Investigation of ion pairs in electrochemical hexacyanoferrate(II)– hexacyanoferrate(III) system in presence of supporting electrolyte

Abstract

The study of ion pair formation involving redox couples and the cation or anion of supporting electrolytes in solutions is a fundamental question in the field of fluid mechanics and electrochemistry. In this research, density functional theory (DFT) calculations were performed to investigate the ion pair reactions of hexacyanoferrate(III) and hexacyanoferrate(II) with varying numbers of potassium ions from the supporting electrolyte KCl. The binding energy and Gibbs free energy for the ion pair reactions were evaluated. The DFT results revealed a non-monotonic behavior in the binding energy and Gibbs free energy for the ion pair reaction of hexacyanoferrate(III) with different numbers of K ions.Further exploration of the chemical structures of ion pair compounds of hexacyanoferrate(III) and hexacyanoferrate(II) with varying numbers of K ions was conducted using DFT methods. Notably, in the first step of the ion pair reaction with K ions, the iron-potassium chemical bond length in hexacyanoferrate(III) was observed to be smaller than in subsequent steps.The DFT results also provided insights into the capacity of hexacyanoferrate(III) and hexacyanoferrate(II) to accept potassium ions for forming ion pair structures, showing that the former has a lower acceptance capacity than the latter. Furthermore, DFT calculations of equilibrium constants confirmed experimental results based on reduction electrical conductivity data obtained from ion pair formation in the solution.