The Measurement of pH

The nature of the water molecule is to dissociate in aqueous solutions into two ions

H2O ⇔ H⁺ + OH^-  

The H+ is called hydrogen ion or proton, the OH- is the so-called hydroxide ion.

pH is the mathematical description of the activity of the hydrogen ions. It goes from about -1 to about + 15. Corresponding to this scale liquids are classified as acid, alkaline or neutral: If a solution is neither acid nor alkaline it is called neutral. This corresponds on the scale to pH 7. Acid means a high activity of hydrogen ions and  pH lower than 7. Alkaline solutions are charaterized by a lower activity of the hydrogen ions respectively a higher activity of the hydroxide ions and  a pH above 7.  The grahic below illustrates with some examples the pH scale.
The pH scale is a logarithmic scale. The change of one unit means in fact a ten times increase or decrease of the activity of the hydrogen ions.  This explains why the corrosiveness with increasing distance from the neutral point is strongly growing.

For determination or measurement of the pH it is necessary to have suitable equipment. This is provided by electro chemical measurement systems, litmus paper, indicators and photometers. Accurate results for pH are only provided by the electro chemical methods. The appropriate sensor is the pH electrode.

The pH electrode is an electrochemical sensor which consists of a measurement electrode and a reference electrode.The measurement electrode is made of a special glass which is due to its surface properties very sensitive for hydrogen ions. It is filled with a buffer with pH 7. Immersing this electrode into the sample leads to a change of the voltage  at the measurement electrode in comparison to the reference electrode. This change will be detected and calculated by the meter directly into pH. Modern digital IDS electrodes do the raw signal processing directly in the sensor. This improves the accuracy and optimizes documentation.