Carina M. M. Machado

Challenges in modelling and optimisation of stability constants in the study of Cu-(TAPS)(x)-(OH)(y) system by polarography.

This work describes the application of polarography, a technique scarcely used for modelling and optimisation of stability constants, in the study of copper complexes with [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS). Direct current polarography (DCP), using low total copper ion and large total ligand to total copper concentration, enabled the full characterization of Cu-(TAPS)(x)-(OH)(y) system, whose complexation occurs in the pH range of copper hydrolysis and Cu(OH)(2) precipitation. Cu-(TAPS)(x)-(OH)(y) system was studied by DCP and glass electrode potentiometry (GEP) in aqueous solution at fixed total ligand to total metal concentrations ratios and varied pH values (25.0 degrees C; I=0.1M, KNO(3)). The predicted model, as well as the overall stability constants values, are (as logbeta): CuL(+)=4.2, CuL(2)=7.8, CuL(2)(OH)(-)=13.9 and CuL(2)(OH)(2)(2-)=18.94. GEP only allowed confirming the stability constants for CuL(+) and CuL(2) and was used to determine the pK(a) of TAPS, 8.342. Finally, a briefly comparative analysis between TAPS and other structural related buffers was done. Evaluation based on logbeta(CuL) versus pK(a) revealed that TES, TRIS, TAPS and AMPSO coordinated via amino and hydroxymethylgroups forming a five-membered chelate ring. For BIS-TRIS and TAPSO, and possibly DIPSO, one or more five-membered chelate rings involving additional hydroxyl groups are also likely formed.

Modelling of Pb–(TAPS)x–(OH)y system and refinement of stability constants in the region of lead hydrolysis and lead hydroxide precipitation

The influence of [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) on solutions containing lead(II) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP). The readings were taken at fixed total TAPS to total lead(II) concentration ratios and various pH values, at 25.0+/-0.1 degrees C and ionic strength 0.1M KNO(3). Due to the basic pK(a) of the ligand, which occurs in the pH range where large amount of lead polynuclear species are formed, and the occurrence of ligand adsorption, that disabled the use of high concentrations of TAPS on DCP experiments, GEP and DCP experimental conditions were put to the limit in order to provide the correct Pb-TAPS-OH model and reliable stability constants. The proposed final model is: PbL, PbL(2), PbL(2)(OH) and PbL(2)(OH)(2) with overall stability constants values, as logbeta, 3.27+/-0.06, 6.5+/-0.1, 12.7+/-0.1 and 17.27+/-0.06, respectively. A comparative analysis of the strength of complexation of TAPS and a structural related buffer, 2-hydroxy-3-[tris(hydroxymethyl)methylamino]-1-propanesulfonic acid (TAPSO), with lead is also discussed.

Interpretation of non–Nernstian slopes in graphic analysis of data collected in pH range close to deprotonation of a ligand: Part I. A glass electrode potentiometric and polarographic study of Cd–(TAPSO)x–(OH)y and Zn–(TAPSO)x–(OH)y systems

In this work, the complexation of cadmium and zinc ions by 3-[N-tris(hydroxymethyl)methylamine]-2-hydroxypropanesulfonic acid (TAPSO), a commercial biological buffer, was evaluated using three electrochemical techniques, at fixed total-ligand and total-metal concentration ratio and varied pH, at 25.0+/-0.1 degrees C and ionic strength set to 0.1M KNO(3). For both metal-ligand systems, complexation was evidenced in the pH range close to deprotonation of the ligand and the final models were optimised after a meticulous graphical analysis. For Cd-(TAPSO)(x)-(OH)(y) system, two complexes, CdL and CdL(2), were identified in the buffering region of the ligand. The proposed final model for this system is: CdL, CdL(2) and CdL(2)(OH) with stability constants, as logbeta, of 2.2, 4.2 and 8.6, respectively. For Zn-(TAPSO)(x)-(OH)(y) system, the complex ZnL is the main species formed in the buffering pH range. The proposed final model is ZnL, ZnL(OH) and ZnL(OH)(2) with overall refined stability constants (as logbeta) to be: 2.5, 7.2 and 13.2, respectively.

Recycling of aluminum and caustic soda solution from waste effluents generated during the cleaning of the extruder matrixes of the aluminum industry

Anodising industries use a concentrated caustic soda solution to remove aluminum from extruder matrixes. This procedure produces very alkaline effluents containing high amounts of aluminum. The work reported here was focussed on recycling aluminum, as aluminum hydroxide, from these effluents and regenerating an alkaline sodium hydroxide solution. Briefly, the method comprises a dilution step (necessary for reducing the viscosity of the effluent and allowing the subsequent filtration) followed by a filtration to eliminate a substantial amount of the insoluble iron. Then, sulphuric acid was added to neutralize the waste solution down to pH 12 and induce aluminum precipitation. The purity of the aluminum salt was improved after washing the precipitate with deionised water. The characterization of the solid recovered, performed by thermogravimetric analysis, Fourier transform infrared spectroscopy and X-ray diffraction, indicated characteristics typical of bayerite. The proposal method allowed recovering 82% of the aluminum present in the wastewater with high purity (99.5%). Additionally, a sufficiently concentrated caustic soda solution was also recovered, which can be reused in the anodising industries. This procedure can be easily implemented and ensures economy by recycling reagents (concentrated caustic soda solution) and by recovering commercial by-products (aluminum hydroxide), while avoiding environmental pollution.

Electrochemical Determination of Methyltin Compounds

Abstract. The electrochemical behaviour of monomethyltin, dimethyltin and trimethyltin compounds in 20% (V/V) methanol/water solution, 0.05 mol/L in tetraethylammonium perchlorate at pH 2.5, has been investigated by differential pulse polarography and differential pulse anodic stripping voltammetry. In differential pulse polarography, dimethyltin and trimethyltin gave one reversible wave with peak potentials at −0.70 V and −1.07 V, respectively. Detection limits were 6.6 × 10−7 mol/L for dimethyltin and 4.1 ×  10−6 mol/L for trimethyltin. The electrochemistry of monomethyltin was found to be more complex. By differential pulse anodic stripping voltammetry, monomethyltin, dimethyltin and trimethyltin produced distinct stripping peaks (−0.39 V, −0.75 V and −1.14 V, respectively), which allow to determine these compounds at trace levels. Using this new method, detection limits were: 1.2 × 10−7 mol/L for monomethyltin, 1.7 × 10−7 mol/L for dimethyltin and 1.4 × 10−6 mol/ L for trimethyltin. For monomethyltin, a second peak (Ep = −0.60 V), less sensitive (detection limit of 2.5 × 10−6 mol/L), was also observed at concentrations above 4.2 × 10−7 mol/L.Recoveries of methyltin compounds added separately to tap water samples at the 0.42–16.9 µmol per litre level ranged from 84.5 to 99.8% depending upon the methyltin species.