Disinfection - Public Water Systems - page 619

Forms of Chlorine

Chlorine Gas. Upon addition to water, chlorine gas reacts rapidly with water to form
hypochlorous acid (HOCl) per the following reaction:

Cl HO ⇒ HOCl H Cl (19-1)

2(g) 2

Chlorine gas addition to water produces a hydrogen ion that reduces the pH of the
water.
Hypochlorous acid is a weak acid that dissociates slightly into hydrogen and hypo-
chlorite (OCl ) ions:

HOCl ⇔ H

OCl

(19-2)

This dissociation is incomplete between a pH of 6.5 and 8.5, resulting in the presence
of both HOCl and OCl species. Very little dissociation of HOCl occurs below a pH
of 6.5, while complete dissociation to OCl occurs above a pH of 8.5. Because the
disinfection efficacy of HOCl is significantly higher than that of OCl , chlorination at
lower pH is preferred for disinfection.

Sodium Hypochlorite. Bulk commercial sodium hypochlorite solution typically con-
tains 12.5 percent available chlorine. One gallon of 12.5 percent sodium hypochlorite
solution typically contains the equivalent of one pound of chlorine.
Sodium hypochlorite is formed by dissolving chlorine gas in a sodium hydroxide
solution. The addition of sodium hypochlorite solution to water results in the formation
of HOCl similar to chlorine. The reaction is shown in the following expression:

NaOCl HO ⇒ HOCl Na

OH

(19-3)

2

Note that the addition of sodium hypochlorite to water yields a hydroxyl ion (OH )
that increases the pH of the water, as opposed to chlorine addition, which decreases
pH. In addition, the pH of the water is further increased due to excess sodium hy-
droxide used to manufacture sodium hypochlorite.
Sodium hypochlorite degrades over time. The sodium hypochlorite solution stability
depends on the hypochlorite concentration, the storage temperature, and the length of
storage time before use. The degradation of sodium hypochlorite is catalyzed by the
impurities of the solution and exposure to sunlight. Sodium hypochlorite decomposi-
tion affects the feed rate and dosage over time.
Dilute sodium hypochlorite solutions (0.8 percent) can be generated electrochemi-
cally on-site from salt brine solution (see Chapter 24, ''Chemical Storage and Feeding
Systems,'' for details). At this dilute concentration, the sodium hypochlorite is more
stable than the bulk 12.5 percent solution.

Calcium Hypochlorite. Calcium hypochlorite is supplied in solid form and typically
contains 65 to 70 percent available chlorine. The addition of calcium hypochlorite to
water also results in the formation of HOCl. The reaction is shown in the following
expression: