The pH sensitive glass electrode

A pH electrode is composed of two main parts, a glass electrode and a reference electrode. The glass electrode, or with a different name the measuring electrode, is the part that is actually measuring pH.

Let's take a closer look at the function of the electrode.

pH sensitive glass

The most fragile part of the measuring electrode is the hydrogen sensitive glass tip.

The electrical potential created at the glass tip is related to the pH value of the sample. And that is what we want to measure!

Reference element

To be able to connect the electrode to a pH meter (voltmeter) you need to electrically connect to the internal buffer (inside of the glass bulb). A silver (Ag) wire, on to which is coated a layer of silver chloride (AgCl), has shown to be a good solution. Silver is the best electrical conductor of all metals. A common name for the Ag/AgCl wire is reference element.

Sensitive measurements

The pH electrode has a very high internal resistance, typically of the order of 100 MOhm. To be able to make reliable potential measurements the input resistance of the pH meter should be at least 10000 times larger than the resistance of the sensor.

Deviations from the ideal electrode

Nothing is perfect, not even a pH electrode. It has a couple of shortcomings:

Different sources of error in the output voltage of a pH electrode. The pH measurement is a potentiometric measurement of a very small potential difference between a measuring electrode and a reference electrode. It's a sensitive measurement, where there are several sources of error inherent in the electrode.

Temperature is an important factor and a source of error in pH measurement.

In strongly acidic and alkaline medium the electrode reacts not only to hydrogen activity.

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