pH and ORP Measurement in Water Treatment: Sensor Selection and Maintenance Guide

pH and ORP (oxidation-reduction potential) are two of the most important parameters in water treatment process control. Accurate, reliable measurement of these parameters is essential for disinfection efficiency, corrosion control, and regulatory compliance.

pH measures the hydrogen ion concentration in a solution on a scale from 0 to 14. In water treatment, pH control is critical for coagulation and flocculation efficiency (optimal pH 6.5–7.5), chlorine disinfection effectiveness (hypochlorous acid dominates below pH 7.5), and corrosion inhibition in distribution systems. A pH shift of 0.5 units can significantly affect disinfection byproduct formation and pipe corrosion rates.

ORP measures the tendency of a solution to accept or donate electrons, expressed in millivolts (mV). In disinfection applications, ORP provides a direct indication of the oxidizing power of the water, integrating the effects of chlorine concentration, pH, and temperature. An ORP value above +650 mV is generally considered sufficient for effective disinfection in swimming pools; municipal drinking water treatment typically targets +750 to +850 mV.

Sensor selection considerations: For clean water applications (drinking water, swimming pools), a standard glass pH electrode with a single-junction reference is sufficient. For wastewater or industrial process streams containing suspended solids, oils, or aggressive chemicals, choose a flat-membrane or open-junction electrode with a pressure-equalized reference to prevent clogging and contamination. For high-temperature applications (above 60°C), select a high-temperature electrode with appropriate temperature compensation.

Maintenance best practices: Rinse the electrode with clean water after each measurement or at regular intervals in online installations. Calibrate with fresh buffer solutions (pH 4.01 and 7.00 for two-point calibration) at least weekly for critical applications. Inspect the reference junction for clogging or discoloration monthly. Replace the electrolyte in refillable electrodes every 3–6 months. Store electrodes in pH 4 buffer or 3M KCl solution when not in use — never in distilled water. TL Environment TL-PH Series pH/ORP sensors include a built-in temperature sensor for automatic temperature compensation and are compatible with all major online water quality analyzers.