A process and system for measuring biocide concentration in biocide treated seawater in an oilfield pipeline are provided. The system includes a seawater plant, a plurality of seawater sampling locations throughout the oilfield pipeline immediately downstream from the seawater plant, a surge tank, a water supply plant, a pH monitoring system, and an autosampler. Each of the plurality of seawater sampling locations has a pH monitoring system and an autosampler. The autosampler is used to collect a plurality of samples of the biocide treated seawater from the oilfield pipeline. The method includes measuring pH of the biocide treated seawater in the oilfield pipeline, when the pH of the biocide treated seawater is below 6.8, collecting a plurality of samples using an autosampler, correlating the pH of the biocide treated seawater with the biocide concentration, and stopping collection of the plurality of samples when the pH is 7.8 or higher.