Abstract: An electrochemical sensor apparatus and electrochemical sensing method are described, using one or more working electrodes (110) of boron doped diamond (BDD). A cathodic reduction process provides a cathodic measurement and, substantially simultaneously, an anodic oxidation process provides an anodic measurement. A sum of a content of two equilibrium species within an aqueous system is obtained using both the cathodic measurement and the anodic measurement. One example measures total free chlorine by simultaneously measuring hypochlorous acid (HOCl) and hypochlorite ion (OCl-). The BDD working electrode (110) comprises at least one ablated region (115) which introduces non-diamond carbon sp2 material. The ablated region (115) may comprise one or more grooves (114) which are cut into the working surface (112), e.g. by a laser.