Standard Exchange Current Density Research Articles

Highly selective electrochemical sensor for new generation targeted-anticancer drug Ibrutinib using newly synthesized nanomaterial GO-NH-B(OH)2@AgNPs modified glassy carbon electrode

Ibrutinib (IBR) is a small molecule new generation smart anti-cancer drug that is Bruton's tyrosine kinase (BTK) inhibitor. In this study, the sensitive and selective electrochemical sensor based on a modification of a glassy carbon electrode with a new GO-NH-B(OH)2 (graphene oxide boramidic acid) nanoparticles and Ag nanoparticles (AgNPs) is proposed for IBR determination. GO-NH-B(OH)2@AgNPs/GCE was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The performance of the electrode was evaluated with the parameters of electrode surface area (A), heterogeneous rate constant (ket), standard exchange current density (I0). The electrochemical determination of IBR in 0.1 M HNO3 solution was investigated at the surface of GO-NH-B(OH)2@AgNPs/GCE. Under optimized square wave voltammetric (SWV) conditions, peaks current at GO-NH-B(OH)2@AgNPs/GCE showed good linearity with the concentration of IBR in the operating range of 0.025–1.00 μg mL−1 (5.7 × 10−8 – 2.3 × 10−6 M) with a detection limit (LOD) of 0.006 μg mL−1 (1.36 × 10−8 M). Besides its high stability and reproducibility, the response of IBR at GO-NH-B(OH)2@AgNPs/GCE was not influenced by the presence of dopamine, uric acid, and ascorbic acid. The sensor has successfully caught IBR in pharmaceutical and human urine samples.

Evidence of a Magnetic Effect on the Electron Transfer of the Hydrogen Evolution Reaction (HER)

In 1972, Trasatti carefully compiled the rates of hydrogen evolution reaction (HER) on 31 metal electrodes near pH 0 [1]. The HER is a surface reaction that can be represented as Hsoln⁺ ⇌ Hads⁺ Hads⁺ + e ⇌ Hads ∙ 2 Hads ∙ ⇌ H2,ads H2,ads ⇌ H2,soln Trasatti also defined the electrochemical work function Φ, which is the energy needed to remove an electron from the electrode surface to a redox species immediately at the electrode surface. The rates of the HER reaction are embedded in the standard exchange current density j0 (A/cm2) that measures the standard heterogeneous rate of electron transfer, k0. Trasatti demonstrated that plot of log j0 versus Φ yields two parallel lines as shown, where regression yields log j₀ = (6.4₄ ± 0.2₄) Φ (eV) - (35.₄±1.₁) with R² = 0.98 for the higher rate line and log j₀ = (6.5 ± 0.5₆) Φ (eV) - (38.₅ ± 2.₄) with R² = 0.93 for the lower rate line. The slopes are the same but the intercepts differ by 3. For a given Φ, metals on the upper line have HER rates that are 1000 x higher than metals on the lower line. In this presentation, the magnetic properties of the electrodes are shown to describe the binary segregation of the data into two discrete lines. The magnetic properties of the electrodes are set by paired and unpaired electron spins. The results are evidence of a magnetic effect on electron transfer.

Redefining the Electrochemical Kinetics of Redox Reactions on Passive Metals

The kinetics of redox reactions are described by specifying a standard exchange current density and a transfer coefficient that locates the position along the reaction coordinate at which the activated complex (transition state) occurs. However, this formulation was originally developed to describe charge transfer reactions on bare metal surfaces. In the case of a passive metal, however, the barrier layer of the passive film represents a barrier to the quantum mechanical tunneling of charge carriers (electrons and electron holes) between the metal and the redox reaction center and theory shows that this barrier has a profound effect upon the tunneling probability and hence upon the values of the exchange current density and the Tafel constant. In this presentation, the role of quantum-mechanical tunneling of charge carriers through the barrier oxide layer on a passive metal, in determining the electrochemical kinetic parameters is explored theoretically. The exchange current density for a redox reaction occurring on the passive metal surface is assumed to be the product of the exchange current density on the bare metal surface and the quantum mechanical tunneling probability (QMTP) of charge carriers through the barrier layer, which is inversely proportional to the exponential of the barrier thickness and the tunneling constant that in turn is proportional to the square root of the product of barrier height and the effective electron mass. The QMTP is used to redefine the exchange current density for the cathodic reaction (e.g., reduction of oxygen), as measured on the passive surface, to that on the (hypothetical) bare metal alone. Thus, knowing the voltage, the barrier layer thickness can be calculated from the Point Defect Model (PDM), which in turn can be used to estimate the QMTP for that particular set of conditions. Then, the exchange current density for the redox reaction on the passive metal surface is calculated and used, if the barrier height and the effective electron mass are constants, in the generalized Butler-Volmer equation to describe the current/voltage characteristics of the redox reaction. In doing so, it becomes evident that the kinetics of a redox reaction are fully determined by specifying a (hypothetical) exchange current density and transfer coefficient of the reaction on the bare metal (“hypothetical” because the bare metal condition may not be realizable experimentally), the QMTP, the electric field strength within the barrier layer, the polarizability of the barrier layer/solution interface, and the potential of zero thickness of the barrier layer. The application of this theory is illustrated with respect to the hydrogen electrode reaction (HER) on passive carbon steel and on aluminum and in predicting the corrosion potentials of stainless steels and nickel alloys in the primary coolant circuits of water-cooled nuclear reactors.

Application of a One-Dimensional Transient Electrorefiner Model to Predict Partitioning of Plutonium from Curium in a Pyrochemical Spent Fuel Treatment Process

It has previously been proposed by safeguards experts that curium will track plutonium through a spent fuel pyroprocessing facility, enabling nondestructive assaying of plutonium via counting neutron emissions from 244Cm. This is a critical assumption for the neutron balance approach to safeguards. If Cm and Pu were to behave chemically the same, counting neutrons could be used to estimate Pu concentrations. In this study, plutonium tracking with curium has been investigated using Enhanced REFIN with Anodic Dissolution (ERAD), a one-dimensional transient electrorefiner model based on fundamental electrochemical equations. The model was used to simulate simultaneous deposition of uranium, plutonium, and curium onto a solid metal cathode. Chemical/physical properties used by the model were either obtained from the literature or assumed. The standard exchange current density of curium was estimated by analyzing published cyclic voltammetry data for LiCl-KCl-CmCl3. The focus of the ERAD calculations was on verifying that Pu and Cm could codeposit onto the cathode along with U and to determine if the Pu/Cm ratio would be the same between the salt pool and cathode deposit. It was determined that Cm largely resists cathode deposition, while Pu can be driven to codeposit at sufficiently high current densities. The expected concentration of Cm in the salt would not support any deposition of Cm onto the cathode. It would need to be raised to ~1 wt% before small gram quantities of Cm will deposit onto the cathode. Even then, the Pu/Cm ratio of the cathode was found to be three orders of magnitude higher than the ratio in the salt. It is, thus, concluded that the neutron balance approach would be ineffective at safeguarding a nuclear fuel pyroprocessing facility.

Electrochemical Kinetics of CuCl(aq)/HCl(aq) Electrolyzer for Hydrogen Production via a Cu-Cl Thermochemical Cycle

An electrochemical kinetics investigation of the CuCl(aq)/HCl(aq) electrolyzer identified methods to significantly reduce the platinum loadings required to achieve a high cell current density of 0.5 A/cm2 at 0.7 V. As the CuCl(aq)/HCl(aq) electrolyzer is a key component of the Cu-Cl thermochemical cycle, the economic viability of the Cu-Cl thermochemical cycle was significantly improved by reducing the loading required to achieve 0.5 A/cm2. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) with a rotating disc electrode were employed to investigate the kinetics of the aqueous CuII/CuI chloride complexes reaction on platinum and glassy carbon using a three-electrode cell. It was found that the standard exchange current density of the anodic CuII/CuI electrochemical reaction on platinum, 4–12 A/cm2, was significantly larger than the values reported for the HER cathodic reaction thus far. In addition, SEM was used to observe the effectiveness of different catalyst application techniques. Through SEM observations, and electrochemical data analysis, the amount of platinum used in a laboratory scale CuCl(aq)/HCl(aq) electrolyzer was reduced from 0.8 mg/cm2 applied to both electrodes to 0.4 mg/cm2 on the cathode and zero at the anode while still maintaining a current density of 0.5 A/cm2 at 0.7 V of applied potential difference.

On the shape of stress corrosion cracks in sensitized Type 304 SS in Boiling Water Reactor primary coolant piping at 288 °C

Evolution of the shape of surface cracks in sensitized Type 304 SS in Boiling Water Reactor primary coolant circuit piping at the reactor operating temperature of 288°C is explored as a function of various environmental variables, such as electrochemical potential (ECP), solution conductivity, flow velocity, and multiplier for the oxygen reduction reaction (ORR) standard exchange current density (SECD), using the coupled environment fracture model (CEFM). For this work, the CEFM was upgraded by incorporating Shoji’s model for calculating the crack tip strain rate and more advanced expressions were used for estimating the stress intensity factor for semi-elliptical surface cracks. This revised CEFM accurately predicts the dependence of the crack growth rate on stress intensity factor and offers an alternative explanation for the development of semi-elliptical cracks than that provided by fracture mechanics alone. The evolution of surface crack semi-elliptical shape depends strongly upon various environmental variables identified above, and the CEFM predicts that the minor axis of the ellipse should be oriented perpendicular to the surface, in agreement with observation. The development of the observed semi-elliptical cracks with the minor axis perpendicular to the surface is therefore attributed to the dependence of the crack growth rate on the electrochemical crack length.

Charge and Mass Transfer on a La2O3 Added Li/Na or Li/K Molten Carbonate Meniscus Electrode of LaNiO3 Coated Au Ring for Oxygen Reduction Reaction

Introduction The LaNiO3 is an alternative cathode material of NiO for molten carbonate fuel cells (MCFCs) because of extremely higher stability than NiO in molten carbonates.1 In our previous study, the activity of the LaNiO3 was not less than that of NiO for the oxygen reduction reaction (ORR) on a meniscus electrode.2However, the activity should separate charge transfer and mass transfer enough.In this study, the charge and mass transfer of the ORR on LaNiO3 and NiO have been evaluated by the potential step method (PSM) and the steady method (SM) on a meniscus electrode to discuss the ORR activity and mechanism of the LaNiO3for MCFC application. Theoretical background The apparent exchange current density: i 0, and the cathodic symmetry factor: a c were determined by mass transfer free polarization curves with the Allen-Hickling plot of following the Butler-Volmer equation. The dependence of i 0on gas composition is as follows i 0 = i 0 0 p O2 ap CO2 b (1)where i 0 0, a, and b were the standard exchange current density and the apparent reaction orders for O2 and CO2, respectively.The limiting current density: i lc, was determined with the polarization curves with mass transfer resistance and the i 0from the Butler-Volmer equation as follows. h = {1/(Rf i 0)+1/i lc}iRT/(nF) (2)where Rf and i were roughness factor and current density per geometric area. The dependence of i lcon gas composition is as follows i lc = i lc 0 p O2 a’p CO2 b’ (3)where i lc 0, a’, b’ were the standard limiting current density, and the apparent limiting current order for O2 and CO2, respectively. Experimental Li/Na (=52/48 mol%) or Li/K (=62/38 mol%) molten carbonate saturated La2O3 (1.5mol%) were used as an electrolyte. The working electrode was the LaNiO3or NiO coated Au ring whose height was controlled by a micrometer. Reference electrode was a reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode.The LaNiO3 powder was prepared by the thermal decomposition of the mixture of lanthanum nitrate hexahydrate and nickel acetate tetrahydrate at 1073 K for 12 h in air.3 The LaNiO3 powder was sintered on a gold ring at 1273 K for 48 h while NiO (wako, 99.9%) was sintered at 1073 K for 12h. Specific surface area of the electrode was evaluated with the BET specific surface area of the LaNiO3or NiO powder after sintering at each condition.The ORR was determined in ambient pressure with oxygen partial pressure (p O2) range from 0.02 to 0.9 atm, and carbon dioxide pressure (p CO2) range from 0.001 to 0.9 atm in an argon-balance. The temperature was at 923 K. The chronoamperometry (CA) from 10 to -10 mV vs. ROE was performed for 600s. The meniscus height was h=2mm of the PSM for mass transfer free polarization. The initial current density was evaluated by extrapolation to t=0 with the linear relation between current and the square root of time. The meniscus height was h=4mm of the SM with mass transfer resistance. The current density was evaluated by the average value for 500~600 s of CA. Results and discussion The i 0 and as a function of the p O2 or p CO2 on LaNiO3 or NiO electrode in Li/Na or Li/K molten carbonate were shown in Fig. 1. The i 0 on LaNiO3 electrode was not less than that on NiO electrode in both electrolytes. The a, and b were 0.39 ± 0.08 and -0.18 ± 0.04 for both the electrolytes and the electrodes, respectively. Since the percabonate path mechanism (PCP) whose a and b, were 0.5 and 0 while those of the peroxide path mechanism (POP) were 0.5 and -1.0, the reaction on LaNiO3for the ORR might be mixed mechanism of POP and PCP. The i lc and as a function of the p O2 or p CO2 on LaNiO3 or NiO electrode in Li/Na or Li/K molten carbonate were shown in Fig. 2. The i lc in Li/K molten carbonate was higher than that in Li/Na molten carbonate. O2 solubility in Li/K molten carbonate was twice as much as that in Li/Na molten carbonate.4 Therefore, the i lcwhich is flux that passed through the meniscus for Li/K would be larger than that for Li/Na. Reference 1. K. Matsuzawa, Y. Akinaga, S. Mitsushima, and K. Ota, J. Power Sources, 196, 5007 (2011)2. K. Matsuzawa, Y. Esaki, Y. Takeuchi, K. Watanabe, Y. Kohno, K. Ota and S. Mitsushima, Proc. 4th Asian Conf. Molten Salt Chem. Tech., p.285 (2012).3. T. Noda, K. Komaki, and T. Kawasaki, Panasonic Tech. J., 55, 108 (2009) [in Japanese].4. T. Nishina, Y. Masuda and I. Uchida, Molten Salt Chem., 93, 424 (1993).

Numerical simulation and experiment in the systems of labile complexes of metals. Silver reduction from the thiourea electrolyte

Voltammograms of silver reduction from complex thiourea electrolytes with the ligand to metal concentration ratio varied from 1 to 50 were measured on the freshly prepared surface of silver electrode. The results were analyzed using the numerical model of this electrode process which takes into account homogeneous chemical reactions proceeding in solution, diffusion–migration transfer of the species present in solution, and electrode reactions participated by electroactive components of the electrolyte. We estimated the values of standard exchange current density for partial electrode reactions in which silver complexes with different number of ligands can take part. The highest electrochemical activity is exhibited in solutions with the low concentration of complex-forming agent – by silver aqua complexes, while in solutions with high thiourea concentration – by silver complexes with one ligand. It was found that kinetic hindrances to discharge of the thiourea silver complexes unlikely to be due to the adsorption of thiourea molecules blocking the growth sites or to the adsorption of complex silver ions. The results admit interpretation of the mechanism of electrode process either from the viewpoint of simultaneous discharge of all the complex silver ions present or from the viewpoint of discharge of silver complex with one ligand in combination with rapid chemical reactions of dissociation of other, non-discharging silver complexes.

Kinetic Monte Carlo simulation of Pt discontinuous thin film formation adsorbed on Au

The evolution of a non-uniform Pt film adsorbed on Au, in contact with a solution containing [PtCl 6] 2− ions was studied by means of lattice gas/kinetic Monte Carlo simulations with pair potentials at 300 K in the canonical ensemble. We found that the evolution of the morphology takes place through two types of exchange: surface diffusion and diffusion mediated by the solution, which occur with different probabilities or rates. These two types of moves were included in the simulation. By means of this dynamic description, it was possible to estimate the exchange rate of Pt atoms on a Au flat surface. A linear dependence was found between the exchange rate and the reciprocal of the number of particles. This function was used to extrapolate the exchange rate to a system of macroscopic size. The standard exchange current density calculated from this value was 5.24 × 10 −6 A cm −2. The morphological evolution of the film as a function of the time was also studied.

Hardness of metals from electron transfer reactions at electrode surfaces

The standard exchange current densities pertaining to electron transfer processes at electrodes are employed to estimate chemical hardness of various metals. This is accomplished by deriving a new parametric relation for hardness in terms of the work function and surface potential of electrons. Hydrogen evolution and ferric/ferrous redox reactions are considered as examples to extract chemical hardness from electrode kinetic data. The surface potential is calculated for a large number of metals using phenomenological thermodynamic considerations. The significance of the methodology is also illustrated by calculating the potential of zero charge of metal/solution interfaces, thus demonstrating that equilibrium as well as kinetic studies in electrochemistry are capable of yielding the hardness of metals.

Hydrogen Evolution Reaction on Electrodes: Influence of Work Function, Dipolar Adsorption, and Desolvation Energies

The dependence of the standard exchange current density of the hydrogen evolution reaction on electrode surfaces has been investigated using the work function of metals, free energy of bond formation with adsorbed hydrogen atoms, surface potential of the reactant, and adsorption characteristics of solvent dipoles. The explicit influence of the parameters is derived using a postulated transition state and a satisfactory agreement with experimental data is noticed for a large number of sp and d metals.

Electron transfer reactions at metal electrodes: Influence of work function on free energy of activation and exchange current density

The dependence of the free energy of activation on the work function of electrodes, solvation energies, and surface potentials of the reactant species pertaining to electron transfer reactions at metal/solution interfaces is derived using thermodynamic considerations. The standard exchange current density is calculated for Fe3++e↔Fe2+ at different metal electrodes and compared with experimental data as well as molecular dynamics simulations.

Kinetic and mechanistic study of hydroxyl ion electrosorption at the Pt(111) surface in alkaline media

Abstract The adsorption of OH− ions on the Pt(111) plane has been studied by fast cyclic voltammetry in sodium hydroxide solutions (0.03 to 1 M), under quasi-equilibrium and Tafel approximation conditions. It was shown that the OH− ion adsorption is an electrosorption process with one electron exchanged between an OH− ion and the platinum surface. The electrosorption process follows the Frumkin adsorption isotherm with low intensity repulsive interactions of the adsorbed species (f=2–3). The estimated values of the standard electrochemical rate constant (k°=5.6×10−4 cm s−1) and the standard exchange current density (joo=5.45×10−2 A cm−2) indicate a rather fast electrochemical process.

Application of the redox potential for controling a sulfide oxidizing bioreactor

The investigations described show that the formation of elemental sulfur from the biological oxidation of sulfide can be optimized by controling the redox state of the solution. The nonsoluble sulfur can be removed by gravity sedimentation and re-used as a raw material, i.e., in bioleaching processes. It was shown that, by supplying an almost stoichiometrical amount of oxygen to the recirculated gas phase, the formation of sulfate is minimized. The redox potential is mainly determined by the sulfide concentration because this compound has a high standard exchange current density with the platinum electrode surface. By maintaining a particular redox setpoint value, in fact, the reactor becomes a "sulfide-stat." It was shown that in a sulfide-oxidizing bioreactor the measured redox potential, using a polished redox electrode, is kinetically determined rather than thermodynamically. The optimal redox value for sulfur formation is between -147 and -137 mV (H2 reference electrode, 30 degrees C, pH 8). The presented results are currently used for controling several full-scale installations, which desulfurize biogas and high-pressure natural gas. Copyright 1998 John Wiley & Sons, Inc.

Electrode kinetics of oxygen reduction on gold in molten carbonate

The kinetics of oxygen reduction on gold in pure Li 2CO 3 and Li + K carbonate eutectic melts were studied using various techniques. The steady-state current in the potentiostatic mode was very agitation-dependent, indicating that the mass transfer effect was significant even at low polarization. The stability limits of the carbonate melt and the reaction sequence of oxygen reduction were examined by the potential scan method. The kinetic parameters were determined using the potential step method. The exchange current density was found to be two orders of magnitude higher than the value suggested by potential scan measurements. The standard exchange current density in the eutectic is 11.0 mA/cm 2 at 650°C; that in pure Li 2CO 3 at 750°C is 26.3 mA/cm 2. The exchange reaction orders of O 2 and CO 2, which are similar in the two melts, are quite different from the theoretical values according to the peroxide or superoxide mechanisms. The slow neutralization reaction of oxide by CO 2, which has a strong effect on the electrode potential, is the main reason for this discrepancy.

Charge transfer reactions of oligo-decker sandwich compounds

Electron transfer reactions of oligo-decker sandwich compounds with different transition metal centers (Fe, Ni, Co) and various π-ligands ((C 3B 2)R 5, R = H, CH 3, C 2H 5) have been investigated at platinum and gold electrodes using cyclic voltametry and impedance techniques. The standard exchange current densities of the reactions investigated lie in the range 0.5–30 A cm −2 and are independent of the nature of the electrode metal; within the experimental accuracy the transfer coefficients are 0.5 for all systems. For one particular system the temperature dependence of the reaction rate was measured in two different solvents, dichloromethane and acetonitrile. Both the activation energy and the frequency factor indicate a significant participation of inner sphere modes.

The nickel electrode in liquid ammonia

The formal standard potential of nickel in 1 M ammonium nitrate solution in liquid ammonia is E o' (Ni 2+/Ni) = 6 (±12) mV vs. a standard hydrogen electrode. The absorption spectra of nickel(II) in liquid ammonia are very similar to those of the complex Ni(NH 3) 2+ 6 in aqueous solution. The charge-transfer rates of dissolution and deposition of nickel in solutions of ammonium chloride, ammonium nitrate or lithium perchlorate were independent of the electrolyte composition. The anodic and cathodic transfer coefficients α + = 0.65 and α − = 0.36, respectively, assuming a charge number z = 2, were independent of the temperature. The standard exchange current density in 1 M ammonium nitrate base electrolyte at 273 K was 63 μA/cm 2. The mechanism is characterized by the transfer of Ni 2+ in one step. The corrosion rates of active nickel in several electrolyte solutions were determined. Unlike iron, nickel does not passivate in liquid ammonia, but salt layers prevent unlimited growth of the anodic current density.

Apparent ion-exchange current densities at valinomycin-based potassium ion-selective PVC membranes obtained with an AC-impedance method

The complex ac-impedance spectra of thin (< 70 μm) valinomycin-based potassium-selective PVC membranes symmetrically bathed in KCl, NaCl and LiCl solutions of different concentrations, have been measured with high resolution from 0.01 Hz to 10 kHz. With the exception of those KCl solutions > 10 −4 M, all impedance spectra obtained showed a more or less pronounced and stable second semicircle at medium frequencies, which could be identified as the double-layer capacitance in parallel to a charge-transfer resistance of the corresponding ion equilibrating across the interface PVC membrane/aqueous solution. Using the formalism originally derived for redox reactions at metallic electrodes, apparent standard ion-exchange current densities and consistent charge-transfer coefficients could be obtained with high reproducibility. The apparent standard exchange current density of K + at a valinomycin-based PVC membrane with tetraphenylborate as an additive, lies in the range of about 25 mA cm −2 which is the highest value reported so far for these membranes. A strong correlation was found between the potentiometricall y observed ion selectivity expressed by the selectivity coefficient used in an extended Nernst equation and the apparent exchange current densities of the corresponding ions.

Kinetic criteria for the mechanism of the hydrogen evolution reaction

Additional criteria for distinguishing between the Volmer—Heyrovský and the Volmer—Tafel reaction mechanisms for the hydrogen evolution reaction (HER) have been derived from a theoretical treatment of polarization curves under conditions of continuous surface renewal. From data published by Tomashov and Vershinina for the HER on Pd, Ni, Fe, Sn and Pb, estimates of hydrogen coverages and rate constants have been made. Approximate values of standard exchange current densities could thus be predicted for the two reaction mechanisms; these have been compared with experimental data. In order to reconcile the various kinetic criteria with each other, it has been proposed that the Volmer—Heyrovský reaction mechanisms is operative on the majority of metals. Alternative interpretations within the Volmer—Heyrovský mechanism have been presented for Tafel slopes in the region of −30 mV dec −1 and for the relative positions of volcano curves, which might otherwise have been considered indicative of the Volmer—Tafel reaction mechanism.

Rate constants, activation free enthalpies and activation entropies of the electrochemical reduction of 1,4-diazines in DMF

The standard exchange current densities of nine 1,4-diazines at gold electrodes were measured in DMF in the temperature range of +25 up to −59°C. From the resulting free enthalphy of activation of pyrazine as a function of temperature an activation entropy of −1.5 k is calculated. It is compared with the activation entropy of −1.1 k following from the theory of Marcus. The relative change of the free enthalpy of activation with temperature is only a property of the temperature dependences of refractive index and dielectric constant of the solvent. It is used to obtain the free enthalpies of activation for the standard temperature of 298 K. The experimental values for the compounds are compared with theoretical values, calculated from the inner and outer reorganization energies λ i and λ 0. For λ i the bond lengths and force constants were obtained from Hückel calculations, for λ o different approximations according to Marcus and Peover were used. The calculated and the measured values coincide within less than 10%. The relation between the free solvation enthalpy and the outer reorganization energy is discussed.