A novel potassium ion sensing based on Prussian blue thin films

A novel method for sensing potassium ions in aqueous solutions is presented. This method is based on the Prussian blue (PB) modified tin oxide (TO) electrode. Thin films of PB (KFeFe(CN)6), deposited onto transparent TO substrate, are reduced to Everitt’s salt (ES, or K2FeFe(CN)6) in solutions containing potassium ions. Using an in situ measurement of the absorbance changes in PB thin film while applying different reductive potentials to the electrode, the PB mole fraction within the thin film, x, can be obtained for solutions containing various concentrations of potassium ions. By applying either the theory of regular solution (0.25<x<1) or the Margules equation (0<x<0.25) to the Nernst equation, one can construct a special function, which depends mainly on the PB mole fraction and the interaction parameters involved. This sensing method, which eliminates the need for pre-conditioning the electrode to a fixed potential, as proposed previously by other researchers, is based on a linear log–log plot of this special function against the potassium ion concentrations. Such a relationship was observed when the K+ concentrations ranged from 3.3×10−4 M to 1.0 M. The interferences of Na+ and Ba2+ on the K+ sensing are also studied.