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

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

19

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 22

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] 43

22