Direct measurement of pH within marine biofilms on passive metals

Direct measurements of the pH within natural marine biofilms grown on platinum have been made with iridium/iridium oxide solid-state microelectrodes. The data show that the pH is highly variable, with steep gradients both perpendicular to and along the metal surface. Measured values ranged from 8 in the outer portions of the biofilm to below 2 in localized areas near the metal surface. It was verified that bulk water flowing along the hydrophilic outer sheath of the microelectrode could effect the measured pH values. However, meaningful data could still be taken because the response time of the electrode was sufficiently less than the time for water ingression. The data presented are consistent with recent heterogeneous biofilm models and with the idea that localized areas of low pH within biofilms can be a contributing factor in biologically influenced corrosion processes.