Iron hexacyanoferrate films : spectroelectrochemical distinction and electrodeposition sequence of 'soluble' (K+-containing) and 'insoluble' (K+-free) Prussian Blue, and composition changes in polyelectrochromic switching

Films of Prussian Blue (PB) deposited by electroreduction in the absence of K + have absorbance maxima at 730 nm which narrow and shift on voltammetric cycling in KCI solution to 690 nm, these absorptions being deemed diagnostic of 'insoluble' and 'soluble' PB respectively. Spectroelectro- chemistry shows that the initial deposit even in K+ solution is invariably the insoluble form, being then converted into the soluble, irrespective of electrochemical conditions or solution composition ; this resolves several problems in the current literature. Electrochemical oxidation of PB gives continuous mixed-valence compositions up to complete oxidation ; by contrast, reduction to Prussian White involves clean conversion of one into the other at a critical potential