Jg Hoogland

The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer II

Abstract In order to elucidate the mechanism determining the mass transfer at a gas evolving electrode, the thickness σ of the Nernst diffusion layer was determined as a function of the current density for both hydrogen and oxygen evolving on horizontal and vertical electrodes in acid as well as in alkaline solutions. Also the diameter of the bubbles and the diameter of the bubble street were determined. It was found that both σ and the slope h of the log σ/log i curve strongly depend on the nature of the gas evolved and on the nature of the electrolyte and that the position of the electrode has only a small effect on σ and on h . At i 2 the diameter of free bubbles is practically independent of the current density. At i > 30 mA/cm 2 coalescence of bubbles occurs very often with exception of the hydrogen bubbles in alkaline solution. From the experimental results it was concluded that the mass transfer at a gas evolving electrode is determined by the flow of solution along the electrode surface, this flow being caused by detaching and rising bubbles and by coalescence of bubbles.

The iodine/iodide redox couple at a platinum electrode☆

The I/iodide redox couple was studied on Pt in 0.5M H2SO4 by measuring the impedance as a function of frequency. From these measurements, the exchange c.d. j0 was derived according to Sluyters. The dependence of j0 on the reversible potential and the I and the iodide concns. was established. By comparing the theoretical slopes with the exptl. slopes, it is shown that the electrode process consists of the 3 reactions: I- -> Iad + e; Iad + I- -> I2 + e; I2 + I- -> I3-. The 2nd reaction det. the exchange c.d. The transfer coeff. is 0.5, the standard exchange c.d. is 400 ma./cm.2, and the energy of activation is 15 kcal./mole. [on SciFinder (R)]

The electrolysis of an acidic NaCl solution with a graphite anode : III. mechanism of chlorine evolution

Abstract The mechanism of chlorine evolution on a graphite anode in acid chloride solution has been investigated. Chloride ions are discharged according to the Volmer reaction Cl − → Cl ad + e. Molecular chlorine is formed by discharge of chloride ions on chlorine atoms according to the Heyrovsky reaction Cl − + Cl ad → Cl 2 + e. For an aged electrode the Heyrovsky reaction is the rate-determining step, whereas for a new electrode both the Volmer and the Heyrovsky reaction determine the relation between potential and cd. For chlorine evolution on an aged electrode in a solution of 4 M NaCl and 1 M HCl, the activation energy of the Heyrovsky reaction and also of the chlorine evolution at the reversible potential at 1 atm chlorine is 8·4 ± 0·3 Kcal/mole molecular chlorine; the transfer coefficient of the Heyrovsky reaction is 0·5; the exchange cd of the Heyrovsky reaction at 25°C is 0·009 ± 0·002 mA/cm 2 real surface area; the exchange cd of the Volmer reaction at 25°C is 0·18 ± 0·04 mA/cm 2 real surface area; and the degree of coverage with atomic chlorine is 0·05. For the chlorine evolution on a new electrode, the ratio between the exchange cd of the Volmer reaction and that of the Heyrovsky reaction lies between 1 and 10; and the total exchange cd at 25°C is 0.12 mA/cm 2 real surface area.

The mechanism of the anodic formation of the peroxodisulphate ion on platinum—I. establishment of the participating anion☆

Abstract From the correlation of the current efficiency for peroxidisulphate (persulphate) formation in solutions of sulphuric acid, of K 2 SO 4 and (NH 4 ) 2 SO 4 with the actual SO 4 2− concentration, the conclusion is drawn that exclusively SO 4 2− ions are involved in the persulphate formation. The discharge of HSO 4 − ions in concentrated sulphuric acid solutions leads to the formation of peroxomonosulphuric acid (Caros acid).

Electrolysis of acidic NaCl solution with a graphite anode—I. The graphite electrode

Abstract A graphite anode evolving chlorine from a chloride solution is slowly oxidized to CO and CO 2 . This oxidation causes a change in the characteristics of the electrode—an ageing, comprising a change of the nature of the graphite surface and an increase of the surface area. It appears that a new graphite electrode is covered with a stable oxide that protects it against attack. During continued anodic polarization this stable oxide disappears at a potential of 1·72 V, probably with formation of CO and/or CO 2 . The roughness of the surface then increases, attaining a maximum value of about 17 times that of a new graphite electrode, which has a roughness factor of 30. The electrochemically active surface areas of a new and of a 2000-h aged graphite electrode are 2 and 33 times the geometrical surface area respectively.

The mechanism of the anodic formation of the peroxodisulphate ion on platinum—III. elaboration of experimental results

Abstract From results obtained previously, we conclude that at sufficiently high potentials a platinum anode placed in an SO 4 2− -ion containing solution becomes partly covered by sulphate species the oxide-covered platinum becoming confined to certain patches. On the sulphate-covered parts persulphate formation ( via the discharge of SO 4 2− ions) takes place almost exclusively, while oxygen is evolved on the oxide-covered patches. Peroxomonosulphate is formed at the boundaries of the patches according to the reaction SO 4 − + OH → HSO 5 − .

The electrolysis of an acidic NaCl solution with a graphite anode-II. atomic chlorine present in a graphite electrode

Abstract A graphite electrode at which chlorine is evolved takes up chlorine, this being the cause of a gradually decreasing chlorine evolution after the current has been switched off. From the fact the by immersion in alkali oxygen is evolved it must be concluded that this chlorine is present in atomic form. Determination of the amount taken up as a function of electrode dimension, temperature, cd and the time it has been in use as anode, leads to the conclusion that the atomic chlorine probably is adsorbed on the surface of the crystallites inside the graphite. Its diffusion coefficient is D = 2.4 X 10 t-3 exp (-4500/ RT ) cm 2 /s.

The mechanism of the anodic formation of peroxodisulphate ion on platinum—II. Time dependence of the anode potential

Abstract At constant cd the potential vs log (time) graphs consist of sections with different slopes; so do the steady-state potential vs SO 4 2− concentration graphs. At the same total cd, the abrupt changes in slope occur at practically the same potentials. The change of potential with time is thus interpreted as being caused by the adsorption of sulphate species, which then become inactive in persulphate formation. This process can be described by first-order kinetics in terms of the active sulphate ions.

Electrolysis of an acidic NaCl solution with a graphite anode : IV. Chlorine evolution at a graphite electrode after switching off current

Abstract During electrolysis of an acid chloride solution, atomic chlorine is taken up by a graphite anode. After switching off the current the evolution of molecular chlorine continues. This phenomenon we call residual gas evolution (r.g.e.) It is established that the molecular chlorine is formed according to the Volmer-Heyrovsky mechanism, The rate of r.g.e. is determined by an equilibrium between the diffusion of atomic chlorine out of the graphite and the formation of molecular chlorine. During the first seconds it is the latter that mainly determines the rate of r.g.e.; thereafter the diffusion becomes more and more the rate-determining step.

Mechanism of bromine evolution at a graphite electrode

Abstract The mechanism of the electrochemical bromine evolution at a graphite electrode is elucidated. Bromine is formed according to the Volmer-Heyrovsky mechanism, the Heyrovsky reaction being the rate-determining step, For a solution containing 4 M NaBr, 0.1 M Br 2 and 1 M H 2 SO 4 , the activation energy at the reversible potential is 9 ± 2 Kcal/mole; the geometric exchange cd is 0.2 mA/cm 2 at 25°C; the real exchange cds at 25°C of the Volmer and of the Heyrovsky reaction are 2.0 mA/cm 2 and 0.1 mA/cm 2 respectively; the degree of coverage of graphite by atomic bromine at the reversible potential is 0.17; and the transfer coefficient of the Heyrovsky reaction is 0.5.