Sensors for pH Measurement in the Laboratory and in Industrial Processes

pH Probes

A Broad Portfolio of Laboratory and In-Line Analytical pH Probes

A pH probe is an analytical instrument that measures hydrogen-ion activity in solutions and provides the acidity or alkalinity as a pH value. They are designed to deliver precise performance across various industries, such as pharmaceutical, chemical, food and beverage, water, and wastewater treatment. METTLER TOLEDO is a global manufacturer of pH probes, offering a highly reliable and diverse range suitable for use in laboratory and industrial processes.

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What is pH?

On a logarithmic scale, pH (potential of hydrogen) represents the acidity or alkalinity of a water-based solution. On this scale, 7 represents neutrality, lower values indicate acidity, and higher values indicate alkalinity.

What are pH probes and how do they work?

A pH probe allows a user to determine the alkalinity or acidity of a solution. pH probes work by measuring a solution's hydrogen-ion activity, which is detected by a sensitive glass membrane at the end of the probe.

When exposed to an aqueous solution, the exterior surface of the glass membrane develops a gel layer. Because the probe is filled with an aqueous electrolyte solution, a similar gel layer forms on the inner side of the glass membrane. Depending on the pH value, the H+ ions in and around the gel layer can diffuse into or out of this layer. As a result, the solution's H+ ion concentration is determined.

If the solution is alkaline, H+ ions diffuse out of the layer and a negative charge forms on the membrane's outer surface. If the solution is acidic, H+ ions diffuse into the layer and a positive charge forms on the membrane's outer surface. Because the probe contains an internal buffer with a constant pH, the potential on the inner surface of the membrane remains constant throughout the measurement. As a result, the pH probe potential is the difference between the inner and outer charges of the membrane.

To learn more about the theory of pH Measurement, we recommend watching our webinar What is pH?

how pH probes work
how pH probes work

What components does a pH probe have?

A pH probe is made up of several components with the primary ones being:

  • Glass body/shaft
  • pH-sensitive glass membrane filled with an internal buffer solution
  • Reference electrode
  • Reference electrolyte
  • Junction or diaphragm

pH probe components
pH probe components

Why is a reference electrode needed while measuring pH with a pH probe?

A reference electrode is needed while measuring pH with a pH probe to provide a defined stable reference potential against which the pH probe potential will be measured. To do this, the reference electrode needs to be made of glass, which is not sensitive to the H+ ions in the solution. It must also be open to the sample environment into which it is dipped. To achieve this, an opening or junction is made in the shaft of the reference electrode, allowing the inner solution or reference electrolyte to flow into the sample environment. The reference electrode and pH probe (half-cell) must be in the same solution for correct measurements.

reference electrodes with pH probes
reference electrodes with pH probes

How often should you clean your pH probe?

pH probes should be cleaned before calibration and after each measurement. To clean your pH probe, rinse it thoroughly with distilled water and dab it dry with a clean tissue.

In order to prolong the lifetime of your pH probe and to achieve the most accurate results, it is important to clean and maintain it properly. Probes that are not cleaned after use or are neglected for a long time lose their accuracy, lowering the measurement precision of the whole system. To learn more, refer to our pH Essentials guide.