Development of a separation/determination method based on the quantitative controlled potential electrolysis at the aqueous/organic solution interface and its application to coulometry of ions

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The coulometric determination had been applied only to species which are easily reduced or oxidized before the present work. The purpose of the present work was to expand the field of coulometry to ionic species which are not easily reduced or oxidized by referring to the recently developed electrochemical method, voltammetry for ion transfer at the interface between aqueous (W) and organic (O) solutions.(1) Theoretical considerationThe rapid electrolysis was considered to be essential for the precise and accurate coulometric electrolysis, since the inaccuracy due to the background current could be reduced when the electrolysis was rapid. Here, the rapid electrolysis is performed when the ratio of the area (A) of the W/O interface to the volume (V) of the sample solution (W and/or O) is large and the thickness (δ) of the diffusion layer at the interface is thin.(2) Development of electrolytic cellsIt was found that the ratio of A/V was extremely large and δ was considerably small when … More the W/O interface was stirred directly, and an "electrolytic cell with interfacial stirring (ECIS)" was developed. Another cell developed was a "electrolytic cell of flow type (ECFT)". The ETFC was composed of a porous Teflon【○R】 tube of 1 mm in inner diameter, 2 mm in outer diameter and 50 cm in length. A silver wire of 0.8 mm in outer diameter and 60 cm in length which was coated with silver chloride was inserted into Teflon^【○R】 tube. The silver wire worked as a working/reference electrode. The counter electrode was a silver wire wound around the tube. The electrolysis was carried out by dipping the tube with silver electrodes in an organic solution (O) and forcing an aqueous sample solution (W) to flow into the gap between the tube and the silver working/reference electrode. A tetraphenylborate ion selective electrode set in O near to the tube worked as a reference electrode in O. The V/A ratio is large with ECFT and δ 6 is thin since W flows in ECFT.(3) Evaluation of capabilities of ECIS and ECFTThe capabilities were evaluated under various conditions by changing objective ions, organic solvents, supporting electrolytes, dimensions of cells, etc, and it was found that the coulometric transfer of not only hydrophobic ions but also hydrophilic ions from W to 0 could be attained when ionophors were added in O. When the concentration of an objective ion in W was 10^<-3> M, the quantitative electrolysis (the transfer of more than 99 % of the ion from W to O) could be attained with a precision of less than 1 % within 20 min by ECIS and within 1 min by ECFT. This result indicates that the field of coulometry has been expanded to the ion transfer at the W/O interface. Less