Charles W. Tobias

The solubility and diffusion coefficient of oxygen in potassium hydroxide solutions

The solubility of oxygen in aqueous KOH solutions has been measured by a Van Slyke apparatus and by an adsorption technique developed by Hildebrand. In the range of concentration of KOH between 0 and 12 N, at 25°C, the two methods yielded identical results; at 760 torr oxygen partial pressure, log S = log 1·26 × 10−3 − 0.1746 C, where 0·1746 is the solubility coefficient, S the concentration of oxygen, g-mol/l, and C the concentration of KOH, g-mol/l. Between 0° and 60°C both the solubility and the solubility coefficient decrease with increasing temperature. Diffusion coefficients of oxygen in aqueous KOH were evaluated from the limiting current of oxygen on a rotating disk electrode, and also by a stagnant tube technique similar to that used by von Stackelberg. The diffusivity drops sharply with increasing KOH concentration, and increases with temperature. At 25°C and for KOH concentrations between 2 and 4 N, the product of the diffusivity and the viscosity is constant: Dμ = 1·3 × 10−7 g.cm/s2, where D is the diffusivity, cm2/s, and μ is the viscosity in poise. At 60°C and for 1 < N KOH < 8, the value of this product is Dμ = 1·9 × 10−7 g.cm/s2.

Theoretical Analysis of Current Distribution in Porous Electrodes

General equations describing the behavior of porous electrodes are developed. These equations are used to determine the initial and the steady‐state conditions in one‐dimensional porous electrodes of uniform geometry and polarization parameters. In particular, it is shown that the current and reaction distributions in the depth of the electrode are strongly influenced by the type of activation polarization and by mass transport of the reacting ionic species, in addition to the effective conductivities of the two phases. It is found that a linear approximation to a Tafel curve leads to an inadequate description of actual behavior when the reaction is distributed nonuniformly in the depth of the electrode.

Mass-Transfer Measurements by the Limiting-Current Technique

Publisher Summary Limiting-current density refers to the maximum rate at 100% current efficiency, at which a particular electrode reaction can proceed in the steady state. This rate is determined by the composition and transport properties of the electrolytic solution and by the hydrodynamic condition at the electrode surface. It has been observed that in many cases mass transfer is not the sole cause of unsteady-state limiting currents, observed when a fast current ramp is imposed on an elongated electrode. Major conditions for valid measurement and interpretation of limiting currents include reaction characterized by slow surface kinetics, progress of the electrode position reaction, use of low concentrations of the reacting ionic species in free convection studies. This chapter outlines the underlying principles, discusses the conditions of validity, and highlights some selected applications and basic features of the limiting-current phenomenon. The chapter also illustrates a synopsis of electrochemical mass-transfer theory, to the extent required for analysis of limiting-current conditions. Various complicating factors—migration effects, the choice of appropriate diffusivity values, unsteady-state conditions, current distribution below and at the limiting current, and the effect of formation of rough metallic deposits—in the interpretation of limiting-current measurements are considered.

Resistance to Potential Flow through a Cubical Array of Spheres

Precise conductivity measurements on models sectioned out from a cubic lattice of spheres in a continuous medium indicate that the effective conductance of such a system deviates from the values predicted by Lord Rayleighs analytic solution of this potential distribution problem. Deviations become particularly significant when the spheres approach close packing, and when the conductance of spheres is much greater than that of the continuum. By use of a different function for potential, and by consideration of higher terms in the series expression for the potential in the continuous phase, Rayleighs results are modified, yielding an analytical expression that represents effective conductance satisfactorily in the concentration region approaching close packing.

On the Conductivity of Dispersions

Experiments on suspensions of glass beads in electrolytes indicate that Bruggemanns approximation represents the dependence of effective conductance on volume fraction very satisfactorily when the dispersed phase contains a broad range of particle sizes. Data on narrow size ranges fall in between values predicted by the Maxwell and Bruggemann equations. These findings are consistent with the physical assumptions implicit in both theoretical developments.

Mass transfer by free convection at horizontal electrodes

Limiting currents were measured in an unstirred cell at horizontal cathodes facing upward. Electrolyte composition ranged from 0·01 to 0·7 M CuSO4 in 1·5 M H2SO4. Cathode sizes varied from 0.1–30 by 10 cm, and the free height above the electrode from 1–16 cm. Limiting currents for deposition of copper ranged from 0·68–129 mA/cm2. For electrode width larger than 20 mm the data is well represented by the general correlation Nu′ = 0.19 (Sc. Gr)13, where Nu′, Sc, and Gr are the Nusselt number for mass transfer, the Schmidt, and Grashof numbers, respectively. The experimental range used in the correlation included 108 < (Sc. Gr) < 1·4 × 1012 The results indicate that the boundary layer is turbulent, as it is in the case of the analogous heat-transfer model.

The Anodic Evolution of Ozone

Trends of the anodic ozone evolution reaction are characterized for several different anode material and electrolyte combinations. The effect of electrolyte temperature and concentration, current density, and additions of small quantities of the fluoride ion are explored. The general behavior of ozone current efficiencies are rationalized in terms of coverage of the anode surface by adsorbed anionic material and the electronegativity of the electrolyte anions. At 0°C ozone current efficiencies of over 50% are reported for hexafluorophosphoric acid electrolyte which represent substantially higher yields than any previously achieved.

Conductivities in Emulsions

The dependence of the conductivity of water‐propylenecarbonate emulsions on volume fraction of the dispersed phase, on the conductivities of the continuous phase , and of the discontinuous phase , has been measured in the ranges of and . The results indicate that when neither Maxwells equation nor Bruggemanns approximation represents the behavior of data satisfactorily. In an attempt to take into account the interaction of fields around particles of the dispersed phase, a simple approximation is proposed, based on Maxwells equation. In the range of conductivities investigated in the present study the new equation represents the conductivity of emulsions with reasonable accuracy.

Selected Physical Properties of Ternary Electrolytes Employed in Ionic Mass Transfer Studies

Densities and viscosities of CuSO4-H2SO4, AgC104-HC104, and Ks[Fe(CN)6]:K4[Fe(CN)~]-NaOH solutions in water have been measured over a range of compositions. Experimental values of the integral diffusion coefficients are also given for silver perchlorate in aqueous perchloric acid, and for K3[Fe(CN)6] and K4[Fe(CN)6] in aqueous NaOH.

Chemical Surface Modification of Porous Silicon

Resistance to room temperature oxidation and control over wetting properties can be achieved by chemical modification of a porous‐silicon surface. Fourier transform infrared spectroscopy was used in the transmission mode to monitor the surface chemistry of both treated and untreated porous‐silicon samples before and after exposure to humid air at room temperature. Surface modification methods investigated include: (i) vapor‐phase silation using either hexamethyl‐disilazane or trimethylchlorosilane, and (ii) rapid thermal annealing in nitrogen, ammonia, or argon ambients. The silation treatments, carried out in the presence of trace moisture, were successful both in creating surface trimethylsilyl groups and in suppressing room temperature oxidation. Rapid thermal annealing at temperatures as low as 500°C for 30 s eliminates all silicon hydrides. Nitrided porous‐silicon layers are formed at 1100°C in either ammonia or nitrogen; in both cases the silicon nitride infrared absorption peaks scale with the porous layer thickness, indicating that the compounds are distributed throughout the porous layer.