Characterisation of the Carbon Paste Electrode / Carmine Indigo Aqueous Solution Interface for Electrochemical Sensor Application

Electrochemical sensors are used for regular monitoring of water, environmental and food matrices for trace metal elements (TMEs). These devices have on their surface a layer of active substances that complex ETMS such as mercury (II). Their effectiveness lies in the control of chemical reactions, the effect of the solvent and processes at the interface. The electrochemical behaviour of the indigo carmine dye was determined using cyclic voltametry at pH = 3 and pH = 10 on the carbon paste electrode (CPE). The acidic medium shows four current peaks giving two oxidising/reducing pairs: Indigo carmine (IC)/Leuco-indigo (LIC) Carmine and Dehydroindigo Carmine (DHIC)/Indigo Carmine (IC). In a basic medium, the voltamogram shows the existence of a single peak, that of dehydroindigo carmine. The mercury (II) sensor is optimal in a basic medium at pH = 10. Dehydroindigo carmine is a positive bipolaron which adsorbs to the surface of the working electrode by the internal sphere mode. Its structure contains groups enabling it to form 1:2 and 1:1 stoichiometric complexes. The sensor interface is influenced by current and mass transport phenomena. Their electrical parameters enable the equivalent electrical circuit of the interface to be constructed.