1-11. ACID-BASE POOL CHEMISTRY
a. Ion. An ion is an electrically charged particle. When a compound separates
into ions, it is said to ionize. The process of ion formation (ionization) is critical in acid-
base pool chemistry.
b. Acid. An acid is a compound that ionizes to produce an excess of hydrogen
ions. A base is a compound that ionizes to produce an excess of hydroxyl ions. A
neutral compound ionizes to produce an equal number of hydrogen and hydroxyl ions
and is neither an acid nor a base. Water is an example of a neutral compound.
c. pH. The symbol pH is an expression of how acidic or alkaline (basic) a
substance is. The values for pH range from 0 (very acidic) to 14 (very basic). The pH
values relate inversely to the hydrogen ion concentration of a solution: the higher the
pH value, the lower the hydrogen ion concentration and the less acid and the more
basic is the substance. Pure water has a pH value of 7, which is neutral. A pH value
below 7 indicates an acid solution, and a pH value above 7 indicates a basic solution.
d. Effect of pH on Swimming Pools. Swimming pools must maintain a slightly
basic pH value between 7.2 and 8.4. The pH is not a measure of the amount of acid or
base in a solution but measures only how much is ionized. Thus, a solution could
contain a large amount of a unionized compound that would not affect the pH until it is
ionized. The pH of swimming pool water affects pool operation in several ways. In
particular it has an effect on the efficiency of chlorine, which is added to pool water as a
(1) Effect on bacterial kill rate. The percent of chlorine that remains in pool
water in its most effective form for killing bacteria depends upon the pH. As the pH
value rises and the water becomes more basic, chlorine becomes less effective as a
disinfectant. Refer to paragraph 1-12e for a more detailed discussion of this
(2) Eye irritation. Generally, a pool with a pH of a 7.6 to 8.0 causes the
least eye irritation. If the water is too acidic or too basic, eye irritation results.
(3) Corrosion of pipes and deterioration of mortar. If the pH is allowed to
remain at 7.0 or below, the water can corrode metal pipes and can act as an acid in
dissolving mortar and concrete in the pool walls.
(4) Algae growth. Water with a low pH may encourage growth of algae. A
pH of 8.0 will tend to inhibit this growth. Maintaining required chlorine levels would also
inhibit the growth of algae.