Piotr K. Wrona

EMPIRICAL PARAMETERS OF LEWIS ACIDITY AND BASICITY FOR AQUEOUS BINARY SOLVENT MIXTURES

Abstract Empirical parameters of Lewis acidity, E N T , introduced by Reichardt et al., and Lewis basicity, B KT , introduced by Kamlet and Taft, have been determined for mixtures of water with ten organic solvents. In the case of water/alcohol mixtures a distinct dependence between these acidity and basicity parameters have been found. For the other solvent mixtures the E N T on B KT dependence is more complex even if these parameters are purified from non-specific solute/solvent interactions.

Capacitance of the gold electrode in 0.5 M H2SO4 solution: a.c. impedance studies

Abstract Impedance spectroscopy was used to measure the properties of the gold electrode in 0.5 M H 2 SO 4 solution in the double-layer region (−0.25 to 1.05 V vs. sodium chloride saturated calomel electrode (SSCE)) and the region of oxide monolayer formation (1.05–1.4 V vs. SSCE). The dependence of the capacitance on potential was analysed. In both regions equivalent circuits composed of a resistor corresponding to the resistivity of the solution and a capacitor were proposed. It was found that in the double-layer region the capacitance reflected the properties of the pure gold electrode, whereas in the oxide region it depended on the coverage of the surface, corresponding partially to pure gold, MOH dipoles, oxide or a reconstructed gold surface. The improvements made by introducing a constant phase element instead of the capacitor and adding a parallel resistor in the oxide region were analysed. The roughness of the gold electrode was also estimated. The results obtained by impedance measurements were complemented by voltammetric studies to clarify the process, to confirm the measured values and to explain the meanings of the calculated parameters.

Oxidation of Nitrites on Solid Electrodes I. Determination of the Reaction Mechanism on the Pure Electrode Surface

The electrochemical behavior of nitrite ions on gold, platinum, and glassy carbon (GC) rotating electrodes was investigated. It was observed that the oxide layer formed on the platinum and gold electrode surfaces hinders the nitrite oxidation process. The use of different solid electrodes allowed the determination of the conditions under which an elucidation of the reaction mechanism is possible. Those are the conditions when the oxide layer does not exist or its effect is negligible. The gold (for pH < 6) and the GC electrode (GCE) (for pH < 12) appeared to he suitable for this purpose. Our investigations confirmed the reaction mechanism in which the overall oxidation of NO - 2 to NO 3 consists of the charge-transfer step NO - 2 → NO 2 + e, followed by the homogeneous disproportionation of NO 2 : 2NO 2 + H 2 O → NO 2 + NO 3 + 2H + . The value of the rate constant of the homogeneous disproportionation reaction, k, was established to be (0.11 ± 0.01) × 10 7 mol 1 dm 3 s -1 . Calculations were based on the experiments performed for different nitrite concentrations and different rotation speeds, over a wide range of pH. The results obtained for the gold and the GCE were almost the same. The obtained values of the formal potential E 0 (NO 2 /NO 2 ) = 0.78 ± 0.006 V, are in agreement with those reported in the literature.

An empirical relationship between the eluant strength parameter ε° and solvent lewis acidity and basicity

Abstract It has been found that Snyders eluant strength parameter e°, used in liquid-solid adsorption chromatography, can be well described by a complementary Lewis acid-base model for solute-solvent interactions: e°=0.152+0.859 · E N T + 0.332 · B KT ( cf Eq.9 ), with a correlation coefficient of r = 0.949 for 28 solvents. This result is in agreement with the theoretical interpretation of e° The correct statistical procedure for estimating the significance of added explanatory parameters in the sequential multiparameter regression analysis is shown ( cf Appendix). A comprehensive list of improved Kamlet-Taft solvent basicity parameters, B KT , is given.

High Affinity of Thallium Ions to Copper Hexacyanoferrate Films

High affinity of thallium(I) ions with respect to a copper hexacyanoferrate (CuHCF) film was found. Interaction between the CuHCF film and thallium(I) ions was investigated using the electrochemical quartz crystal microbalance and cyclic voltammetric techniques. In 0.5 M KNO 3 solution, at submillimolar concentrations of Tl(I) ions, the CuHCF film reversibly transforms, during electrochemical experiments, from the potassium into the thallium form. For slightly higher (several millimoles) Tl(I) concentrations, the electrochemical and gravimetric responses prove that the CuHCF film behaves as the thallium form only. Its formal potential (E f 0 ), calculated from the dependence of the E f 0 on [Tl(I)], is for [Tl(I)] = 1 M, 0.28 V more positive than that found for the potassium form. Experimental results obtained suggest that in both cases, potassium(I) and thallium(I) ions, exchange in interstitial positions takes place. Since the ionic radii and the hydration parameters of both ions are similar, we concluded that this high affinity of thallium(I) ions with respect to the CuHCF film, 250-100 times higher than that of potassium(I) ion, results from chemical interactions. In consequence, the solubility product of a thallium analogue of copper hexacyanoferrate is much smaller than that of the potassium form. For different forms they are equal to (pK values): 37.8 (K 2 Cu 3 [Fe(CN) 6 ] 2 ), 46.3 (Tl 2 Cu 3 [Fe(CN) 6 ] 2 ), and 17.2 (Cu 3 [Fe(CN) 6 ] 2 ).

Catalytic Influence of Commercial Ru, Rh, Pt, and Pd (∼0,1 atomic percent) Intercalated in Graphite on the Hydrogen Evolution Reaction

Graphite electrodes intercalated with various metals (Ni, Co, Pd, Pt, Rh, and Ru) have been bonded with an inorganic polymer and used as cathodes in the hydrogen evolution reactions (HER) in . Four of the most active electrodes showed very good mechanical and electrochemical stability. The overvoltage of the HER at decreased in the following order: −525 mV, for pure graphite and −254, −137, −103, and −58 mV, for the Pd/C, Rh/C, Pt/C, and Ru/C electrodes, respectively. The kinetics of the HER for two electrodes (Ru/C and Pd/C) were measured with the use of an ac impedance technique. In both cases, the HER proceeds via the Volmer‐Heyrovsky mechanism. The results obtained proved that the Pd/C electrode had a larger active surface area than the Ru/C one did.

An extension of the Kamlet–Taft basicity scale of solvents

A critical analysis is given of currently popular scales of solvent Lewis basicity (Koppel–Palm, Paju–Koppel, and Gutmann) with particular consideration to and extension of the Kamlet–Taft scale, BKT. Measurements for 22 new solvents were carried out and BKT values for 70 solvents are reported. A rough agreement between BKT and other parameters for solvent basicity has been found. A linear dependence of the Koppel–Palm E values with ET was shown for solvents of higher polarity (ET > 42).

Oxidation of Nitrite on Solid Electrodes II. Determination of the Reaction Mechanism on Surfaces Covered by an Oxide Layer

The nitrite oxidation process was reinvestigated at rotating gold and platinum electrodes covered by an oxide layer. The electrochemical properties of the surface oxides were determined by recording the coverage curves of the gold and platinum electrodes at various pH values and for different experiment durations. The reaction mechanism on the oxidized surface was found to be the same as that on the oxide-free surface. The sole effect of the surface oxide was a drop of the standard rate constant of the electrode reaction which was a function of the surface coverage by the oxide. Nitrite oxidation on the oxide-covered electrodes was found to proceed via the innersphere mechanism with inhibitor adsorption on an energetically uniform surface. The kinetic results obtained for the gold and the platinum electrode were almost the same. The complicated shape of rotating disk voltammograms was explained in terms of the increase of the rate of the electrode reaction with increasing electrode potential and the drop of the standard rate constant of the electrode reaction with increasing electrode potential due to the increase of the oxide coverage with electrode potential, the latter process being dependent on the electrode material, pH, and time (scan rate).

On the correlations between empirical lewis acid-base solvent parameters and the thermodynamic parameters of ion solvation

Abstract Assuming a straight-line dependence of the chemical potential of anions (cations) on empirical Lewis acid (base) parameters of solvents, the values of the change of the chemical potential of ions and the change of the surface potential between water and non-aqueous solvents have been calculated and briefly discussed. On the basis of the results obtained and literature data the dependence of the standard emf of the cell Pt,H2 |HCl|AgCl|Ag|Pt′ on the function of empirical Lewis acid-base parameters has been analysed and discussed.

Electrode processes of chloramines in aqueous solutions

Abstract Electroreduction of chloramines (H2NCl, HNCl2 and NCl3) at the platinum electrode was investigated in aqueous solutions of varying pH. The two former compounds are reduced irreversibly and their cathodic peak potentials are pH dependent (Epc (H2NCl)=−0.25 to −0.15 V (pH 7–4), Epc (HNCl2)=0.35–0.45 V (pH 4.5–2.5), at scan rate ν=0.05 V s−1). Trichloramine is reduced at the same potential at which Cl2 is formed, and the whole system is reversible. From the dependence of cathodic peak potentials on pH the protonation equilibrium constants of H2NCl and HNCl2 were determined. They were found to be (pK) 6.4±0.1 and 3.41±0.05, respectively. The extinction coefficients of protonated species are lower than that of non-protonated ones. The formal potential of the electrode reaction: NCl3+4H++6e−=NH4++3Cl− extrapolated to pH 0 equals 1.44 V (pH 0, SHE), thus the Gibbs energy of formation of NCl3, ΔGf=362.1 kJ mol−1.