The electrocatalytic activity of an N,N-diethylaniline-grafted carbon (DEA/PFC) electrode was utilized to improve the sensitivity in the cathodic detection of free chlorine. Voltammetric curves showed a well-defined cathodic peak of free chlorine on DEA/PFC, lowering the overlap with that of dissolved oxygen. Amperometric measurements were applied for the determination of free chlorine for a concentration range of 4 orders of magnitude. The results showed that measurements of the amperometric response with DEA/PFC can be used as the basis for a simple, accurate, and rapid method to determine free chlorine.