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A novel method using a micro-ion-selective electrode (micro-ISE) technique was developed for in situ lead monitoring at the water metal interface of a brass-leaded solder galvanic joint in a prepared chlorinated drinking water environment. The developed lead micro-ISE (100 pm tip diameter) showed excellent performance toward soluble lead (Pb2+) with sensitivity of 22.2 +/- 0.5 mV decade(-1) and limit of detection (LOD) of 1.22 x 10(-6) M (0.25 mg L-1). The response time was less than 10 s with a working pH range of 2.0-7.0. Using the lead micro-ISE, lead concentration microprofiles were measured from the bulk to the metal surface (within 50 pm) over time. Combined with two-dimensional (2D) pH mapping, this work clearly demonstrated that Pb2+ ions build-up across the lead anode surface was substantial, nonuniform, and dependent on local surface pH. A large pH gradient (Delta pH = 6.0) developed across the brass and leaded-tin solder joint coupon. Local pH decreases were observed above the leaded solder to a pH as low as 4.0, indicating it was anodic relative to the brass. The low pH above the leaded solder supported elevated lead levels where even small local pH differences of 0.6 units (Delta pH = 0.6) resulted in about four times higher surface lead concentrations (42.9 vs 11.6 mg L-1) and 5 times higher fluxes (18.5 X 10(-6) vs 3.5 X 10(-6) mg cm(-2) s(-1)). Continuous surface lead leaching monitoring was also conducted for 16 h.