Sensors for pH Measurement in the Laboratory and in Industrial Processes

pH Sensors

Laboratory & Process Analytical pH Sensors

A pH sensor determines the alkalinity or acidity of a solution. METTLER TOLEDO offers a broad portfolio of pH sensors for various industries, such as pharmaceutical, chemical, food and beverage, energy, and semiconductor, as well as for water and wastewater treatment. Whether you need a pH sensor in the laboratory or for in-line use, we have suitable sensors that meet all your application requirements.

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FAQs

What is a pH sensor?

A pH sensor, also called probe or electrode, is an important tool that allows a user to determine the alkalinity or acidity of a solution. The glass membrane at the end is sensitive to H+ ions. Additionally, many of our pH sensors also offer a redox measurement.

How does a pH sensor work?

The outside of the glass membrane forms a gel layer when it encounters an aqueous solution. A similar gel layer is also formed on the inner side of the glass membrane, since the sensor is filled with an aqueous electrolyte solution. The H+ ions in and around the gel layer can either diffuse into or out of this layer, depending on the pH value. Thus, the H+ ion concentration of the solution is measured. If the solution is alkaline, H+ ions diffuse out of the layer and a negative charge is established on the outer side of the membrane. Since the glass electrode has an internal buffer with a constant pH value, the potential on the inner surface of the membrane remains constant during the measurement. The pH sensor potential is therefore the difference between the inner and outer charge of the membrane.

Why is a reference sensor needed while measuring pH using a pH sensor?

The purpose of the reference sensor is to provide a defined stable reference potential against which the potential of the pH sensor will be measured. To be able to do this the reference sensor needs to be made of a glass that 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 sensor through which the inner solution or reference electrolyte can flow out into the sample. The reference sensor and pH sensor (half-cell) must be in the same solution for correct measurements.

Which reference sensor is used when measuring pH?

There are several reference systems available. These include silver/silver chloride, iodine/iodide and mercury/calomel systems, as well as some adaptations. However, the silver/silver chloride system is almost always used in modern pH measurements. The potential of this reference system is defined by the reference electrolyte and the silver/silver chloride reference element. It is important that the reference electrolyte has a high ion concentration, which results in low electrical resistance.

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