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
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