chlorine residuals measurement · 2018-04-04 · 1 chlorine residuals measurement 1 terry...
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Chlorine Residuals Measurement
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Terry EngelhardtApplication Development Manager –Drinking Water Hach Company
• 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-
Reaction with Water
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HOCl vs. OCl-
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• Chlorine existing in water as hypochlorous acid (HOCl) or the hypochlorite ion (OCl-) is defined as free available chlorine
Free Available Chlorine
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• Salts used for chlorination include– Lithium hypochlorite LiOCl
LiOCl + H20 Li+ + HOCl + OH-
– Sodium hypochlorite NaOClNaOCl + H20 Na+ + HOCl + OH-
– Calcium hypochlorite Ca(OCl)2
Ca(OCl)2 + 2H20 Ca2+ + 2HOCl + 2OH-
Hypochlorite Salts
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• 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
Combined Chlorine - Chloramination
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Breakpoint Curve
To
tal C
hlo
rin
e R
es
idu
al
Chlorine Added
Bre
akpo
int
Monochloramine predominates
Dichloramine predominates
Free Residual
Cl2:N < 5:1 Cl2:N > 5:1 Cl2:N > 9:1
A B C
• 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
Breakpoint Curve Considerations
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• 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
Chloramination
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• Monochloramine - NH2ClNH3 + HOCl NH2Cl + H2O
• Dichloramine - NHCl2NH2Cl + HOCl NHCl2 + H2O
• Tricholoramine (Nitrogen Trichloride) - NCl3NHCl2 + HOCl NCl3 + H2O
• Chloramines are not as effective disinfectants as free chlorine
Chloramine Formation
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• Ammonia in solution as
– NH3 Free ammonia gas dissolved in water or;
– NH4- The ammonium ion
Definition of Unreacted Ammonia
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To
tal a
nd
Fre
e A
mm
on
ia
Chlorine Added
To
tal R
es
idu
al C
hlo
rin
e
Cl2:N <5:1 Cl2:N >5:1 Cl2:N >9:1
Monochloramine Formation
Free Ammonia
Total Ammonia Dichloramine Formation
Free Residual Chlorine
Breakpoint curve for chlorination and chloramination
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Calculate Ratio as Cl2:N!
Ammonia, NH3
Atomic mass
Atoms/ molecule
Nitrogen 14 X 1 = 14
Hydrogen 1 X 3 = 3
Molecular Weight (Mass)
= 17
Percent Hydrogen3 x 100
= 17.617
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If the feed rate is calculated on NH3 as NH3 instead of as N, the feed rate is off 17.6%!
Effect of pH on Chloramine Species
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0
10
20
30
40
50
60
70
80
90
100
5 6 7 8 9
pH
% D
ich
lora
min
e
0
10
20
30
40
50
60
70
80
90
100
% M
on
och
lora
min
e
• Distribution of chloramine species is effected by:
– pH
– Ammonia concentration (see breakpoint curve)
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Comparison of Methods
Method Range mg/l
Detection Level*
%RSD Use Skill
DPD colorimetric 0-5 0.005 1-2 F & T 1 Ultra low-range DPD colorimetric
0-0.500 0.002 5-6 T 2
DPD titration 0-3 0.018 2-7 F & T 2 Iodometric Up to
4% 1 NR
Total Oxidants
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Amperometric Titration - Forward
Up to 10 0.015 1-2 F & T 3
-Back 0.006-1 0.006 15 T 3
Electrode 0-1 0.05 10
Total Oxidants
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Monochlor-F W WW
0-4.5 0-10 0.09 2
Mono-chloramine 1
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Skill Level: 1= Minimal training; 3 = Experienced
*Under ideal laboratory conditions. Practical limit for all methods is really about 0.02
DPD-Chlorine Reaction Products
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N+
H HH
N+
HEtEt
+Cl2
N+
H HH
N+
EtEtN+
EtEt
N+
HH
AMINE (colorless)
WÜRSTER DYE (magenta colored)
IMINE (colorless)
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DPD Würster Dye Absorbance Curve
0.2500
0.1500
0.0500
400.00 440.00 480.00 520.00 560.00 600.00
Wavelength, nanometers
Abs
orba
nce
512 nm 553 nmMaximum sensitivity 510-515 nm
530 nm
Colorimetric Methods – Lab or Field Use
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�Chlorine – DPD
�Chloramination – MonoChlor F
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• Free residual measurement– Add sample to sample
cell– Blank– Add reagents– Read within 1 minute
• Total residual measurement– Add sample to sample
cell– Blank– Add reagents– Wait 3 minutes– Read within 3-5
minutes
Measuring Free and Total Residual
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Test Kits
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Compara-tors
Test Strips
Chlorine X X
MonoChloramine NA NA
Avoid use of color comparators for regulatory reporting due to subjective errors
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Common Interferences
• Other oxidants: ClO2, O3, Br2, H2O2, I2,
KMnO4
• Disinfection by-products, 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)
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Compensating for Manganese Interference
• Split sample. Analyze first portion as usual
• Second Portion:– Adjust pH w/1N
sulfuric acid– Add drops of 30 g/l
potassium iodide; wait one minute
– Add drops of 5 g/l sodium arsenite
– Add DPD and complete test
• Subtract result of second portion from first portion
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Sample Size 5 ml 10 ml 25 ml
H2SO4, 1N Adjust to pH 6-7
Adjust to pH 6-7
Adjust to pH 6-7
Potassium Iodide, 30 g/l
2 drops 2 drops 3 drops
Sodium Arsenite, 5 g/l
2 drops 2 drops 3 drops
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• For total chlorine determinations, KI is oxidized by chlorine andchloramines, at pH 4, to form tri-iodide:
• Then the two half reactions are :
• Stoichiometry is thus 2:2 (titrant : sample)
Chemistry of Amperometric Titration
−−−→+ IeI 32
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( ) −+++→+ eOHOHPhAsOOHPhAsO 224
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−+−++→+ ClKIKICl 233
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• 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
Forward Titration
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• 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
Forward Titration
Potential Applied
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• 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.
Forward Titration
+ -Chlorine reduced
(or iodine for total
chlorine determination)
PAO Oxidized
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• Amperometry– The more chlorine (or oxidant) in solution, the greater
the amount of current flow.
Forward Titration
Current
• Amperometry– As the PAO titrant is added, the PAO reduces the
chlorine, and the chlorine concentration decreases.
Forward Titration
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• Amperometry – As the chlorine concentration decreases, the amount
of current also decreases.
Forward Titration
Current
• Amperometry – When all of the chlorine has been reduced by the
PAO, the amount of current falls to near zero.
Forward Titration
Current
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• Amperometry– The chlorine concentration is calculated based on the
amount of PAO added to reduce the measured current to zero.
Forward Titration
Typical Amperometric Titration System
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Microampere meter
1.123
Magnetic
Stirrer
Titrant delivery system
Dual Platinum or
Silver/Platinum Electrode
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• Titration curves and calculations
Forward Titration
On-line Chlorine Measurement
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Colorimetric DPD – Free or Total Chlorine
Amperometric Probe – Free or Total Chlorine
Monochlor F –Monochloramine, Free and Total Ammonia
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Online Chlorine Monitoring –
Major Technologies
Colorimetric:
measuring intensity of color developed by reaction of chlorine with indicator (chemical compound, e.g. DPD). The deeper color, the higher chlorine concentration.
Main Differentiators:
• Independent of major sample parameters (pH, flow, temperature),
• Established calibration curve
Amperometric:
measuring electrical currentgenerated in a circuitry by reaction of chlorine with electrodes . The larger current value, the higher chlorine concentration.
Main Differentiators:
• No chemical reagents required
• Fast response to analyte concentration changes
Really?
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Online Chlorine Monitoring Comparison
Colorimetric Amperometric
Pros
•Accuracy - no calibration•Unattended operation (up to 30 days)•Predictable and simple maintenance•Results independent of changes in sample pH, temperature, conductivity, sample pressure
Pros
•Fast response
•Reagentless technology
•No waste stream?
Really?
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Online Chlorine Monitoring Comparison
Colorimetric Amperometric
Cons
•Reagents and waste stream management
Cons
•Greater influence from sample pH, temperature, flow, pressure, Cl2concentration, etc.
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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.
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CL17 Chlorine Analyzer Pocket Colorimeter
Use a portable colorimeter to verify operation of on-line chlorine analyzers. Do not use color comparitors
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%
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Terry EngelhardtHach CompanyApplication Development Manager –Drinking Water 800-227-4224 [email protected]
Contact Information
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N Illinois
Paul Gauger
Hach Company
800-227-4224 X2060
S Illinois
Brad Baldwin
Hach Company
800-227-4224 X2327