How to Measure the pH Value?

In any water a certain number of the water molecules are dissociated, this means that H+ and OH- ions exist side by side. The pH value expresses only the activity of the hydrogen ions (the negative decadic logarithm).With a glass probe the pH value is measured by comparison of the tension between the reference electrode in a solution with a determined pH value and the measuring electrode made of glass in the water.

The characteristic of the glass probe is its especially designed glass membrane. If it contacts water it builds up a special gel-like layer, sometimes called swell layer. By the absorption of water, H+ ions are set free which can move freely in the glass membrane and in the adjoining water, thus turning the glass into an ion conductor. The glass electrode has a buffer effect filled with material at a constant pH value. This "buffer" contains the reference electrode leading off the potential. The reference electrode is made of silver and silverchlorid and the inside buffer consists of a KCl solution or a KCl gel.

The silver wire coated with silver chlorid reacts to the activity of chlorid ions in the adjacent solution. The separation of the inside buffer and the measured water is made by a special capillary connection, the diaphragm. Contrary to the redox measurement the measuring value of the pH electrode has to be calibrated at regular intervals. This is done with two calibrating solutions as well as an adjustment screw at the measuring unit. Special care should be taken of the regular service and the operating conditions of the pH-electrode. The operating conditions are described under the heading "redox electrode".