A novel mechanism of amplifying pH oscillations by pH-dependent EDTA-metal ion complexation is proposed. If there is a metal ion involved in the H+ consuming reactions in the pH oscillator and the metal's complex formation constant is big enough, the nonlinear coupling of the original oscillator with the complexation induced metal ion oscillation can finally amplify the pH oscillation. This effect is demonstrated in H2O2-S2O32--Cu2+ system and further discussed in three other systems. Since pH oscillation is widely used in many areas as a spontaneous periodical driving force at the molecular level, this work may help to broaden the driving range of pH oscillationrs.