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Quartz crystal microbalance respond not only to the deposited mass on its surface, liquid property in contact with its surface, but also respond to the mechanical property change of its coating material. The mechanical property of the coating material which affects the sensor resonance frequency is the modulus elasticity change, mass density change and conformational change of the coating layer. It is known that the polymer mechanical property can be affected by the gas or liquid in contact with the polymer. Gas or liquid absorption leads to the change in the modulus elasticity of the film which absorb the gas or liquid compare to the initial film modulus elasticity. Ionic interaction may also affect the polymer film mechanical property. Potassium chloride solution in water has ionic strength related to its concentration. This work shows that the frequency respond of the QCM sensor coated with polystyrene was affected by the potassium concentration in water. The frequency change of the QCM sensor coated with polystyrene film to the potassium chloride concentration from was presented.