This document discusses chlorine measurement techniques for water disinfection. It explains that chlorine reacts with water to form hypochlorous acid (HOCl) and hydrochloric acid. HOCl is a stronger disinfectant than hypochlorite ion (OCl-), and its effectiveness depends on pH. The document describes colorimetric and amperometric methods for measuring free chlorine. It also discusses challenges like pH dependence and how to measure total chlorine. Common applications of chlorine measurement in water treatment plants and distribution systems are outlined.

1. CHLORINE MEASUREMENT
Praful Hanmante

2.  Most commonly used for
disinfection of water.
CHLORINE AS DISINFECT

3. Cl2 when applied to water  forms hypochlorous acid and
hydrochloric acid
Cl2 + H2O ↔ HOCl + H+ +Cl- [pH dependent reaction]
Ca(OCl)2 + 2H2O ↔ 2HOCl + Ca(OH)2
NaOCl + H2O ↔ HOCl+NaOH
HOCl = Hypochlorous Acid
Ca(OCl)2 = Calcium hypochlorite
Ca(OH)2 = Calcium Hydroxide
NaOCl = Sodium hypochlorite

4. HOCl ↔ H+ + OCl- [pH dependent reaction]
 Killing effiency of HOCl is 40 -80 times larger than OCl - .
Lower pH favors HOCl.
 HOCl = Hypochlorous acid
 OCl - = Hypochloride ion
 Free chlorine = HOCl + OCl-
FREE CHLORINE

5.  HOCl + NH3 ↔ NH2Cl (monochloramine) + H2O
 HOCl + NH2Cl ↔ NHCl2 (dichloramine) + H2O
 HOCl + NHCl2 ↔ NCl3 (trichloramine) + H2O
 Combined chlorine=monochloramine+dichloramine+
trichloramine
 Total chlorine = combined chlorine + free chlorine
COMBINED CHLORINE

6. BREAKPOINT

7. MEASUREMENT TECHNIQUES
 DPD Colorimetric Method
Amperometric Method

8. Add buffer and indicator solution to a known volume of water.
After mixing, measure wavelength of light transmitted through sample
to determine colour change and thus, amount of free residual chlorine
in the sample.
Advantages
Price.
High pH is not a limiting factor.
EPA approved DPD measurement.
Disadvantages
Uses reagents..
High maintenance.
No pH or temperature output.
COLORIMETRIC METHOD

9. COLORIMETRIC METHOD

10.  Free Residual Chlorine— (DPD), The reaction takes place at a
buffered pH of 6.3 to 6.6.
 Total Residual Chlorine— By adding potassium iodide to the
sample, chloramines in the sample oxidize iodide to iodine,
which then oxidizes the DPD indicator to the magenta color at
a buffered pH of 5.1.

11. A sensor consisting of a membrane (which allows HOCL to
migrate through it), two dissimilar metal electrodes, and an
electrolyte are submersed into the water sample.
A free residual chlorine reading is derived based on mV
changes experienced by the sensor.
Advantages
Ease of use
No reagents or buffers added
Disadvantages
pH dependence
AMPEROMETRIC METHOD

12. How does the chlorine sensor work
 Current is proportional to
diffusion rate, which is
proportional to concentration
cathode
membrane
anode
(Ag/AgCl)
electrolyte
(KCl)
electrons
HOCL
HOCL + H+ + 2e- Cl- + H2O
2Ag + 2Cl - AgCl + 2e-
Amperometric sensor measures
only HOCL

13. PH DEPEDENACE

14. FREE CHLORINE AND THE PH PROBLEM
 Three ways to solve the
problem...
 Restrict applications to
only those having
constant pH
 Use acid to adjust the
pH of the sample
 Measure pH continuously
and perform automatic
correction
350
400
450
500
550
600
650
7.0 7.5 8.0 8.5 9.0 9.5
sensitivity,nA/ppm
pH at 25 C

15. 15
The reagent solution...
 Add an acid to the sample to lower
pH to about 6.
 Acid converts OCl to HOCl, which
the sensor measures.
 Disadvantages
– requires sample conditioning
system
– requires reagents acid
reagent
bottle
reagent
pump
sensor
HOCl + OCl HOCl

16. INTERNAL PH CORRECTION

17. The Rosemount Analytical approach
Use continuous pH correction
Good between pH 6.0 and 9.5.
– Below pH 6.0, don’t need correction.
– Above pH 9.5, usually there is too little
HOCl available to measure.
If pH varies less than 0.2 peak-to-peak,
don’t need continuous pH correction.
If pH varies more than 0.2 peak-to-peak,
use continuous pH correction.
free chlorine
sensor
pH
sensor

18. Free chlorine sensor - 499ACL-01
 Easy to replace membrane - no tools
required
 Range: 0 - 20 ppm
 Linear range: 0 - 6 ppm
 Linearity: 2% (typical). Following calibration
in the linear range, the measured
concentration of any point in the linear range
is within about ±0.1 ppm of the expected
value.
 Electrolyte life: about three months
 Variopol quick disconnect option makes
replacing the sensor easy.
cable
cable grip
machined
plug,
receives
cable grip
O-rings
MNPT
threads,
pipe tape
not shown

19. 19
Determination of total chlorine
 No single sensor can be used to determine all the different forms
of total chlorine.
 The only way to measure total chlorine is to convert it into a form
the sensor can measure.
 Requires a sample conditioning system. Most manufacturers use
an off-line system.
acetic acid buffer
with potassium iodide
total chlorine iodine
iodine sensor
to waste
The concentration of iodine produced by the reaction between total chlorine
and potassium iodide is directly proportional to the concentra-tion of total
chlorine in the sample.

20.  Can be used as disinfectant
 Chloramines: effective vs. bacteria but NOT viruses.
 Chloramines not powerful enough to form THMs.
 Last a lot longer in the mains than free chlorine,
20
CHLORAMINES

21. Activated carbon
 C + 2Cl2 + 2H2O → 4HCl + CO2
 When dissolved in water, sodium bisulfite (SBS) is formed
from SMBS (Sodium metabisulfite) :
Na2S2O5 + H2O → 2 NaHSO3
2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4
DECHLORINATION

22. 22
Dechlorination
Remember: You’re measuring the absence of chlorine, not chlorine.
chlorine
sensor
dechlorination
chemicals
chlorinated water
chlorinated water sample
(for calibrating and checking
sensor response)
dechlorinated
water sample

23. Installation
Linear velocity in basin is
typically not high enough
for a good measurement.
Basins and tanks:
Rarely is there enough flow
through a tank to provide
adequate stirring.
better
sample point
Best practice is to use an
extracted sample and the
FCL arrangement for flow
control

24.  FLOW
 Loss of flow
 Air bubbles
 Membrane fouling
FACTOR AFFECTING

25. Maintenance
Current depends on diffusion rate through
the membrane.
chlorine
membrane
cathode
clean
surface
fouled
surface
A dirty or fouled membrane blocks the passage of chlorine. The result is low
readings and sluggish response to concentration changes.

26. 26
Sample flow
 Diffusion rate (ie sensor current) depends on the concentration of
chlorine at the surface of the membrane.
 As the sensor operates, it depletes the chlorine at the membrane
surface.
 Diffusion from bulk liquid is not adequate to replace loss, so
current drops.
cathode
membrane
sample
cathode
membrane
sample
cathode
membrane
sample
NO FLOW

27. 27
Sample flow (cont’d)…
 Flowing sample keeps the concentration of chlorine at the
surface equal to the bulk concentration.
 Sensor current is proportional to the concentration of chlorine in
the bulk solution.
cathode
membrane
sample
FLOW
cathode
membrane
sample

28. 28
Sample flow is important
 Normally, sensor current is a non-linear function of flow.
 There is a minimum flow above which the sensor current is more or
less independent of flow.
 Each type of sensor has an optimum flow.
sensor
current
flow
minimum flow rate

29. 29
Automatic cleaning systems
wastewater
aeration basin
oxygen
sensor sensor
sensor cable
spray nozzle
membrane
water or air line
normal operation
spray cleaner activated
recovery period
normal operation
Commonly used with oxygen sensors.

30. 30
3 ELECTRODE

31. CAMPARISION

33. 33
Drinking water filter plant
T
OZ
ozone
generator
screens
raw water
flocculant
ozone destruct
T
ozone contact
chamber
settling basin
backwash
pump
to screens
backwash reclaim pond
NaOH
NaOCl
pHCl
T PC
mixer
contact
tank
T

34. 34
Distribution system
filter plant
NaOCl
Cl
Cl

35. 35
Groundwater source
NaOCl
Cl
Cl
well

36. 36
Desalination/wastewater reclamation plant
NaOCl
media filter
T
C
C
C
Cl
RO modules
calcite
filter
dCl
NaHSO3
membrane filter
pH Cl T
reject
*SCS921A
**FCL
*
**
PC

37. 37
Waste treatment plant
raw
waste
grit chamber
primary sedimentation
tank
screens
to sludge system
to sludge
system
aeration basin
secondary
sedimentation
tank
dechlorination
tank
blower
NaOCl
activated
sludge
to sludge
system
to
aeration
basin
chlorine contact
chamber
DODO
CldCl
T
Cl typically total chlorine