An apparatus for determining the endpoint in a chemical mechanical polishing operation used for polishing a metal-containing material includes an electrochemical cell and an electronic circuit. An acidic polishing slurry is used to oxidize the metal and the oxidized metal is included in an effluent slurry stream, a sample of which is provided to the apparatus. The apparatus includes a liquid-phase working electrode, a reference electrode and a solid electrolyte which allows for the interchange of ions between the electrodes. An electronic circuit is coupled to the electrode for monitoring the component activity of the effluent slurry stream by measuring the electric potential across the electrodes. When the measured electric potential changes, indicating a change in the composition of the effluent slurry, endpoint is indicated.