pH meter

Louvain-La-Neuve

The pH meter generally consists of two elements: an electronic box allowing the display of the numerical value of the hydrogen potential "pH" and a probe which measures this value. This glass probe contains two electrodes, one for measurement and one for reference. These electrodes are in the form of glass tubes, one contains a pH 7 buffer and the other contains a saturated solution of potassium chloride. These two electrodes can be combined (the most common today) or separate.

The bulb of the measuring electrode is made of porous glass or a permeable glass membrane covered with silica and metal salts. There are two silver threads covered with silver chloride; one which is immersed in a pH 7 buffer in the bulb and the second is immersed in the saturated solution of potassium chloride in the reference electrode.

When the probe is placed in a solution to be analyzed, the hydrogen ions accumulate around the bulb and replace the metal ions of the latter. This ion exchange generates an electrical flow which is captured by the silver wire. The voltage of this electrical flow is measured by comparison with the voltage generated with the reference electrode and is converted into a pH value by the pH meter. The porosity of the glass membrane decreases with its continuous use, thus the performance of the probe.

Increasing the acidity of the solution and therefore the concentration of hydrogen ions relative to the internal solution of the electrode increases the voltage, which has the consequence of lowering the pH value. It will be the opposite in an alkaline medium because the voltage will decrease and cause the pH value to increase.

The pH meter is calibrated before each first measurement with two buffer solutions (either pH 7 and 4 in an acid medium, or pH 7 and 10 in a basic medium). It is important to calibrate at the same temperature as that of the analysis solution otherwise there would be an impact on the accuracy of the measurements.

This device measures a global potential or voltage which is based on the ratio between the concentration of H3O+ ions and the difference in electrochemical potential which is established in the glass electrode. The reference electrode provides a stable voltage since it has a fixed concentration of potassium chloride solution. While the potential of the measuring electrode only depends on the pH of the solution to be analyzed. It is the difference in potential (voltage) generated by the ion exchange through the glass membrane of the measuring electrode and the reference electrode which is measured by the pH meter.