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.

Chlorine added to Water

• 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)

O

H HClCl

O

H ClClH

OCl

Na

O

H H

O

H Cl

Na

H

O

water + chlorine gas hypochlorous acid + hydrogen ion +chloride ion

water + sodium hypochlorite hypochlorous acid + sodium hydroxide

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OC l Hi

[ ]

[ ][ ]

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OC l Hi

[ ]

[ ][ ]

K i [ ]

[ ][ . ]

4

4 0 5

4

22

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OC l Hi

[ ]

[ ][ ]

Decrease 1 pH unit

K i [ ]

[ ][5].

4

4

4

2 00 0 2 2

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OC l Hi

[ ]

[ ][ ]

K i [ ]

[ ][5].

6

2

6

1 00 6 2

Decrease 1 pH unit

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OCL Hi

[ ]

[ ][ ]

K i [ ]

[ ][5].

7

1

7

51 4 2

Decrease 1 pH unit

Dissociation of HOCl

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

O

H ClH

OCl

KHOC l

OC l Hi

[ ]

[ ][ ]

K i [ . ]

[ . ][5]

.

.

7 2 7 3

0 7 2 7

7 2 7 3

3 6 3 52

Decrease 1 pH unit

Percent HOCl vs pH in Water vs pH @ 15oC

0

10

20

30

40

50

60

70

80

90

100

5 6 7 8 9 10

pH (negative log of [H+])

% H

OC

l (h

ypo

chlo

rou

s ac

id)

KHOC l

OCL Hi

[ ]

[ ][ ]

decrease pH 1 unit

Increase [H+]x10

Percent HOCl vs pH in Water vs pH @ 15oC

0

10

20

30

40

50

60

70

80

90

100

5 6 7 8 9 10

pH (negative log of [H+])

% H

OC

l (h

ypo

chlo

rou

s ac

id)

zero celcius

30oC

KHOC l

OCL Hi

[ ]

[ ][ ]

decrease pH 1 unit

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