Measurement of Changing Ferrous Iron Concentration Using Redox Potential

Abstract

A method of determining the oxygenation rates of Fe(II) to Fe(III) was developed in relation to the mechanism of phosphate release from sediments because it is related to the redox reaction of Fe(II)-Fe(III).
The method uses observed redox potential (Eh_obs) to determine Fe(II) concentration in high pH region, because it is too low to be determined by spectrophotometry.
The redox potential is based on the redox reaction Fe³⁺+e^-_??_Fe²⁺ and expressed by Eh=Eh⁰-0.059 log([Fe²⁺]/[Fe³⁺])(25°C). On condition that produced ferric hydroxide is in solution equilibrium, the change of Fe²⁺ concentration with time was expressed as log([Fe²⁺]_t=t/[Fe²⁺] _t=0)={(Eh_obs)_t=0-(Eh_obs)_t=t}/0.059(25°C). Here, the solution equilibrium of Fe³⁺ and OH^- was found to be in an apparent equilibrium [Fe³⁺][OH^-]^2.68=10^-35.8.
The redox potential in the test solutions was found to be also affected with the redox equilibrium between NO₂^- and NO₃^-, formed from the absorbed NO₂ gas. Therefore, it is necessary to use the initial part of the observed redox potential changes to determine the oxygenation rates of Fe(II).