The electrochemical pH sensor

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How the pH sensor works

The most common pH sensor is the glass electrode. It's used in many industry applications and in a wide variety of fields. The glass-electrode method has high reproducibility, and it can measure pH of various solutions.

Electrochemical sensor

A pH electrode is a potentiometric or electrochemical sensor that has a voltage output. A potentiometric sensor consists of two electrochemical cells or electrodes:

- the glass electrode, sometimes called the measuring electrode or active electrode.

- and the reference electrode.

The electric potential created between the glass electrode, and the reference electrode is a function of the pH value of the measured solution. So once the potential difference has been measured you can calculate the pH value. How? Nernst equation, see below, will explain more.

"A battery with a variable voltage"

When the glass and the reference electrode, are immersed in a solution, a small galvanic cell is established. If you use a voltmeter and measure the potential between these two electrodes, you will see a potential difference.

The pH sensor is essentially a simple single cell battery. It can be thought of as a battery, with a voltage that varies with the pH of the measured solution.

Combination electrodes

The majority of pH electrodes available commercially are combination electrodes that have both the glass electrode and the reference electrode conveniently placed in one housing. While the reference electrode and the pH measuring electrode are physically combined into one electrode, the electrodes still function independently, and characteristic problems remain.

To learn more about how the pH sensor works, please continue to read:

• Nernst equation - mathematical description of the pH electrode.

• Glass electrode - why the electrical potential of the glass electrode indicate the pH value.

• Reference electrode - makes contact with the sample solution and (should) deliver a stable reference potential.

• pH versus temperature - always take into account the temperature at pH measurements.

• Different potentials of a pH electrode - the measured pH electrode potential is affected by a number of factors.

• Combination electrode - modern electrodes combine the reference and standard electrodes into one unit.

• Nonglass pH electrode - an alternative to the glass electrode where an unbreakable sensor is required.

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Optical pH sensors
One technique to measure pH is to use a fiber optic sensor. Maybe an uncommon pH technique, but I think it's important to know it exists. The development ...

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