Term used to describe hydrogen-ion activity. It is equal to -log a_H⁺ here a_H⁺ is the activity of the system. The pH-value is an indicator, for example, for the way an aqueous solution reacts:

Aqueous solutions with pH < 0 are called high-acid solutions, those with pH > 14 are high-alkaline. Examples of pH-values at temperatures of around 25°C:

The pH-value is also conditional on temperature. For example, calcium hydroxide, Ca (OH)₂, has a pH-value of 13.4/12.6/12.0/11.5 at temperatures of 0/20/40/60 °C. Specification of the process medium's pH-value provides valuable pointers for the choice of stable materials. As is clear from the above table, even small additions of strong acids or alkalines can cause a medium's pH-value to fall or rise appreciably. For example, adding just 0.365 g/l HCI to any alkaline solution with pH 8 will reduce the pH-value to 3, turning the weak alkaline solution into a strong acid one. Let us assume that in the chosen example it is planned to use a single mechanical seal with hard face materials because the solution also contains solids. If the enquirer considered the HCI content of a "mere" 0.03% to be negligible and omitted it therefore from his specification, the seal manufacturer might recommend using a single-acting mechanical seal with seal face and stationary seat made of tungsten carbide,

soldered on a carrier made of 1.4122,  with secondary seals made of EP rubber and the remaining metal parts made of Cr(Ni) steel. A mechanical seal with this material selection is bound to fail prematurely because the face materials and the other metal parts will corrode in the strong acid solution. If the enquirer specifies the process medium correctly, including details of temperature, concentration, pH-value and the other operating conditions, the seal manufacturer could recommend using a single-acting mechanical seal with seal face and stationary seat made of silicon carbide, secondary seals made of fluorocarbon rubber and metal parts made of Hastelloy C-4. This fictitious but realistic example indicates the great influence which the pH-value can exert on the material recommendation.