An electrochemical viewpoint on the solubility of silver halides in water
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ORIGINAL PAPER
An electrochemical viewpoint on the solubility of silver halides in water ˇ Samec2 Takashi Kakiuchi1 · Zdenek Received: 7 July 2020 / Revised: 28 July 2020 / Accepted: 28 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract An electrochemical view on the solubility product of sparingly soluble salts in water, which Nernst introduced in 1892, is helpful for better understanding of the precipitation equilibria. Specifically exemplified is the solubility of silver halides in terms of the partitioning of the salt-constituting ions between the ionic crystal and the adjacent solution. Keywords Silver halides · Solubility product · Nernst · Standard ion transfer potential · Solubility · Distribution potential · Ion transfer
Introduction The solubility of sparingly soluble salts in water is a classical example of thermodynamic equilibria that is taught in high school or undergraduate-level classes of chemistry. The solubility product, Ksp , in addition to the condition of the charge balance in the solution phase, can describe almost all what is happening in the solution that may contain foreign electrolytes in addition to the sparingly soluble salt, without paying attention to the salt phase side. It may seem that there is no room where electrochemistry, a heavy user of Ag|AgX reference electrodes, plays any meaningful role in determining the physicochemical properties of the system. In fact, the state of the solid salt phase is of fundamental importance in colloid chemistry [1–3]; the surface states of the salt particles, in particular, electrical charge, in the solution phase determine a variety of surface phenomena, such as colloid stability, electrical conductivity, and electrophoretic mobility. Another telling example, we encounter in elementary chemistry, indicating the significance of the electrical state of the charged particles, is argentometric titration, where the end point can deviate from the equivalence point, depending on the method of end point detection
Takashi Kakiuchi
[email protected] 1
pH Science and Technology Laboratory, Kinomoto 1058, Wakayama, 640-8453, Japan
2
J. Heyrovsky Institute of Physical Chemistry, Praha 8-Libeˇn, Czechia
[4]. About a century ago when dye indicators were introduced for end point detection in argentometric titration, Fajans proposed a model for the color change on the basis of the adsorption of silver and/or halide ions on the AgX particles [5, 6]. His model or similar interpretations have survived to date in textbooks of, e.g., analytical chemistry [7, 8]. Similar adsorption models have been employed in colloid chemistry to interpret the coagulation behavior of silver halide particles in response to the silver and halide ion concentrations, focusing on the change in the electrical double layer on the solution side of the colloid-solution interface [9]. In those adsorption models, the presence of the electrical double layer in the ionic crystal side and its structural change in response to the change in
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