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pH Electrodes for the laboratory

We offer pH electrodes or combination electrodes for pH measuring devices from Agilent Technologies, Mettler-Toledo and WTW. The electrodes can be used in a wide range of applications such as pH measurements in food testing. The chemical composition, homogeneity, temperature, process pressure and the pH range of the sample are important criteria when choosing the right electrode. Contact our customer service to assist you on selecting the right electrode.

A pH electrode selection guide can be found here.

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  1. Mettler-Toledo

    pH electrode LE438 (plastic, 3-in-1)

    sku: MR51340242
  2. Mettler-Toledo

    pH combination electrode InLab Micro

    sku: MR51343160
  3. Mettler-Toledo

    pH-Elektrode InLab Solids Pro IP67

    sku: MR51343156
  4. Mettler-Toledo

    pH electrode LE407 (plastic, gel

    sku: MR51340330
  5. Mettler-Toledo

    pH combination electrode InLab Easy

    sku: MR51343010
  6. Mettler-Toledo

    pH combination electrode InLab Routine Pro

    sku: MR51343054
  7. Mettler-Toledo

    pH combination electrode InLab Science

    sku: MR51343070
  8. Mettler-Toledo

    pH combination electrode InLab Routine Expert Pro

    sku: MR51343101
  9. Mettler-Toledo

    pH combination electrode InLab Hydrofluoric

    sku: MR51343176
  10. Mettler-Toledo

    InLab® Ultra Micro-ISM

    sku: MR30244732
  11. Mettler-Toledo

    pH-Elektrode LE438-IP67

    sku: MR30247153
  12. WTW

    SenTix® 51 combined pH-electrode

    sku: WW103651
  13. WTW

    SenTix® 52 combined pH-electrode

    sku: WW103652
  14. WTW

    SenTix Mic-D combined pH-electrode

    sku: WW103660
  15. WTW

    SenTix® Ag ORP-electrode

    sku: WW103664
  16. WTW

    SenTix® Au combined ORP-electrode

    sku: WW103665
  17. WTW

    SenTix PtR combined ORP-electrode

    sku: WW103666
  18. WTW

    SenTix 91 combined pH-sensor

    sku: WW103695
  19. WTW

    pH-Einstabmesskette SenTix SP-DIN

    sku: WW103730
  20. WTW

    SenTix HWD pH electrode

    sku: WW103731
  21. WTW

    SenTix® 945 ORP electrode

    sku: WW103743
  22. WTW

    SenTix® 940-P

    sku: WW103760
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pH electrodes selection guide

What is required for pH measurements?

A special measuring tool for hydronium ions which the pH value refers to, is required to measure the pH value. The principle of the measurement is based on a sensor with a glass membrane, which selectively detects hydronium ions and registers the reaction between the membrane and a sample solution. This glass electrode potential alone does not provide sufficient information, a second sensor is thus required which provides the reference potential for the pH sensor. To determine the pH value of the solution, the potential difference between these electrodes must be measured.

The reaction of the pH-sensitive glass electrode depends on the concentration of the H+ ions (or H3O+). Hence, pH is a measurement of both acidity and alkalinity, a selective measurement of hydronium ion activity. On the other hand, the reference electrode does not react to the concentration of the H+ ions in the sample solution and always delivers the same constant potential which is used to measure the potential of the pH sensor. The difference in potential between the two electrodes determines the pH of the solution. Since this potential is a linear function of the hydronium ion concentration in the solution, the concentration can be measured quantitatively.

Nowadays, the pH sensor and the reference sensor are very often housed in one electrode. This combination of reference and pH electrode is called a combined pH electrode. Each of these three types of electrodes is different and has its own characteristics and properties.

pH Electrodes

The pH electrode is the component that actually registers the pH of the solution. It consists of a glass shaft with a thin glass membrane at the lower end that is H+ ion sensitive. When this membrane comes into contact with an aqueous solution, the outside of the membrane glass forms a gel layer. A similar gel layer is also formed on the inside of the membrane glass as the electrode is filled with an internal aqueous electrolyte solution.

The H+ ions in and around the gel layer can - depending on the pH value and thus the H+ ion concentration of the measurement solution - either diffuse into or out of the layer. If the solution is alkaline, the H+ ions diffuse out of the layer and a negative charge is created on the outside 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 potential of the pH electrode is therefore the difference between the inner and the outer charge of the membrane.

Reference electrodes

The task of the reference electrode is to provide a defined stable reference potential against which the potential of the pH sensor can be measured. To achieve this, the reference electrode must be made of glass, which is not sensitive to the H+ ions in the solution. It must also be open to the sample environment in which it is immersed. An opening or a diaphragm is made in the shaft of the reference electrode through which the inner solution or the reference electrolyte can flow into the sample. To obtain correct measurements, the reference electrode and the pH half-cell must be in the same solution.

The electrode construction consists of an inner reference element that is immersed in a defined reference buffer solution and has indirect contact with the sample solution via the diaphragm. This contact chain ensures a stable potential. There are several reference systems available, but the silver/silver chloride system is now used almost exclusively in laboratories. The potential of this reference system is defined by the reference electrolyte and the silver/silver chloride reference element. It is important for the reference electrolyte to have a high ion concentration and a low electrical resistance.

As the reference electrolyte flows into the sample during the measurement, pay attention to possible reactions between the reference electrolyte and the sample solution. Such reactions could affect the electrode and measurement.

Combined electrodes

Combined electrodes are much easier to handle than two separate electrodes and are therefore used very frequently today. In the combined electrode, the pH-sensitive glass electrode is surrounded concentrically by the reference electrode, which is filled with the reference electrolyte. The pH and reference components of the combined electrode have the same properties as the corresponding separate electrodes. The only difference is that they are integrated in a single electrode for easier handling. The use of individual pH and reference electrodes instead of a single combined electrode is only recommended if the two components of the combined electrode have a very different expected life span.

To further simplify pH measurements, a temperature sensor can be used together with the pH and the reference element in the same shaft. This enables temperature compensation while measurung pH. Such electrodes are also referred to as 3-in-1 electrodes.

pH and redox electrodes for every application

Electrodes may consist of different materials (very common are platinum, calomel or hydrogene electrodes) and are hence very versatile. They are knowned as single-rod measuring cells which are used to determine the pH value. However, the standard hydrogene electrode serves here as the reference electrode to determine the standard potential of redox couples. The most common ones are solid state electrodes containing of a metal and its dilution. Liquid electrodes such as quicksilver electrodes can also be used. 

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