CHLORINE MEASUREMENT

Praful Hanmante

Most commonly used for

disinfection of water.

CHLORINE AS DISINFECT

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

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

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

BREAKPOINT

MEASUREMENT TECHNIQUES

DPD Colorimetric Method

Amperometric Method

Add buf fer and indicator solution to a known volume of water.

Af ter mixing, measure wavelength of l ight transmitted through sample to determine colour change and thus, amount of free residual chlorine in the sample.

AdvantagesPrice.High pH is not a l imiting factor.EPA approved DPD measurement.

DisadvantagesUses reagents. .High maintenance.No pH or temperature output.

COLORIMETRIC METHOD

COLORIMETRIC METHOD

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.

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

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

PH DEPEDENACE

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

sen

sit

ivit

y, n

A/p

pm

pH at 25 C

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

INTERNAL PH CORRECTION

The Rosemount Analytical approachUse 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

sensorpH

sensor

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

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.

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

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

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

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

FLOW

Loss of flow

Air bubbles

Membrane fouling

FACTOR AFFECTING

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

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

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

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

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

3 ELECTRODE

CAMPARISION

33

Drinking water filter plant

T

OZozone

generator

screens

raw water

flocculantozone destruct

T

ozone contact

chamber

settling basin

backwash

pump

to screens

backwash reclaim pond

NaOH

NaOClpHCl

T PC

mixer

contact

tank

T

34

Distribution system

filter plant

NaOCl

Cl

Cl

35

Groundwater source

NaOCl

Cl

Cl

well

36

Desalination/wastewater reclamation plant

NaOCl

media filter

TC

C

C

Cl

RO modules

calcite

filter

dCl

NaHSO3

membrane filter

pH Cl T

reject

*SCS921A

**FCL

*

**PC

37

Waste treatment plant

raw

waste

grit chamber

primary sedimentation

tankscreens

to sludge systemto sludge

system

aeration basin

secondary

sedimentation

tank

dechlorination

tank

blower

NaOCl

activated

sludgeto sludge

system

to

aeration

basin

chlorine contact

chamber

DODO

CldCl

T

Cl typically total chlorine