We report on amperometric solid-contact potassium-selective electrodes based on catalytically synthesized Prussian Blue nanoparticles. Both the films and nanoparticles are able to generate amperometric response in flow-injection mode under constant potential. However, nanoparticles provide higher sensitivity than the films. Moreover, the intrinsic permeability of Prussian blue for potassium ions allows for selective detection of potassium in comparison to sodium even without ion-selective membrane. The results of Electrochemical impedance spectroscopy were used to manage the sensitivity by controlling the charge transfer resistance for ion-selective membranes of different density. By performing measurements in Bis-Tris as a carrier buffer containing no interfering cations an additional increase in selectivity was enabled. The record selectivity coefficient logK _Na⁺_/K⁺ is -2.7 for amperometric flow-injection mode was achieved. The obtained results indicate the prospects for analysis of biological fluids since Na⁺/K⁺ ratio is approximately 20-40.