Optical chemical sensors have attracted much attention in the past few years for their simplicity, fast response and precision in analyte detection. Ionophore based optical sensors (IBOS) are being extensively researched for possibility of choosing and assembling the components for the purpose of better selectivity. Ionophore based optical sensors contain an ionophore and fluorophore/chromophore as an indicator. Large number of iminocoumarine derivatives are used as fluorophores, due to their ability of fluorescence quenching or fluorescence amplification upon reaction with the analyte. Immobilisation into films adds to the stability and firmness of sensor systems. This work represents the development of a film that contains ionophore (valinomycin) and fluorophore (7-N,N-diethylamino-2-imino-3-(2-N-methylbenzimidazolyl)chromen (IM-2)) immobilised in a PVC matrix, which functions as an ion-selective optode (optical sensor) for potassium ions. The purpose of this work was spectrophotometric (UV-visible absorption and fluorescence) characterisation of the film containing the fluorophore IM-2 as a fluorescent unit and to evaluate the possibility of usage in optical ion sensors. Sensor sensitivity, influence of pH, selectivity and stability were evaluated. Sensor shows good analytical parameters showing the highest sensitivity at pH 8 at which various potassium concentrations ranging from 3.16*10^-4 mol/dm^-3 to 1 mol/dm^-3 can be detected. Sensor shows great stability and potassium selectivity among alkali metals and alkaline earth metals whereas trivalent cations interfere with sensor’s functionality.