Chlorine has the electronic configuration of [Ne]3s 3p and is located in Group VIIA of the Periodic Table in the third period. [Ne] means that this element has the electronic configuration of the noble gas neon. The letters p and s refer to the p and s orbitals; the superscripts indicate the number of electrons that the orbitals contain. Thus, the p orbital contains 5 electrons and the 5 orbital contains two electrons, making a total of seven electrons in its valence shell. This means that the chlorine atom needs to acquire only one more electron to attain the neon configuration for stability. This makes chlorine a very good oxidizer. In fact, it is a characteristic of Group VIIA to attain a charge of -1 when the members of this group oxidizes other substances. The members of this group starting from the strongest oxidizer to the least are fluorine, chlorine, bromine, iodine, and astatine. This group forms the family of elements called the halogen family.

All the chlorine disinfectants reduce to the chloride ion (Cl-) when they oxidize other substances, which must, of course, be reducing substances. The chlorine starts with an oxidation state of zero and ends up with a -1; it only needs one reduction step. One the other hand, the hypochlorites start with oxidation states of +1 and end up with also a -1; thus, they need two reduction steps. Because the chlorine atom only needs one reduction step, while the hypochlorites need two, the chlorine atom is a stronger oxidizer than the hypochlorites. As a stronger oxidizer, it is also a stronger disinfectant.

Hydrolysis and optimum pH range of chlorination. As previously mentioned, chlorine is supplied in the form of liquefied chlorine. The liquid must then be evaporated into a gas. As the gas, Cl^, is applied into the water or wastewater, it dissolves into aqueous chlorine, Cl2(aq), as follows:

Cl2(aq) then hydrolyzes, one of the chlorine atoms being oxidized to +1 and the other reduced to -1. This reaction is called disproportionation. The reaction is as follows:

Cl2(aq) + H2O ^ HOCl + H+ + Cl-1 Kh = 4.0( 10-4) (17.9)

From Equation (17.9), the hypochlorous acid, HOCl, is formed, which is one of the chlorine disinfectants. If its formula is analyzed, it will be found that the chlorine has an oxidation state of +1, as we mentioned before. Note also that hydrochloric acid is formed. This is a characteristic in the use of the chlorine gas as a disinfectant. The water becomes acidic. Also, as we have mentioned, the chlorine molecule is a much stronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant. From Equation (17.9), if the water is intentionally made acidic, the reaction will be driven to the left, producing more of the chlorine molecule. This condition will then produce more disinfecting power. As will be shown later, however, this condition, where the chlorine molecule will exist, is at a very low pH hovering around zero. This makes the chlorine molecule useless as a disinfectant.

HOCl further reacts to produce the following dissociation reaction:

Using Equation (17.9), let us calculate the distribution of Cl2(aq) and HOCl. Expressing in the form of equilibrium equation,

Taking logarithms, rearranging, and simplifying,

[Cl2(aq)] _ 10<PKH-pH+!og[CT1]} _ {3.40-pH+log[Cl-1]} (1712)

pKH is the negative logarithm to the base 10 of KH.

Table 17.3 shows the ratios of [Cl2(aq)]/[HOCl] and [HOCl]/[Cl2(a?)] as functions of pH and the chloride concentrations, using Equation (17.12). The concentration of 1.0 gmmole/L of chloride is 35,500 mg/L. This will never be encountered in the normal treatment of water and wastewater. Disregarding this entry in the table, the


as Functions pH

and Chloride Concentration

[CI-] (gmmoles/L)

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