MIRAGE DETECTION OF COUNTERION FLUX BETWEEN PRUSSIAN BLUE FILMS AND ELECTROLYTE-SOLUTIONS

Mirage detection is used to monitor the flux of counter-ions between an electrode of Prussian Blue (PB) (FeIII, FeII(CN)6( mixed valence complex) and an aqueous electrolyte during voltammetric experiments. Mirage detection is the measurement of the deflection of a laser probe passing through the solution layer close to the electrode. This layer has a refractive index gradient due to the concentration gradient of the counter-ions which enter or leave the PB film for charge compensation. With this technique, it is shown that, on reduction, electroneutrality of the PB film occurs by incorporation of K+ or Na+ in the neutral MA electrolyte (A-=Cl-, Br-, ClO4 -, SO4 -, NO3 -), while in acidic KCl electrolyte, H+ ions enter the film concurrently with K+ ions. No confirmation of the "insoluble" → "soluble" transformation, taking into account the first voltanunetric cycles, could be found. During the oxidation step of the PB films, one does not observe anion incorporation for charge compensation inside the film, but expulsion of some species. Complementary tests show that Fe3+ ions leave the film. © 1990.