Development of a Gold Microelectrode and its Application for Evaluating Free Chlorine Consumption by Metal Surfaces

An amperometric gold microelectrode for in situ free chlorine measurement was newly fabricated, characterized, and successfully applied for evaluating free chlorine consumption by ductile iron coupons at the micro level in a simulated drinking water distribution system. The developed gold microelectrode showed a linear relationship with various free chlorine concentrations (0-4 mg Cl2/L) at an applied potential of +150 mV vs. Ag/AgCl. The response time was less than 5 seconds and the limit of detection was 0.08 ± 0.008 mg Cl2 /L. However, the gold microelectrode showed a pH interference on free chlorine measurement, requiring construction of calibration curves at different pHs. After measuring pH and free chlorine concentration micro profiles simultaneously at the same depth, pH was compensated for determining accurate free chlorine concentrations. Two examples of pH compensation were demonstrated with increasing pH and decreasing pH conditions near the metal surface. The welldefined pH compensated free chlorine micro profiles provided more accurate corrosion kinetic parameters such as flux (J), reaction rate (k), and free chlorine concentration at the metal surface (Cs). The developed gold microelectrode will be a useful experimental tool for evaluating localized corrosion processes.