Acidity (pH)

Definition

Acidity (pH) is numerically equal to the negative decimal logarithm of the activity of (a_H+) or the concentration [H⁺] of hydrogen ions (in gram ions per liter).

This concept was introduced in 1909 by the Danish chemist Sørensen. pH reflects the first letters of Latin words potentia hydrogeni – the power of hydrogen, or pondus hydrogenii – weight of hydrogen.

For low mineralized water, the difference between activity and concentration of hydrogen ions is not geochemically significant, but for high mineralized water the identification of activity and concentration is essential.

The introduction of pH as an indicator of acid-base properties of aqueous solutions was founded on the ability of water to dissociate into ions according to the scheme H₂O = H⁺+OH⁻. In connection with this reaction and using the concept of ionic product of water, K_W = a_H++ a_OH- (where K_{W -} ionic product of water, a_H++ and a_OH- activities of H⁺ and OH⁻ respectively). K_W at 22 °C is equal 10⁻¹⁴. If the...