CHLORINE 143
4. CHEMICAL AND PHYSICAL INFORMATION
4.1 CHEMICAL IDENTITY
Information regarding the chemical identity of chlorine is located in Table 4-1. This information includes
synonyms, chemical formula and structure, and identification numbers. For the purpose of
disambiguation, terms that are commonly used in reference to chlorinated water are defined in Table 4-2.
4.2 PHYSICAL AND CHEMICAL PROPERTIES
Information regarding the physical and chemical properties of chlorine is located in Table 4-3.
Chlorine (Cl
2
) is a heavier-than-air, greenish-yellow gas with a pungent, irritating odor (HSDB 2009).
The odor threshold for chlorine in air is generally between 0.2 and 0.4 ppm (Amoore and Hautala 1983;
The Chlorine Institute 1998). Perceivable sensory irritation occurs at 1.0 ppm in air (EPA 1999).
Chlorine is a nonflammable gas; however, it is a very strong oxidizing agent, reacting explosively or
forming explosive compounds or mixtures with many common chemicals (O'Neil et al. 2001). Chlorine
reacts directly with nearly all of the elements to form chlorides (Lide 2005; O'Neil et al. 2001). Chlorine
is stored and transported as a liquid in pressurized containers (EPA 1999). Chlorine is transported as
either a liquid or a gas through pipelines within chemical plants or over distances of several kilometers
(Schmittinger et al. 2006).
Chlorine hydrolyzes rapidly and almost completely in water to form hydrochloric acid, hypochlorous
acid, and hypochlorite as follows:
Cl
2
+ H
2
O
HOCl + H
+
+ Cl
-
H
+
+ OCl
-
HOCl
The equilibrium constants for these reactions are represented by:
[ HOCl ][H
+
][Cl
−
]
K
1
=
[ Cl
2
]
(1)
CHLORINE 144
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-1. Chemical Identity of Chlorine
a
Characteristic
Information
Chemical name
Chlorine
Synonyms/trade names
Chlorine gas
b
, Bertholite, molecular chlorine, chlorine mol, dichlorine
Chemical formula Cl
2
Chemical structure
Cl Cl
Identification numbers:
CAS registry 7782-50-5
NIOSH RTECS FO2100000
c
EPA hazardous waste
No data
DOT/UN/NA/IMCO shipping UN1017; IMO 2.0
HSDB
206
EINECS 231-959-5
c
NCI No data
a
All information obtained from HSDB 2009 except where noted.
b
EPA 1999
c
IPCS 1996
CAS = Chemical Abstracts Service; DOT/UN/NA/IMO = Department of Transportation/United Nations/North
America/International Maritime Dangerous Goods Code; EINECS = European Inventory of Existing Commercial
Chemical Substances; EPA = Environmental Protection Agency; HSDB = Hazardous Substances Data Bank; NCI =
National Cancer Institute; NIOSH = National Institute for Occupational Safety and Health; RTECS = Registry of Toxic
Effects of Chemical Substances
CHLORINE 145
4. CHEMICAL AND PHYSICAL INFORMATION
Table 4-2. Commonly Used Terms Related to Chlorinated Water
Term
Meaning
Chlorinated water
The solution that results when molecular chlorine or a hypochlorite salt is
added to water for the purpose of water disinfection. Molecular chlorine
reacts rapidly with water under environmental conditions to form
hypochlorous acid, hypochlorite, and hydrochloric acid; therefore,
chlorinated water does not contain molecular chlorine under normal
conditions. Some other substances that are commonly formed in
chlorinated water include oxidized inorganics, chloramines, and
trihalomethanes.
Free chlorine
The combination of the equilibrium species molecular chlorine,
hypochlorous acid, and the hypochlorite ion in chlorinated water. Since
molecular chlorine is usually not present in water samples, this term
usually refers to the amount of hypochlorous acid and hypochlorite in
water.
Combined chlorine
The amount of chloramines (chlorine combined with nitrogen) present in
chlorinated water.
Total chlorine
The amount of free chlorine (hypochlorous acid and hypochlorite) plus
combined chlorine (chloramines) present in chlorinated water.
Available chlorine
A measure of the oxidizing strength of a solution. It is equal to the
amount of molecular chlorine that when added to water would produce a
solution with equivalent oxidizing power. It is commonly reported as
weight percent.
Residual chlorine
The amount of free chlorine remaining in a chlorinated water sample that
has been collected at a point of use. This indicates whether the water
has retained its disinfection properties.
Aqueous chlorine
A term that usually has a meaning similar to that of free chlorine. In this
sense, “aqueous chlorine” should not be misunderstood as the amount of
molecular chlorine in water (aqueous molecular chlorine). The term
“aqueous chlorine” is commonly used in reference to a prepared aqueous
solution of hypochlorite and hypochlorous acid.
Sources: APHA 1998a; Edstrom Industries 2003; Fukayama et al. 1986; IARC 1991; The Chlorine Institute 2006;
Westerhoff et al. 2004; WHO 2007.
CHLORINE 146
4. CHEMICAL AND PHYSICAL INFORMATION
Property Information
Molecular weight 70.905
Color
Greenish-yellow
Physical state Gas
Melting point
-101.00 °C
Boiling point
-34.04 °C
Density in air
2.482 (air=1)
b
Density, as liquid
20 °C, 6.864 atm
1.4085 g/mL
c
-35 °C, 0.9949 atm
1.5648 g/mL
c
Odor Pungent, irritating
Odor threshold:
Water
Not applicable
d
Air 0.2–0.4 ppm
c,e,f
Solubility:
Water
14.6 g/L at 0 °C; 7.3 g/L at 20 °C
c,g
Other solvents
Glacial acetic acid, dimethylformamide, nitrobenzene, phosphoryl
chloride, carbon tetrachloride, tetrachloroethane, pentachloroethane,
hexachlorobutadiene, and chlorobenzene
h
Partition coefficients:
Log K
ow
Not applicable
Log K
oc
Not applicable
Vapor pressure at 25 °C 5,830 mm Hg
Henry's law constant
1.17x10
-2
atm-m
3
/mol
i
Autoignition temperature
Not applicable
Flashpoint Not applicable
Reactivity
Strong oxidizer; reacts explosively with many materials
Conversion factors 1 ppm=2.9 mg/m
3
; 1 mg/m
3
=0.344 ppm
c
a
All information obtained from HSDB 2009, except where noted.
b
O'Neil et al. 2001
c
EPA 1999
d
Amoore and Hautala (1983) reported an odor threshold of 0.002 ppm for chlorine in water; however, these authors
state that this solution lacks enough persistence for this value to be used for reference purposes.
e
Amoore and Hautala 1983
f
CI 1998
g
EPA 1994b
h
Schmittinger et al. 1996
i
CHLORINE 147
4. CHEMICAL AND PHYSICAL INFORMATION
(2)
The relative percentage of Cl
2
, HOCl, and OCl
-
at some fixed concentration of Cl
-
can be expressed as:
(3)
(4)
(5)
Using the expressions for the equilibrium constants in Equations 1 and 2 and the relationship that pH is
equivalent to the negative logarithm of the hydronium ion concentration, Equations 3–5 can be re-written
as:
(6)
1+ 10 + 10
[Cl
−
] [Cl
−
]
(7)
−
1
%OCl =
−
(8)
[Cl ] 1
1+ +
2 pH pH
K K
1 2
10 K
2
10
[ H
+
][OCl
−
]
K
2
=
[ HOCl ]
][][][
][
%
2
2
2
−
++
=
OCl HOCl Cl
Cl
Cl
][][][
][
%
2
−
++
=
OCl HOCl Cl
HOCl
HOCl
][][][
][
%
2
−
−
−
++
=
OCl HOCl Cl
OCl
OCl
pHpH
KKK
Cl
2
211
2
1
% =
pH
pH
K
K
Cl
HOCl
10
10
][
1
1
%
2
1
++
=
−
Figure 4-1 illustrates the speciation as a function of pH using values for K
1
= 3.9x10
-4
M
2
(Cotton et al.
1999; Farr et al. 2003) and K
2
= 2.9x10
-8
M (Farr et al. 2003) at 25 °C. This figure shows the
CHLORINE 148
4. CHEMICAL AND PHYSICAL INFORMATION
Figure 4-1. Speciation of Cl
2
, HOCl, and OCl
-
as a Function of pH
100
90
80
70
60
50
Cl2
HOCl
40
OCl-
30
20
10
0
pH
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
% Species
CHLORINE 149
4. CHEMICAL AND PHYSICAL INFORMATION
pH-dependant relationship between molecular chlorine, hypochlorous acid, and hypochlorite in aqueous
solution. Sodium hypochlorite bleach solutions typically have a pH of 11–13 (The Chlorine Institute
2006). As illustrated in Figure 4-1, the addition of acid to a hypochlorite solution (e.g., mixing of sodium
hypochlorite bleach with acid drain cleaner) can drive the pH low enough to result in the release of
dangerous amounts of molecular chlorine gas.