871A pH and ORP Sensors and Accessories

The Foxboro® brand 871A Electrochemical Sensors, when used with 871A Accessories, are suitable for many process pH and ORP measurement applications. As symbolized by the “CE” marking, these Sensors conform to all applicable European Union directives.

These sensors are used with Foxboro 876PH Intelligent Transmitters, 875PH Intelligent Analyzers, 870PH Analog Transmitters, and 873PH, 873DPX,  Electrochemical Analyzers.

Measurement of pH by the 871A Sensor is accomplished by immersing a flat glass electrode, with a double-junction, gel-filled reference electrode, into a process solution. The glass electrode produces an electric potential proportional to the hydrogen ion activity. The reference electrode completes the circuit and provides a stable reference for the glass electrode.

These two electrodes constitute an electrolytic cell having a millivolt output that is proportional to the pH of the solution.

ORP is related to the logarithm of the ratio of oxidation activity to reduction activity, just as pH is related to the logarithm of the hydrogen ion activity. A solution that contains an excess of oxidizing agent has a positive ORP, while one containing an excess of reducing agent has a negative ORP. Typical ORP applications include treatment of chromate and cyanide wastes produced by plating, chemical, and metal treatment plants. ORP monitoring enables industry to determine reaction end points and to meet today’s stringent environmental requirements.


Document title Document type Document Date Download
Specification Sheet 01/11/2015