Determination of concentration of saturated ferrocene in aqueous solution

Chun Ouyang, Koichi Jeremiah Aoki, Jingyuan Chen, Toyohiko Nishiumi, Bo Wang

Department of Applied Physics, University of Fukui, Bunkyo, Fukui, Japan

Abstract: The solubility of ferrocene in aqueous solution is known to be approximately 0.04 mmol/dm³. The solubility values determined by voltammetry have been overestimated because of adsorption on electrodes. This work deals with discerning diffusion from adsorption by altering not only the voltammetric time scale but also the solvents used. Fast voltammetric responses by differential pulse voltammetry and fast scan voltammetry exhibited adsorption behavior. In contrast, quasi steady-state voltammetry showed the diffusion-control, the current of which seemed to evaluate the saturated concentration accurately. However, the currents in the solution including a small amount of organic solvent were smaller than those in the aqueous solution although the concentrations were identical. Solutions including organic solvents have often been used to obtain calibration curves. Therefore, the concentration evaluated from the calibration curve was estimated to be larger than the true concentration. The current in the organic solvent was explained in terms of the extra solvation energy by supersaturation, which was dissipated to low concentrated domains by diffusion. It was formulated in the form of diffusion coefficients. The true concentration was evaluated to be 0.01 mmol/dm³ by slow scan voltammetry in the solution without calibration curves.

Keywords: adsorption, ferrocene, aqueous solution, voltammetry, supersaturation