Single-ion activity: a nonthermodynamically measurable quantity
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FEATURE ARTICLE
Single-ion activity: a nonthermodynamically measurable quantity Takashi Kakiuchi1 Received: 6 July 2020 / Revised: 6 July 2020 / Accepted: 7 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Ionic liquid salt bridge (ILSB) that effectively minimizes the liquid junction potential in a galvanic cell articulates the nonthermodynamic measurability of single-ion activity. Points for further improvement of the stability of ILSB are summarized, and associated intriguing phenomena at the ILSB-W interface are described. Keywords Liquid junction potential · Salt bridge · Ionic liquid salt bridge · Galvanic cell · Instability · Spontaneous emulsification
Nonthermodynamic nature of galvanic cell and single-ion activity measurements A galvanic cell is nonthermodynamic; the separation of a redox reaction into two half-cell reactions necessarily introduces a contact, visible or invisible, of two ionic conductors of different chemical compositions. The presence of the contact, usually called liquid junction, though one or both of the two electrolyte phases may not necessarily be in the liquid state, entails the nonthermodynamic nature of a galvanic cell. When the thermodynamic basis of the cell voltage is emphasized, the liquid junction potential at the liquid junction is often set aside as an undesirable factor. However, the uniqueness of galvanic cell, and hence, of electrochemistry, is ascribed to this separation of the redox reaction. Once separated, the electrode with a halfcell reaction allows us to measure the single-ion activity, nonthermodynamically. Recent discussions between Robert de Levie and the present author on this measurability issue down to the level of epistemology as well as the very nature of science timely illustrate the contemporary significance of this measurability issue [3, 4, 13, 14]. There are numerous cases where the single-ion activity plays a key role in determining the physicochemical processes, e.g., a variety of chemical equilibria, such as colligative properties, extraction of metal ions, physicochemical Takashi Kakiuchi
[email protected] 1
pH Science and Technology Laboratory, Kinomoto 1058, Wakayama 640-8453, Japan
processes in environments, and biological energy transductions. For better understanding and controlling these thermodynamic processes, the knowledge of single-ion activity is indispensable [1, 2, 30]. Theoretical and computational studies of electrolyte solutions need reliable experimental data of single-ion activity in higher ionic strength regions [5, 7–9, 21–23]. A practically feasible way to answer such callings is to employ the methods based on a galvanic cell with a salt bridge, as exemplified in the case of pH measurements, in that electrochemistry is the only experimental, practically feasible, way to “measure” the single-ion activity. It is a common practice to measure pH defined in terms of the activity of hydrogen ion, aH+ , using a glass electrode in combination with a salt bridge made of a concen
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