| Abstract
| - The electrochemical behavior of solid microparticles of metal (Ag+, Cd2+, Co2+, Cr2+, Cu2+, Fe2+, Mn2+, Ni2+,Pb2+, and Zn2+) octacyanomolybdates, octacyanotungstates, and hexacyanoferrates has been studied by voltammetry,electrochemical quartz crystal microbalance, and microscopic diffuse reflectance spectroelectrochemical measurements. The solid microparticles have been immobilized on the surface of graphite electrodes prior to theelectrochemical measurements. A comparative study of the cyclic oxidation and reduction of these compounds inthe presence of potassium ions revealed that any interpretation of the electrochemistry requires the solubilityequilibria of the reduced compounds to be taken into account , such as in the case of the silver salts {Ag3K[X]}and {Ag4[X]} (with X = FeII-, MIV- (M = Mo, W)). Because {Ag4[X]} has a lower solubility than{Ag3K[X]}, the electrochemistry is accompanied by a conversion of solid {Ag3K[X]} into solid {Ag4[X]}. Twodistinct voltammetric signal systems are generated by these two compounds according to {Ag3K[X]} ⇄ {Ag3[X]} + K+ + e- and {Ag4[X]} ⇄ {Ag3[X]} + Ag+ + e-. When silver ions are present in the solution adjacentto the microparticles, the silver octacyanometalates and silver hexacyanoferrate show a chemically reversible andvery stable voltammetric behavior. Despite the fact that the electrochemistry is based upon a single-electron/single-ion transfer reaction ({Ag4[X]} ⇄ {Ag3[X]} + Ag+ + e-), more than one electrochemical signal is observedbecause of the simultaneous presence of amorphous and crystalline particles. This study shows that the interplayof solubility equilibria and electrochemical equilibria is generally observed for the other metal octacyanomolybdates,octacyanotungstates, and hexacyanoferrates as well.
- For the first time it is shown that the electrochemical behavior of solid metal octacyanomolybdates, octacyanotungstates, and hexacyanoferrates depends on the solubility equilibria of the reduced compounds, which may incorporate alkali metal ions (cf. scheme for silver cyanometalates), and also on the crystalinity of the solids. These findings allow interpretation of all of the observed electrochemical signals, which is important as solid metal cyanometalates find applications in sensors, electrochemical displays, and electrocatalysis, etc.
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