Potentiometric pH and ORP Measurement

How does pH and ORP measurement work?

pH measurement is potentiometric. A glass membrane pH electrode produces a potential across the membrane dependent on the hydrogen ion activity in the solution. This potential is measured against a reference electrode; according to the Nernst relationship, a change of approximately 59 mV (at 25 °C) occurs per pH unit. Because temperature affects the result, temperature compensation is applied to the measurement.

ORP (oxidation-reduction potential) is the potential difference between an inert metal electrode (generally platinum) and a reference electrode. The result is given in mV and shows the oxidizing or reducing character of the solution.

What potentiometric measurement measures and where it is used

The method is used to monitor pH and ORP continuously:

• Boiler feedwater, demineralized water and condensate lines
• Cooling water and water treatment plants
• Wastewater neutralization and chemical dosing control
• ORP monitoring in disinfection and oxidation processes
• Process waters in the chemical, energy and food industries

Where potentiometric measurement works well and where it struggles

Potentiometric measurement is simple and fast, works over a wide range and can connect directly to the control system. Several sensor channels can be used on one analyzer.

Limitations: electrodes age and become contaminated over time; periodic cleaning and calibration with buffer solutions are required. In ultrapure water, low ionic strength can make the measurement unstable, requiring a special flow cell, electrode or ultrapure water kit. The condition of the reference electrode filling solution and diaphragm affects measurement accuracy.

Choosing an instrument for potentiometric measurement

When selecting, the sample type (ultrapure water, boiler water, wastewater), number of sensors, communication protocol and mounting method are assessed. The interval for electrode cleaning and buffer calibration should be set according to how fouling the sample is. Tetra selects the sensor and flow cell according to the sample's temperature, pressure and fouling properties, and completes calibration and alarm settings together during commissioning.

Tetra and pH/ORP analyzers

Tetra supplies, installs, commissions, calibrates and services pH/ORP analyzers and transmitters in Türkiye. The product range includes the Thermo Orion 2102 pH/ORP analyzer, the AV88 single-sensor analyzer, the AquaPro multi-channel transmitter and the Yokogawa FLXA202 2-wire analyzer. Tetra also builds pH and conductivity analysis systems combining transmitter, sensor, flow cell and sample preparation components.