(63c) Simulation of Rotating Ring Disk Electrode : Effects of Ionic Migration in Presence of Homogeneous Oxidation of Fe2+ in 0.01(M) H2so4 Under O2 Pressure-Disk Results

Presence of excess of supporting electrolyte in an electrolytic solution results into suppression of electric field on account of high conductivity of the solution and as a result migration mode of transport becomes insignificant. However, there are many situations where “excess of supporting electrolyte is not present or the supporting electrolyte concentration is appreciably low [1]”. “In those situations, migration plays an important role for transport of species from the bulk solution to the electrode and hence, current at the electrode changes [1]”. Guha [1] showed how a supporting electrolyte determines the relevance of ionic migration for a Rotating Ring Disk Electrode (RRDE) system. In this work, effects of ionic migration on limiting disk current, concentration boundary layer thickness for the limiting reactant and other important electrochemical parameters for the disk region of a Rotating Ring Disk Electrode (RRDE) system in presence of homogeneous oxidation reaction of Fe²⁺ ions in 0.01(M) sulphuric acid [H₂SO₄] under oxygen pressure have been investigated. Results for the gap region and the ring region of the RRDE system under investigation will be given in future papers. The system chosen for simulation is 0.01(M) solution of ferric sulphate [Fe₂(SO4)₃] with very little (0.01M) sulphuric acid [H₂SO₄] as supporting electrolyte. Oxygen pressure and temperature considered are 150 kPa and 298K respectively. RRDE rotation rate considered for simulation is 1000 rpm. Two cases have been considered for simulation namely “presence of ionic migration along with homogeneous oxidation reaction of Fe²⁺ ions in 0.01(M) sulphuric acid [H₂SO₄] under oxygen pressure” and “absence of ionic migration along with homogeneous oxidation reaction of Fe²⁺ ions in 10 (M) sulphuric acid [H₂SO₄] under oxygen pressure”. The differential equations for the above-mentioned cases have been solved by explicit finite difference numerical technique using a high-speed computer.

Results for disk region indicate that concentration boundary layer thickness of limiting reactant (Fe³⁺) reduces by approximately 16% when migration mode of transport is present as compared to concentration boundary layer thickness of the limiting reactant (Fe³⁺) in absence of migration.

Simulation results show that in presence of migration, steady state limiting disk current is 0.4364 mA which is 34.84% of steady state limiting disk current in absence of migration.

Results also indicate higher concentration of Fe²⁺ ions at steady state in presence of migration as compared to concentration of Fe²⁺ ions in absence of migration throughout the concentration boundary layer of Fe²⁺ ions during electrolysis. This impact of migration leads to higher rate of oxidation of Fe²⁺ ions to Fe³⁺ ions at all rate constant values as compared to rate of oxidation of Fe²⁺ ions to Fe³⁺ ions in absence of migration.

Rate of oxidation of Fe²⁺ ions to Fe³⁺ ions under oxygen pressure follows first order kinetic equation as the concentration of Fe²⁺ ions in this work is 0.02 (M) or 1.1169g/l which is much lesser than 3g/l (at and beyond 3g/l concentration of Fe²⁺ ions, rate of Fe²⁺ ions oxidation is described by second order kinetic equation [2]).Limiting current condition has been assumed in this work.

Literature cited

(1) Guha S. Simulation studies on a rotating ring disk electrode system: role of supporting electrolyte in determination of relevance of ionic migration. AIChE Journal.2013;59(4):1390-1399
(2) Tomasz C, Witold AC. The oxidation of Fe (II) in aqueous sulphuric acid under oxygen pressure. Hydrometallurgy.1984;12:21-30.