What is called a “double junction” ?

Anyone who works with pH electrodes will sooner or later come across the term “double junction.” But what does it mean, and when does using such an electrode actually make sense?

In this article, you will learn how a double-junction electrode is constructed, how it differs from a standard electrode, and for which applications it is particularly well-suited.

How is a pH electrode constructed?

To understand what makes a double-junction electrode special, it helps to first take a brief look at the basic structure of a pH electrode.

A standard pH electrode essentially consists of two parts: the measuring electrode and the reference electrode. In modern devices, these are often combined into a single unit known as a combination electrode.

The measuring electrode reacts to the hydrogen ions in the sample through its glass membrane and generates an electrical signal. The reference electrode provides a stable reference value against which the signal from the measuring electrode is measured.

To allow the reference electrode to make contact with the sample without coming into direct contact with it, it has what is known as a diaphragm: a porous opening through which a small flow of electrolyte solution passes to the sample.

Single Junction vs. Double Junction: What’s the Difference?

A standard electrode, also known as a single-junction electrode, has exactly one diaphragm. The reference electrode is thus in direct contact with the sample via this single diaphragm.

This works well in most applications. However, in challenging samples—such as heavily contaminated water, wastewater, or samples with high protein content—this single diaphragm can quickly become clogged or contaminated by sample components. This leads to inaccurate measurement results and significantly shortens the electrode’s service life.

This is where the double-junction electrode comes into play.

A double-junction electrode has two diaphragms and two electrolyte solutions:

The first diaphragm is located inside the electrode and separates the reference electrode from the external electrolyte. As a result, the reference electrode always remains in a clean, well-defined electrolyte and is not affected by the sample.

The second diaphragm provides the actual connection to the sample solution. It acts as a protective barrier: even if it becomes contaminated by the sample, the reference electrode behind it remains protected.

	Single Junction	Double Junction
Number of diaphragms	1	2
Electrolyte solutions	1	2
Protection of the reference electrode	Go directly to the sample	protected by a second chamber
Suitable for	clean samples	difficult and contaminated samples
Service life	standard	significantly extended
maintenance requirements	normal	lower

When is a double-junction electrode appropriate?

A double-junction electrode is recommended whenever the sample is corrosive, contaminated, or chemically demanding. Specific applications include:

Swimming pools and spas: Chlorinated water with fluctuating chemical conditions places heavy demands on standard electrodes. A double-junction electrode is significantly more durable and requires less maintenance in these environments.

Wastewater and Environmental Monitoring: Samples heavily contaminated with suspended solids, organic compounds, or sediments can quickly clog a single diaphragm. The double protective barrier reliably prevents this from happening.

Food and Fermentation: Samples with high protein, fat, or acid content—such as meat, dairy products, or fermented foods—can contaminate a single diaphragm within a short period of time.

Industrial applications: Highly acidic or alkaline samples, as well as liquids with a high salt content, benefit from the additional protective layer.

For simple applications such as pH measurement in distilled water or clear nutrient solutions in hydroponics, a standard electrode is usually sufficient. The double-junction electrode really comes into its own when the sample places heavy demands on the electrode.

Apera Double Junction Electrodes

Apera offers both pH and ORP electrodes with a double-junction design. They are specifically designed for use with challenging samples and feature an extended service life of up to 80 percent compared to standard electrodes.

All Apera double-junction electrodes are equipped with a BNC connector and are therefore compatible with all standard pH and ORP meters that have a BNC connector.

Overview of all Apera double-junction electrodes

Care and Storage of a Double-Junction Electrode

A double-junction electrode also benefits from proper care. The basic rules are the same as for any pH electrode:

After each measurement, rinse the electrode with distilled water and gently pat it dry; do not rub it. When storing the electrode, always place it in the protective cap with fresh storage solution. Distilled water and tap water are not suitable as storage solutions.

With intensive use in contaminated samples, the outer diaphragm may become clogged over time. In this case, a suitable cleaning solution will help. If cleaning does not resolve the issue and the readings remain consistently unstable, it is advisable to replace the electrode.

Learn more: Proper storage and care of pH electrodes

Conclusion

A double-junction electrode is not a one-size-fits-all solution for every application, but where it is needed, it makes a significant difference. The double protection of the reference electrode ensures more stable measurements, a longer service life, and less maintenance, especially in challenging samples such as wastewater, swimming pool water, or protein-containing foods.

Anyone who regularly takes measurements in such environments should view the additional cost compared to a standard electrode as a long-term investment.

Discover Apera Double Junction Electrodes

Frequently Asked Questions About the Double Junction Electrode

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