxalic acid respectively]. Besides a series of [AILn]3 – 2n complexes with n= 1,2, and 3, the formation of the polynuclear mixed hydroxo-complexes Al3(OH)3L3 and [Al2(OH)2L4]4 – were established. N.m.r. data also indicated the formation of an [AlHL]2+ complex in strongly acidic solutions: pKa([AlHL]2+)≈ 0.0. The significance of the different Al complexes to conditions prevailing in natural waters is discussed, including a model calculation of the solubility of a clay mineral (kaolinite) in the presence of oxalate. Data were analysed using the least-squares computer program LETAGROPVRID.

Equilibrium approaches to natural water systems—6. Acid-base properties of a concentrated bog-water and its complexation reactions with aluminium(III)

Abstract A filtered bog-water, concentrated by means of a freezing technique, has been studied with respect to acid-base properties and aluminium(III) complexation reactions. Sampling was performed during autumn and winter periods with a resulting acidity or alkalinity due to oxic (autumn) or anoxic (winter) conditions. The measurements were performed as potentiometric titrations in constant ionic media (0.02, 0.1 and 0.6 M NaCl) with the use of a glass electrode. The samples show buffer ranges at 3 ⪅ pH ⪅ 5 and pH ⪆ 7.5 . The first is ascribed to the presence of carboxylate groups and is characterized by fast equilibria. The second is due to phenolic OH− groups and precipitation reactions with resulting sluggish equilibria. For one sampling period comprehensive measurements were undertaken to study the possible polyelectrolytic character of the organic acids. Due to the small increase in apparent carboxylate pKa-values with the degree of dissociation at low (0.02 M) as well as at high (0.6 M) ionic strength, the possible polyelectrolytic feature of the acids was neglected. Instead, a good fit to data was obtained by introducing a di-protic acid (H2L) as a model compound. Furthermore the medium dependence of the two acidity constants could satisfactorily be fitted to the expression: log k = log K + aI 1 2 /( 1 + I 1 2 ) + bI , where K is the constant at infinite dilution, a and b parameters of which b has been adjusted to present data. The following K values were obtained: pK1 = 3.65 and pK2 = 4.30. The complexation with Al(III) could be described by the formation of AlL+, AlL2− and the ternary species AlLH−1. The stability constants (log k1 = 4.4 (winter), 4.2 (autumn); log k2 = 4.2, 4.7; pKa (AlL+) = 4.2, 4.2) show no significant trend with sampling period but indicate a stability of the complexes greater than for phtalic acid but lower than for oxalic acid. Finally, the theoretical solubility of the clay mineral kaolinite in the presence of bog-water was computer modelled. The calculations show up on a 10-fold increase with respect to soluble aluminium at pH = 5.

Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. 12. A potentiometric and 29Si-NMR study of silicon tropolonates

Complexation in the H+-Si(OH)4-tropolone (HL) system was studied in 0.6 M (Na)Cl medium at 25° C. Speciation and formation constants were determined from potentiometric (glass electrode) and 29Si-NMR data. Experimental data cover the ranges 1.5 ⩽ - log[H+] ⩽ 8.4, 0.002 ⩽ B ⩽ 0.012 M, and 0 ⩽ C ⩽ 0.060 M (B and C stand for the total concentration of Si and tropolone, respectively). In acid solutions (-log[H+] ⩽ 3) a hexacoordinated cationic complex, SiL3+, is formed with log K(Si(OH)4 + 3HL + H+ XXX SiL3+ + 4H2O) = 7.08 ± 0.03. Furthermore, the formation of a disilicic acid was established from 29Si-NMR data. The dimerization constant of Si(OH)4 was found to be 10 exp (1.2 ± 0.1). In model calculations the solubility of quartz and amorphous SiO2 in the presence of tropolone is demonstrated. Data were analyzed using the least-squares computer program LETAGROPVRID.

Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution—10 : A potentiometric study of aluminium(III) pyrocatecholates and aluminium(III) hydroxo pyrocatecholates in 0.6 M Na(Cl)

Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution : 3. Potentiometric Study of Aluminium(III) Hydrolysis and Aluminium(III) Hydroxo Carbonates in 0.6 M Na(Cl)

Spectroscopic studies of aluminum and gallium complexes with oxalate and malonate in aqueous solution

The local structures of Ga(III) in aqueous oxalate and malonate complexes were studied by means of Ga K-edge EXAFS spectroscopy. Irrespective of the number and type of coordinated ligands, the EXAFS results showed very regular first coordination shells consisting of six oxygen atoms. Scattering paths from more distant atoms revealed that both oxalate and malonate form mononuclear chelate structures where one oxygen from each carboxylate group binds to Ga(III). Again, very little variation in bond distances and no changes in coordination modes were detected as the number of ligands coordinated to Ga(III) was varied. Based on the very close resemblance of IR spectra of oxalate and malonate complexes of Al(III), and the corresponding complexes of Ga(III), it is believed that the local structures of the Al(III) complexes are similar to those of the Ga(III) complexes in terms of ligand coordination modes and distortions. This conclusion was corroborated by results from theoretical frequency calculations.

Catalyst preparation through ion-exchange of zeolite Cu-, Ni-, Pd-, CuNi- and CuPd-ZSM-5

Ion-exchanged zeolite ZSM-5 is the best known catalyst for direct NOx decomposition and a viable candidate for NOx reduction with methane. The preparation is crucial for the efficiency of the conve ...

Equilibrium and structural studies of silicon(IV) and aluminum(III) in aqueous solution. 31. Aqueous complexation between silicic acid and the catecholamines dopamine and L-DOPA

Three component equilibria between H+, silicic acid, and the catecholamines dopamine and L-DOPA were studied by means of potentiometric (glass electrode) measurements in 0.6 M Na(Cl) medium at 25°C. Experimental data cover the ranges 2≤ −log[H+]≤9.5, 0.002≤0.005 M; 0.005 ≤ C ≤ 0.016 M, and 1 ≤ C/B ≤ 6 (B and C stand for the total concentration of Si and catecholamine, respectively). In acidic and near-neutral solutions no interactions were registered while, in slightly alkaline solutions, both systems are characterized by a formation of mononuclear tris-complexes SiL3 containing pyrocatechol-type bonds. The species forming at −log[H+] ≈ 7.5 have fully protonated amine side-chains which, at somewhat higher −log[H+] values, start to deprotonate. Simultaneously, a decomposition of the octahedrally coordinated complex is occurring and the hydrolytic species SiO(OH)3− becomes increasingly important. With dopamine, the equilibrium constants characterizing the system are log K (Si(OH)4 + 3H3L+ α Si(HL)3+ + 2H+ + 4H2O) − 9.70 ± 0.03 and log K (Si(OH)4 + 3H3L+ α SiL(HL)2 + 3H+ + 4H2O) − 19.33 ± 0.03. The corresponding equilibrium reactions with L-DOPA are log K (Si(OH)4 + 3H3L α Si(HL)32− + 2H+ + 4H2O) − 10.08 ± 0.05 and log K (Si(OH)4 + 3H3L α SiL(HL)23− + 3H+ + 4H2O) −19.35 ± 0.07. Data were analyzed using the least-squares computer program LETAGROPVRID. The octahedral oxygen coordination around the silicon atom in these complexes has been verified by using the 29Si NMR method. Via model calculations it is shown that a minimum of ∼ 0.004 M catecholamine is needed to double the aqueous solubility of quartz.