Oscillatory instability induced by buoyancy forces in an electrochemical cell with plane horizontal electrodes is considered. The space between the electrodes is filled with solution containing two types of single-charged cations and one type of single-charged anion. Under conditions of transport limitation of the electrochemical reaction rate, low-frequency convective- concentration oscillations may arise in the system, which are the internal standing waves in the concentration stratified liquid. The conditions for the development of the waves, their frequency and space period are found. A possibility for using internal standing waves for the construction of low-frequency electrochemical resonators is discussed.