Quantitative Evaluation of the Diffusion Coefficients of Aqua Ions in Hydrogels via Iron Corrosion

Abstract

A method to quantitatively evaluate the diffusion coefficient of aqua ions, viz. protons and hydroxide ions, in hydrogels using iron corrosion was developed. In this paper, the locations of the anode and cathode formation, which dominate iron corrosion, were experimentally controlled using pseudosacrificial corrosion protection with conductive anticorrosion ink. First, hydrogen (atomic) generation during the process was confirmed by a chromic gel with iron-doped tungstic acids. Then, the diffusion of aqua ions generated from them was visualized using a BTB-agar hydrogel with BTB (bromothymol blue), and the diffusion coefficient of aqua ions was evaluated using Fick’s law by reading the time variation of luminance. The temperature dependence of the diffusion coefficient was also examined, and the activation energies were evaluated as 19.0 ± 5.9 and 25.9 ± 3.3 kJ/mol for hydroxide ions and proton, respectively. Furthermore, the experimentally evaluated diffusion coefficients of aqua ions were used in a computer simulation to solve the diffusion equation to predict the pH distribution. The diffusion behaviors of aqua ions by the experiment are almost consistent with the simulation results and demonstrate the feasibility of the present method.