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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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.

What are combined pH sensors?

In the combined sensors the pH sensor (glass sensor) and the reference sensor are constructed in the form of two concentric tubes/chambers. The pH electrode envelops the reference electrode they are interconnected to each other via a ceramic junction. These two electrodes, although combined, function separately. The only difference is the ease of handling one sensor, instead of two.

One can also house a temperature sensor in the same body as the pH and reference elements. This allows temperature-compensated measurements to be made. Such electrodes are called 3-in-1 electrodes.

What is the correct way to store pH sensors?

All user manuals provide the necessary information about short- and long-term pH sensor storage.

  • Always: In a reference electrolyte for both short- and long-term storage. This allows immediate use of pH sensor when needed, and ensures a short response time.
  • Sometimes: In buffer pH 4 and 7 between measurements to keep the membrane hydrated.
  • Never: Use deionized water as this will deplete the ion-rich reference electrolyte. Also, do not store the electrode dry as this will damage the membrane, causing the pH sensor to have a longer response time. When stored dry for long periods, many pH sensors must be reactivated by soaking for several hours before installation in order to get the optimal measuring results. If these measures are not sufficient, the sensor may be made functional by treating it with a special reactivation solution followed by subsequent conditioning in the reference electrolyte.

What is the normal lifetime of a pH sensor?

When using and storing a pH sensor as recommended, the expected lifetime is 1-to-3 years. However, a number of factors can decrease the lifetime of a pH sensor. One of them is its use for measuring hot or very alkaline samples. Other factors may be mechanical damage resulting from incorrect storage. Also, if the storage solution is allowed to dry or leaks out due to storage at high temperatures, freezing or other causes, the life of the probe may be reduced significantly.

How can I detect when a pH sensor must be replaced?

The slope and offset of calibration are good indicators of the quality of a pH sensor. When these values surpass certain limits the pH electrode can be regarded as used up. The lower and upper limits for the slope are 85% and 105% and for the offset -35 mV and 35 mV. In addition, please note that an unstable signal or a very long response time in the pH calibration solution indicates an advanced deterioration of a pH sensor. These phenomena are often associated with irregular slopes and offsets.

Some of our in-line digital sensors also offer predictive diagnostics that indicate when a sensor must be replaced.