Three-phase electrochemistry with a hanging drop of water-insoluble liquid: Precipitation of decamethylferrocenium species as a marker of ion transfer route

A recently developed new experimental setup for the three-phase voltammetry was used to detect the follow-up reaction products. Decamethylferrocene was dissolved in a nitrobenzene drop while inorganic salts with various anions were present in the aqueous phase. The third phase was represented by a Pt microcylindrical electrode. The drop was punctured by this electrode to various depths. The reported experiments show how the contribution of migration to the anion transport in the organic phase increases while the puncture depth grows. The use of the new three-phase electrode allowed easy removal of the microcylinder from the cell and optical inspection of its surface. It appeared that the oxidation of decamethylferrocene may be followed by the precipitation of its salts with the corresponding anions. The precipitate was identified by voltammetry and Raman spectroscopy. The length of the wire covered by the precipitate is relatively big in the case of perchlorate and very small (practically the deposit is located at the three-phase boundary) for the nitrate anion, which confirms that ClO4− easily enters nitrobenzene while NO3− does not. In the presence of chloride the situation was entirely different: the precipitation appeared on the part of microelectrode present in the aqueous phase. This proves that the electrooxidation of decamethylferrocene in the nitrobenzene drop leads rather to the transfer of the decamethylferrocenium cation to the aqueous phase and not to penetration of nitrobenzene by the chloride anion.