Water treatment with chlorine (opinion)
The article discusses the interaction of chlorine with water. What substances are formed and how they depend on the pH value of the water.
Chlorine is a diatomic gas Cl2 under normal conditions. When chlorine interacts with water, a reaction of formation of hypochlorous and hydrochloric acid occurs.
Cl2 + H2O = HClO + HCl (1)
Hypochlorous acid, in turn, dissociates into hydrogen and hypochlorite ion.
HClO = H+ + ClO– (2)
These processes depend on the pH of the water. During the formation of hypochlorous acid, hydrochloric acid is formed and thereby the pH of the water decreases. The more hydrochloric acid, the slower the process of interaction of chlorine with water. At a certain pH value, equilibrium is achieved. And if acid is added to the water, the reverse process begins to occur. Chlorine begins to be released from the water. But if the hydrate compensates for the acids formed, then the process of interaction of chlorine with the formation of hypochlorite ion will continue until the complete transition of chlorine into hypochlorite ion.
This can be illustrated using graphs of the dependence of the forms of chlorine content in water on the pH value of water. Figure 1 shows this graph.
They were built in the following way.
The chlorine dissociation constant according to equation (1) is 0.788 *10-3 (at 25 C0). The dissociation constant of hypochlorous acid is 5*10-8 (at 25 C0). Accordingly, the equations it is possible to write are as follows.
Where,
3.1 = – log (0,788*10-3)
7.3 = – log (5*10-8)
According to these equations, we can calculate the pH values depending on different values of chlorine, hypochlorous acid and hypochlorite ion. Graphs of these equations are shown in Figure 1.
Figure 1 Forms of chlorine in water depending on the pH value
Now it is clearly possible to trace in what forms chlorine is in water, depending on its pH value.
Working with materials on the topic of disinfection of water with chlorine, it can be concluded that the greatest disinfecting effect is manifested for the pH range of treated water from 7.0 to 7.6. Although it is indicated that only the hypochlorite ion (ClO-) exhibits disinfecting properties, which begins to appear in treated water from pH = 5.2 and higher. Following this logic, it can be concluded that the higher the pH of the water, the higher the disinfection effect. But practical application shows that the greatest disinfection effect is achieved in the pH range from 7.0 to 7.6.
Why can this be the case?
On the graph, you can see that the pH value = 7.3 corresponds to the same percentage of hypochlorous acid and hypochlorite in the water, i.e. exactly half of the acid was dissociated with the formation of hypochlorite. It is indicated that hypochlorous acid exhibits greater disinfecting properties compared to hypochlorite. Which is quite strange, given the fact that hypochlorite releases atomic oxygen, which is the strongest oxidizer. Or it is also suggested that the hypochlorite ion in an alkaline medium decomposes rapidly to form gaseous oxygen, and because of this, the bactericidal effect decreases with an increase in the pH of the water. Probably, both processes take place, and therefore the presence of hypochlorite ion and hypochlorous acid in water in approximately the same amounts ensures maximum efficiency of the disinfection process. It should also be added that increasing the pH of water contributes to the oxidation of organic acids. Probably, in this case, a significant part of atomic oxygen can be spent on the oxidation of organic matter, and therefore a greater consumption of active chlorine and a longer reaction time will be required. At lower pH values, organic matter is more stable and most of the atomic oxygen of hypochlorite will be spent on disinfection of water.
In any case, taking into account all possible opinions, an approximately equal balance between hypochlorous acid and hypochlorite turns out to be the most effective for water disinfection purposes. This balance is achieved when the pH of the water is in the range of 7.0 – 7.6.
