Harry P. Thun

Influence of changes of platinum electrode surface condition on the kinetics of the oxidation of sodium dithionite and sulfite in alkaline solution.

Abstract The influence of the surface condition of a platinum electrode on the kinetics of the electrochemical oxidation of sodium dithionite and sulfite was investigated. It was found that the oxidation of sulfite does only occur at platinum oxide while the oxidation of dithionite takes place at platinum oxide as well as at platinum hydroxide and bare platinum. However, dithionite cannot be oxidised at rearranged platinum hydroxide, which is formed during the reduction of platinum oxide, a process that is strongly dependent on the applied anodic vertex potential. For both substances a hysteresis effect was observed between forward and backward scan but in both cases the effects could be explained considering the platinum surface condition. As well in the absence as in the present of sulfite and/or dithionite reproducible current-potential curve were obtained after cycling three times between the applied vertex potentials. The observed difference between the first and subsequent scans again could be explained when the variation of the platinum surface condition as a function of potential and number of cycles was taken into consideration.

Solubility of synthetic calcium hydroxyapatites

The solubilities of synthetic calcium hydroxyapatites [Ca(OH)A], Ca5(PO4)3OH, are determined in the ternary system Ca(OH)2+ H3PO4+ H2O at 25°C in the pH range 5–7 by equilibration with dilute H3PO4 solutions. The thermodynamic solubility product Ksp of precipitated Ca(OH)A, prepared by titrating a boiling aqueous suspension of Ca(OH)2 with H3PO4, is (3.25± 0.29)× 10–58 mol9 dm–27. Ca(OH)A prepared by solid state reaction of CaCO3 and CaHPO4 and precipitated Ca(OH)A heated at high temperatures in a water vapour atmosphere showed no constant Ksp. After being washed, these samples equilibrate according to the usual solubility product principles with a Ksp value of (3.44± 0.36)× 10–59 and (2.60± 0.21)× 10–59 mol9 dm–27, respectively.The influence of heat treatment and washing on the solubility behaviour is discussed. Deviations from solubility behaviour which accords with the usual solubility product principles are caused by reactions occurring at the surface of the solid. The latter are induced by a high free energy of the solid and/or the presence of a fraction with high solubility associated with the Ca(OH)A sample.

Electrochemical behaviour of sodium dithionite at a platinum electrode and determination of diffusion coefficient in alkaline solution

The electrochemical behaviour of sodium dithionite at a platinum electrode has been investigated in this paper. Two oxidation waves of sodium dithionite were observed with formation of sulfite as a stable intermediate species. It has been found that the oxidation of sodium dithionite to sulfite, according to the first wave, is controlled by the rate of mass transport, the oxidation of sulfite to sulfate, however, is not purely controlled by transport phenomena. Using the ratio of the limiting currents of both oxidation waves it was possible to determine the diffusion coefficient of sodium dithionite by extrapolation to the small rotation rate of the platinum electrode. From the obtained results it is also advisable to make use of the first oxidation reaction for analytical purposes.

The stability constants of some bivalent metal complexes of α-hydroxyisobutyrate and lactate

Abstract The stability constants of the lactate and α-hydroxyisobutyrate complexes of Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ and UO 2 2+ were determined by potentiometric titration. The average ligand number exceeds 2, indicating the formation of ML + , ML 2 and ML 3 - complexes. The existence of ML 3 - complexes was confirmed by electrophoretic experiments; no polynuclear complexes were formed. α-Hydroxyisobutyrate forms stronger complexes than lactate.

Solubility and Solution Behaviour of Strontiumhydroxyapatite

Crystalline and stoichiometric strontiumhydroxyapatite, Srs(P04)30H, was prepared by solid state reaction. Equilibration of this strontiumphosphate in dilute aqueous H3PO4 at 25 °C in the pH range 5—8 shows that the solid is dissolving stoichiometrically and has an activity solubility product of (4.98 ± 0.31) X 10~52 mol9 Under acidic conditions the dissolving apatite gets coated with a surface layer of /S-SrHPCM which finally determines the solubility behaviour.

The stability constants of the alkaline earth lactate and α-hydroxyisobutyrate complexes

The stability constants of the lactate and α-hydroxyisobutyrate complexes of the alkaline earths were determined by potentiometric titration. The average ligand number exceeds a value of one, indicating the formation of ML+ and ML2 complexes. α-Hydroxyisobutyrate forms stronger complexes than lactate. The stability constants β2 increase as a function of decreasing atomic number for both ligands: Ba< Sr<Ca< Mg, although the constant β1 is greater for Mg than for Ca.

Conductometric study of the ion association of hydrochloric acid in 80 weight per cent 2-methoxyethanol and 20 weight per cent water at temperatures from 20 to 50°C

Abstract The electrical conductance of solutions of hydrochloric acid in a mixture of 80 wt% 2-methoxy-ethanol and 20 wt% water has been studied at temperatures from 20 to 50°C. The results have been interpreted with the Fuoss Onsager theory for associated 1-1 types electrolytes. The values of the association constant K A , the limiting conductivity and the distance parameters have been computed at each temperature.

The effect of water on the conductance mechanism and the solvation of the proton' in 2-methoxyethanol water mixtures at 25°C

Abstract Conductivity measurements at 25°C, on solutions of hydrogen chloride in various mixtures of 2-methoxyethanol and water, are reported. The experimental data were analysed by means of the Fuoss-Onsager equation. The limiting equivalent conductivity and the association constant were determined. This study focusses on the effect of the decreasing amount of water in the solvent mixture, on the ion-solvent interactions and discusses aspects of the solvation and the structure of the electrolytic solutions.