1.principles
Measurement of pH is one of the most important and frequently used tests in water chemistry.
Practically every phase of water supply and wastewater treatment, e.g., acid-base neutralization, water softening, precipitation, coagulation, disinfection, and corrosion control, is pH-dependent. pH is used in alkalinity and carbon dioxide measurements and many other acid-base equilibria. At a given temperature the intensity of the acidic or basic character of a solution is indicated by pH or hydrogen ion activity. Alkalinity and acidity are the acid- and base-neutralizing capacities of a water and usually are expressed as milligrams CaCO3 per liter. Buffer capacity is the amount of strong acid or base, usually expressed in moles per liter, needed to change the pH value of a 1-L sample by 1 unit. pH as defined by Sorenson is –log [H+]; it is the “intensity” factor of acidity. Pure water is very slightly ionized and equilibrium the ion product is
[H+][OH-] = Kw
= 1.01 x 10-14 at 25 ๐C (1)
and
[H+] = [OH-]
= 1.005 x 10-7
where
[H+] = activity of hydrogen ions, moles/L,
[OH-] = activity of hydroxyl ions, moles/L, and
Kw = ion product of water.
Because of ionic interactions in all but very dilute solution, it is necessary to use the “activity” of an ion and not its molar concentration. Use of the term pH assumes that the activity of the hydrogen ion, aH + , is being considered. The approximate equivalence to molarity, [ H+ ] can be presumed only in very dilute solutions ( ionic strength