1. Hypochlorous Acid and Dissociation
Review
Hypochlorous acid “looks” like water
Is the primary disinfectant in used
In this lecture we’ll discuss the importance of pH in the disinfection process.
This will tie directly into the effectiveness of disinfection in a water treatment plant.

2. Chlorine added to WaterCl O H Cl H Cl
water chlorine gas hypochlorous acid + hydrogen ion +chloride ion Na H O O Cl Na O O H Cl H H
water + sodium hypochlorite hypochlorous acid + sodium hydroxide
Hypochlorous acid is the primary disinfectant used in drinking water
Chlorine gas added to water causes a decrease in pH (increase in hydrogen ions)
The chlorine gas reaction is extremely fast (rate constant = 5x1014)
Sodium Hypochlorite added to water increases pH (sodium hydroxide)

3. Dissociation of HOCl This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter

4. Dissociation of HOCl This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter

5. Dissociation of HOCl Decrease 1 pH unit This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
WRONG!
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter
Decrease 1 pH unit

6. Dissociation of HOCl Decrease 1 pH unit This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
STILL
WRONG!
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter
Decrease 1 pH unit

7. Dissociation of HOCl Decrease 1 pH unit This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
GETTING
CLOSE!
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter
Decrease 1 pH unit

8. Dissociation of HOCl Decrease 1 pH unit This reaction is pH dependent
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion
HOCl H OCL-
This reaction is pH dependent
The ionization constant is:
Balanced!
The value of Ki varies with temperature
At 15oC, Ki is 2.32*10-8 moles/liter
Decrease 1 pH unit

9. Increase [H+]x10
decrease pH 1 unit

10. decrease pH 1 unit

11. pH is critical in disinfection
The concentration of HOCl for a given measured free chlorine residual is 80% at pH 7, and decreases to only 30% at pH 8.
At pH 6, 98% of the free chlorine residual is hypochlorous acid.
Disinfection is far more efficient at low pH (6.5) as compared to higher pH (8).
Temperature has an effect as well…but it is offsetting with disinfection mechanism