Chlorine Residuals Measurement
Terry Engelhardt
Application Development Manager –
Drinking Water
Hach Company
1
Reaction with Water
• Forms hydrochloric (HCl) and hypochlorous
(HOCl) acids:
Cl2 + H2O
HOCl + H+ + Cl• Reaction is reversible. Above pH 4, reaction is to
the right
• HOCl dissociates to the hydrogen ion and
hypochlorite ion (OCl-) varying with temperature
and pH
HOCl
H+ + OCl-
2
1
HOCl vs. OCl-
3
Free Available Chlorine
• Chlorine existing in water as hypochlorous acid
(HOCl) or the hypochlorite ion (OCl-) is defined
as free available chlorine
4
2
Hypochlorite Salts
• Salts used for chlorination include
– Lithium hypochlorite LiOCl
LiOCl + H20
Li+ + HOCl + OH– Sodium hypochlorite NaOCl
NaOCl + H20
Na+ + HOCl + OH– Calcium hypochlorite Ca(OCl)2
Ca(OCl)2 + 2H20
Ca2+ + 2HOCl + 2OH-
5
Combined Chlorine - Chloramination
• Chlorine (HOCl and OCl-) reacts with ammonia
to form chloramines, commonly referred to as
‘combined chlorine’
• The predominate species are monochloramine
and dichloramine. A small fraction is
trichloramine or nitrogen trichloride
6
3
Breakpoint Curve
Cl2:N > 5:1
Cl2:N > 9:1
Breakpoint
Total Chlorine Residual
Cl2:N < 5:1
Free Residual
Dichloramine
predominates
Monochloramine
predominates
A
B
C
Chlorine Added
Breakpoint Curve Considerations
• Shape of the curve is dependent upon
– amount of ammonia and other chlorine demand
substances in the water
– temperature
– pH
– contact time
• Most effective disinfection, least taste and odor
occurs with free residual chlorine
• Free chlorine may lead to formation of DBP
8
4
Chloramination
• Chloramination: Purposeful use of chlorine and
ammonia to form monochloramine.
– Minimizes formation of chlorinated organics
– Ammonia to chlorine Ratio is controlled to favor
formation of monochloramine, typically 5:1 Cl2:N
• Total residual chlorine test: All free and
combined chlorine species
9
Chloramine Formation
• Monochloramine - NH2Cl
NH3 + HOCl
NH2Cl + H2O
• Dichloramine - NHCl2
NH2Cl + HOCl
NHCl2 + H2O
• Tricholoramine (Nitrogen Trichloride) - NCl3
NHCl2 + HOCl
NCl3 + H2O
• Chloramines are not as effective disinfectants as
free chlorine
10
5
Definition of Unreacted Ammonia
• Ammonia in solution as
– NH3 Free ammonia gas dissolved in water or;
– NH4- The ammonium ion
11
Breakpoint curve for chlorination and chloramination
Cl2:N <5:1
Cl2:N >5:1
Total Residual Chlorine
Total and Free Ammonia
Total Ammonia
Cl2:N >9:1
Dichloramine
Formation
Free Ammonia
Monochloramine
Formation
Free Residual
Chlorine
Chlorine Added
12
6
Calculate Ratio as Cl2:N!
Atoms/
molecule
Ammonia, Atomic
NH3
mass
Nitrogen
Hydrogen
14
1
X
X
1
3
Molecular
Weight (Mass)
3 x 100
17
Percent Hydrogen
=
=
14
3
=
17
=
17.6
If the feed rate is calculated on NH3 as NH3 instead of as N,
the feed rate is off 17.6%!
13
100
0
90
10
80
20
70
30
60
40
50
50
40
60
30
70
20
80
10
90
0
100
5
6
7
8
% Monochloramine
% Dichloramine
Effect of pH on Chloramine Species
• Distribution of
chloramine
species is
effected by:
– pH
– Ammonia
concentration
(see breakpoint curve)
9
pH
14
7
Comparison of Methods
Method
DPD colorimetric
Ultra low-range
DPD colorimetric
DPD titration
Iodometric
Amperometric
Titration - Forward
-Back
Electrode
Monochlor-F
W
WW
Range
mg/l
Detection
Level*
%RSD
Use
Skill
0-5
0.005
1-2
F&T
1
0-0.500
0.002
5-6
T
2
0-3
Up to
4%
0.018
2-7
2
1
NR
F&T
Total
Oxidants
Up to 10
0.015
1-2
F&T
3
0.006-1
0.006
15
3
0-1
0.05
10
0-4.5
0-10
0.09
2
T
Total
Oxidants
Monochloramine
2
2
1
Skill Level: 1= Minimal training; 3 = Experienced
*Under ideal laboratory conditions. Practical limit for all
methods is really about 0.02
15
DPD-Chlorine Reaction Products
H H H
N+
H H H
N+
Cl2
Et
N+
Et
H
H
N+
+
Et
N+
Et
Et
N+
Et
H
AMINE
(colorless)
WÜRSTER DYE
(magenta colored)
IMINE
(colorless)
16
8
DPD Würster Dye Absorbance Curve
Maximum sensitivity
510-515 nm
530 nm
512 nm
553 nm
Absorbance
0.2500
0.1500
0.0500
400.00
440.00
480.00
520.00
560.00
600.00
Wavelength, nanometers
Colorimetric Methods – Lab or Field Use
Chlorine – DPD
Chloramination – MonoChlor F
18
9
Measuring Free and Total Residual
• Free residual
measurement
• Total residual
measurement
– Add sample to sample
cell
– Blank
– Add reagents
– Read within 1 minute
– Add sample to sample
cell
– Blank
– Add reagents
– Wait 3 minutes
– Read within 3-5
minutes
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Test Kits
Compara Test
-tors
Strips
Chlorine
X
X
MonoChloramine
NA
NA
Avoid use of color comparators for
regulatory reporting due to
subjective errors
20
10
Common Interferences
• Other oxidants: ClO2,
O3, Br2, H2O2, I2,
KMnO4
• Disinfection byproducts, I.e. chlorite
and chlorate
• Particulate
contamination turbidity
•
•
•
•
•
Buffer capacity
Sample color
Mn+3 to Mn+7
Cr +7
Organic N-Cl (organic
chloramines in
wastewater)
21
Compensating for Manganese Interference
• Split sample. Analyze • Subtract result of
first portion as usual
second portion from
• Second Portion:
first portion
– Adjust pH w/1N
Sample
sulfuric acid
Size
5 ml
10 ml
25 ml
– Add drops of 30 g/l
Adjust to Adjust to Adjust to
H2SO4, 1N
potassium iodide;
pH 6-7 pH 6-7 pH 6-7
wait one minute
Potassium
Iodide,
2 drops 2 drops 3 drops
– Add drops of 5 g/l
30
g/l
sodium arsenite
Sodium
– Add DPD and
Arsenite,
2 drops 2 drops 3 drops
complete test
5 g/l
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11
Chemistry of Amperometric Titration
•
For total chlorine determinations, KI is oxidized by chlorine and
chloramines, at pH 4, to form tri-iodide:
Cl2 + 3KI → I 3− + 3K + + 2Cl −
•
Then the two half reactions are :
I 3− + 2e − → 3I −
•
Stoichiometry is thus 2:2 (titrant : sample)
PhAsO + 4 H 2 O → PhAsO(OH )2 + 2 H 3O + + 2e −
Forward Titration
• Amperometry
– Electrochemical technique in which a small electrical
voltage is applied across two electrodes
– Chemical reactions caused by titrant addition cause a
change in current, which is measured and recorded
by the instrument
12
Forward Titration
• Amperometry
– Results are obtained by calculating the current
change as a function of the amount of titrant added
Forward Titration
• Amperometry
– A potential is applied across the electrodes prior to
the titration.
– Buffer is added to the sample and KI is added to total
chlorine samples
Potential Applied
13
Forward Titration
• Amperometry
– Current can flow as long as there is a substance that
can be reduced at the cathode (+) and oxidized at the
anode (-).
+
-
Forward Titration
• Amperometry (for free residual chlorine)
– Chlorine is titrated with PAO titrant. The chlorine is
reduced at the cathode. The PAO is oxidized at the
anode.
+
Chlorine reduced
(or iodine for total
chlorine determination)
-
PAO Oxidized
14
Forward Titration
• Amperometry
– The more chlorine (or oxidant) in solution, the greater
the amount of current flow.
Current
Forward Titration
• Amperometry
– As the PAO titrant is added, the PAO reduces the
chlorine, and the chlorine concentration decreases.
15
Forward Titration
• Amperometry
– As the chlorine concentration decreases, the amount
of current also decreases.
Current
Forward Titration
• Amperometry
– When all of the chlorine has been reduced by the
PAO, the amount of current falls to near zero.
Current
16
Forward Titration
• Amperometry
– The chlorine concentration is calculated based on the
amount of PAO added to reduce the measured
current to zero.
Typical Amperometric Titration System
Titrant
delivery
system
Dual Platinum
or
Silver/Platinum
Electrode
Microampere
meter
1.123
Magnetic
Stirrer
34
17
Forward Titration
• Titration curves and calculations
On-line Chlorine Measurement
Amperometric
Probe – Free or
Total Chlorine
Colorimetric
DPD – Free or
Total Chlorine
Monochlor F –
Monochloramine,
Free and Total
Ammonia
36
18
Online Chlorine Monitoring –
Major Technologies
Colorimetric:
Amperometric:
measuring intensity of color developed
by reaction of chlorine with indicator
(chemical compound, e.g. DPD). The
deeper color, the higher chlorine
concentration.
measuring electrical current
generated in a circuitry by reaction of
chlorine with electrodes . The larger
current value, the higher chlorine
concentration.
Main Differentiators:
Main Differentiators:
• Independent of major sample
parameters (pH, flow, temperature),
• No chemical reagents required
• Established calibration curve
• Fast response to analyte
concentration changes
Really?
37
Online Chlorine Monitoring Comparison
Colorimetric
Amperometric
Pros
Pros
•Accuracy - no calibration •Fast response
•Unattended operation
•Reagentless technology
(up to 30 days)
•No waste stream?
•Predictable and simple
maintenance
•Results independent of
changes in sample pH,
Really?
temperature, conductivity,
sample pressure
38
19
Online Chlorine Monitoring Comparison
Colorimetric
Cons
•Reagents and waste
stream management
Amperometric
Cons
•Greater influence from
sample pH, temperature,
flow, pressure, Cl2
concentration, etc.
39
Keys to Application Success
Steps to choosing your chlorine analyzer:
1. Look at the instrument's major performance
specifications to make your initial decision.
• Chlorine concentration range
• Sample pH range
2. Next, consider each technology's key differentiators to
determine which is preferred for your application.
• Colorimetric
• Amperometric
3. Finally, consider the treatment process details - key to
application success to make sure that your preferred
instrument is right for your application.
40
20
CL17 Chlorine Analyzer
Pocket Colorimeter
Use a portable colorimeter
to verify operation of online chlorine analyzers. Do
not use color comparitors
41
Monitoring Hypochlorite and Aqua
Ammonia Bulk Solutions
• Know what you’re buying
• Know the concentration
being used
• Digital Titration or drop
count (Bleach only)
• 5-15%
42
21
Contact Information
N Illinois
S Illinois
Paul Gauger
Brad Baldwin
Hach Company
Hach Company
800-227-4224 X2060
800-227-4224 X2327
[email protected]
[email protected]
Terry Engelhardt
Hach Company
Application Development Manager –
Drinking Water
800-227-4224 X2327
[email protected]
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