A control vector parameterization approach using the Karhunen-Loeve (K-L) expansion for optimal control is presented. Optimal control profiles at different initial conditions are represented by a truncated K-L expansion and the optimal control is thus transformed into a parametric optimization with the K-L coefficient(s) as the optimization variable(s). Closed-loop optimal control therefore becomes feasible because the number of optimization variables is minimized. The presented approach is demonstrated on a batch electrochemical reactor in which reduction of oxalic acid glyoxalic acid is carried out under conditions with known or unknown disturbance, or electrode deactivation. The closed-loop control with the K-L expansion outperforms the open-loop optimal control with very little computational effort.