tokens: Dissolution of the Pt - 30 % Ir microelectrode tip was observed by scanning electron microscopy at charge densities as low as 200 microC / cm2 X ph ( 1 A / cm2 ) , whereas erosion of activated iridium microelectrodes occurred only at the highest charge and current densities ( 3200 microC / cm2 X ph , 16 A / cm2 ) .