Sensing electrodes derived from the regular silicon wafer were shown to respond sensitively to the part-per-billion level of iron impurities in alkaline hydrogen peroxide solution (frequently referred to as standard clean-1 solution). The potentiometric results clearly indicate that the large positive shift of silicon open-circuit potential mainly originates from the adsorbed iron impurities on the surface oxide layer. Acid treatments by HCl or HF effectively regenerate the iron-contaminated silicon sensing electrode.