Properties Of Chlorine And Its Chemistry

Chlorine (Cl2) is a greenish-yellow-colored gas having a specific gravity of 2.48 as compared to air under standard conditions of temperature and pressure. It was discovered in 1774 from the chemical reaction of manganese dioxide (MnN02) and hydrochloric acid (HC1) by the Swedish chemist, Scheele, who believed it to be a compound containing oxygen. In 1810, it was named by Sir Humphrey Davy, who ins isted it was an element (from the Greek work chloros, meaning greenish-yellow). In nature, it is found in the combined state only, usually with sodium as salt (NaCl), carnallite (KMgCl36H20), and sylvite (Kcl).

Chlorine is a member of the halogen (salt-forming) group of elements and is derived from chlorides by the action of oxidizing agents and, most frequently, by electrolysis. As a gas, it combines directly with nearly all elements. At 10° C, 1 volume of water dissolves about 3.10 volumes of chlorine; at 30° C, only 1.77 volumes of Cl2 are dissolved in 1 volume of water.

In addition to being the most widely used disinfectant for water treatment, chlorine is extensively used in a variety of products, including paper products, dyestuffs, textiles, petroleum products, pharmaceuticals, antiseptics, insecticides, foodstuffs, solvents, paints, and other consumer products. Most chlorine produced is used in the manufacture of chlorinated compounds for sanitation, pulp bleaching, disinfectants, and textile processing. It is also used in the manufacture of chlorates, chloroform, and carbon tetrachloride and in the extraction of bromine. Among other past uses, chlorine served as a war gas during World War I. As a liquid, chlorine is amber colored and is 1.44 times heavier than water. In solid form, it exists as rhombic crystals. Various properties of chlorine are given in Table 2.

Chlorine gas is a highly toxic substance, capable of causing death or permanent injury due to prolonged exposures via inhalation. It is extremely irritating to the mucous membranes of the eyes and the respiratory tract. It will combine with moisture to liberate nascent oxygen to form hydrochloric acid. If both these substances are present in quantity, they can cause inflammation of the tissues with which they come in contact. Pulmonary edema may result if lung tissues are attacked. Chlorine gas has an odor detectable at a concentration as low as 3.55 ppm. Irritation of the throat occurs at 15 ppm. A concentration of 50 ppm is considered dangerous for even short exposures. At or above concentrations of 1,000 ppm, exposure may be fatal. Chlorine can also cause fires or explosions upon contact with various materials. Table 2.5 lists various substances chlorine can react with to create fire hazards. It emits highly toxic fumes when heated and reacts with water or steam to generate toxic and corrosive hydrogen chloride fumes.

Table 2. General properties of chlorine



(as gas)


Atomic Number


Atomic Weight


Melting Point (°C)


Boiling Point (°C)


Liquid Density (0° C and 3.65 atm; g/1)


Vapor Pressure (mmHg @ 20° C)


Vapor Density (@ STP: g/1)


Viscosity (micropoises) at

Temperature = 12.7° C


= 20°


= 50°


= 100°


= 150°


= 200°


In the United States, chlorine was first used as a disinfectant for municipal wastewater treatment in the Jersey City, New Jersey, Boonton reservoir in 1908. This also marked the first legal recognition of chlorine as a disinfectant for public health protection. Chlorine is a strong oxidizing agent and can be used to modify the chemical character of water. For example, it is used to control bacteria, algae, and macroscopic biological-fouling organisms in condenser cooling towers. It is also used to alter the chemical character of some industrial process waters, such as the destruction of sulfur dioxide and ammonia, the reduction of iron and manganese, and the reduction of color (examples include bleaching operations in the pulp and paper industry and oxidation of organic constituents). In water chlorine hydrolyzes to form hypochlorous acid (HOCL), as shown by the following reactions: Cl2 + H20 = HOC1 + H+C1"

The hypochlorous acid undergoes further ionization to form hypochlorite ions

Equilibrium concentrations of HOII and OC1 depend on the pH of the wastewater. Increasing the pH shifts the preceding equilibrium relationships to the right, causing the formation of higher concentrations of HOC1.

Chlorine may also be applied as calcium hypochlorite and sodium hypochlorite. Hypochlorites are salts of hypochlotous acid. Calcium hypochlorite (Ca(OCl)2)

represents the predominant dry form used in the United States. Calcium hypochlorite is commercially available in granular powdered or tablet forms. Either of these forms readily dissolves in water and contains approximately 70 percent available chlorine. Sodium hypochlorite (NaOCl) is commercially available in liquid form at concentrations typically between 5 percent to 15 percent available chlorine. Hypochlorites react in water as follows: NaOCl ->■ Na+OCr Ca(OCl)2 ^ Ca+2 + 20C1" H+ + OCL" ->■ HOC1

The amount of HOC1 plus OC1 in wastewater is referred to as the free available chlorine. Chlorine is a very active oxidizing agent and is therefore highly reactive with readily oxidized compounds such as ammonia. Chlorine readily reacts with ammonia in water to form chloramines.

HOC1 + NH3 -*- H20 + NH2C1 (monochloramine) HOC1 + NH2C1 ^ H20 + NHC12 (dichloramine) HOC1 + NHC12 ^ H20 + NC13 (trichloramine)

The specific reaction products formed depend on the pH of the water, temperature, time, and the initial chlorine-to-ammonia concentration ratio. In general, monochloramine and dichloramine are generated in the pH range of 4.5 to 8.5. Above pH 8.5, monochloramine usually exists alone. However, below pH 4.4, trichloramine is produced. When chlorine is mixed with water containing ammonia, the residuals developed produce a curve similar to the one shown in Figure 2. The positive sloped line from the origin represents the concentration of chlorine applied or the residual chlorine if all of that applied appears as residual. The solid curve represents chlorine residuals corresponding to various dosages that remain after some specified contact time. The chlorine demand at a specified dosage is obtained from the vertical distance between the applied and residual curves. Chlorine demand represents the amount of chlorine reduced in chemical reactions (that is, it is the amount that is no longer available). For molar chlorine to ammonia-nitrogen ratios below 1, monochloramine and dichloramine are formed with their relative amounts dependent on pH and other factors. When higher dosages of chlorine are added, the chlorine-to- nitrogen ratio increases, resulting in an oxidation of the ammonia and a reduction of the chlorine. Three moles of chlorine react with two moles of ammonia, generating nitrogen gas and reducing chlorine to the chloride ion:

Residuals of chloramine decline to a minimum value that is referred to as the breakpoint. When dosages exceed the breakpoint, free chloride residuals result. Breakpoint curves are unique for different water samples since the chlorine demand is a function of the concentration of ammonia, the presence of other reducing agents, and the contact time between chlorine application and residual testing.


Figure 2. Breakpoint chlorination curve.

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