The glass electrode

What the glass membrane does

Strictly speaking, the glass electrode is only the measuring half of a pH cell - the one with the glass membrane. In contact with water, its surface forms a hydrated layer just a few nanometres thick in which hydrogen ions embed themselves. The concentration difference between the sample outside and the inner buffer inside creates the voltage that the meter converts into the pH value. The basics explain how this works in detail.

This hydrated layer is also the reason for two iron rules: the membrane must never dry out and never be rubbed mechanically. Both destroy exactly the layer that makes the measurement possible in the first place.

Membrane shapes for different tasks

The shape of the glass membrane is not a design feature but a functional decision:

• Spherical membrane - the standard shape. Large surface, stable readings, the right choice for most liquid applications.
• Spear-tip membrane - tapers to a point and penetrates solid or semi-solid samples: meat, cheese, fruit, soil. Indispensable in the food industry.
• Flat membrane - measures on surfaces such as skin, paper or agar plates, where nothing can be immersed.

Special glasses

The glass itself is also matched to the application. Standard glass measures reliably between pH 0 and 12; in strongly alkaline solutions, however, sodium ions fake a pH that is too low - the so-called alkaline error. For these cases there are low-alkaline-error special glasses. Temperature-resistant membrane glasses with lower internal resistance exist for hot media, and particularly conductive variants for low temperatures.

Usable on its own?

A pure glass electrode always needs a separate reference electrode, otherwise the comparison potential is missing. In practice, both are almost always combined in one shaft today - as a combination electrode. Separate cells are only found in special cases, for example when measuring and reference systems wear at different rates and are to be replaced individually.

Alkaline and acid error: the edges of the scale

At the very edges, the glass membrane becomes dishonest. Above roughly pH 12, sodium ions crowd into the hydrated layer and are wrongly counted as H⁺ - the reading then comes out too low. This alkaline error grows with temperature and sodium content; for strong caustic solutions there are therefore low-alkaline-error special glasses. At the acidic end, below about pH 1, it tips the other way: the activity of the water itself drops and the reading comes out too high. Anyone measuring regularly at the edges of the scale therefore picks the membrane to match the medium - and reads the data sheet before trusting the decimal place.

Temperature and membrane resistance

The glass membrane is a resistor of several hundred megohms - and that resistance grows roughly tenfold with every temperature drop of 25 to 30 degrees. In cold media the measuring chain becomes noticeably slower and more prone to interference; below about 5 °C, low-resistance membrane glass helps, as offered for chilled and outdoor applications. At the hot end the glass ages faster: continuous duty near the upper temperature limit visibly shortens the lifespan. Why temperature additionally changes the slope is calculated by the electrode check.

Continue to the combination electrode →