Lecture 4: Determination of free chlorine and hypochlorite

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Environmental Engineering Department

Environmental Measurements (EENV 4244)

Introduction

Why we should measure chlorine?

What do we mean by residual chlorine?

What is the source for presence of chlorine in water and wastewater?

Is it mandatory to measure residual chlorine in water and wastewater?

Treated of filtered water is deemed to be fit for consumption only if it is devoid of diseases

producing microorganism.

Chlorination is primarily adopted to destroy or deactivate diseases- producing

microorganisms in the public water supply.

Chlorine is added to water, some of the chlorine reacts first with organic materials and metals

in the water and is not available for disinfection (this is called chlorine demand of the water).

The remaining chlorine concentration after the chlorine demand is accounted for is called total

chlorine.

Chlorine is one of the most widely used disinfectants.

It is very applicable and very effective for the deactivation of pathogenic microorganisms.

Chlorine can be easily applied, measures and controlled. It is relatively cheap.

However, we only started using it as a disinfectants on a wider scale in the nineteenth century,

after Louis Pasteur discovered that microorganisms spread certain diseases.

Chlorine has played an important role in lengthening the life-expectancy of humans.

Chlorine can be used as a disinfectant of the following forms:

Gaseous form Cl2.

Bleaching solution (NaOH): Bleach consists of chlorine gas dissolved in an

alkali-solution, such as sodium hydroxide (NaOH).Chlorine reacts with sodium

hydroxide to sodium hypochlorite (NaOCl).

Bleaching powder Ca(OCl)2: This is produced by directing chlorine through

calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid.

Water borne diseases

Total chlorine

Combined chlorine

Reacts with nitrates and unavailable

for disinfection

Free chlorine

Available to inactivate diseases

causing- organisms

Residual chlorine is usually tested in finished water which is ready to be released into the

distribution system, although operators must also ensure that there is adequate residual at

the extreme ends of the distribution system.

The chlorination process

• Chlorine readily combines with chemicals dissolved in water, microorganisms, small

animals, plant material, tastes, odors, and colors.

• These components "use up" chlorine and comprise the chlorine demand of the

treatment system.

• It is important to add sufficient chlorine to the water to meet the chlorine demand and

provide residual disinfection.

1. Chlorine demand is the amount of chlorine required to kill bacteria, oxidize iron or other

elements in the water.

2. Free available chlorine residual is the amount of chlorine remaining in the water after

the chlorine demand has been met.

3. Contact time is the amount of time that the chlorine is present in the water. The

combination of chlorine residual and contact time determines the effectiveness of the

chlorination treatment.

Combined

Free Breakpoint

SP 4

SP 1

SP 6

SP 3

SP

7

SP 8

SP 9

SP 2

SP 5

Flow from the source

Water distribution network – sampling

locations

y = 5E-08x2 - 0.0002x + 0.4475 R² = 0.9263

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 500 1000 1500 2000 2500

Re

sid

ual

ch

lori

ne

(m

g/l)

distance from chlorination point (m) Average free chlorine

Residual Chlorine vs. distance from chlorination source

Source: H. Sadallan and H. Najar, 2013

y = 623.12x2 - 493.92x + 145.4 R² = 0.8741

45

50

55

60

65

70

75

80

85

90

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Ave

rage

Dia

rrh

oe

a

Average residual chlorine Average chlorine and Diarrhoea

Average free chlorine vs. water borne diseases (Diarrhoea)

Source: H. Sadallan and H. Najar, 2013

Kinetics of Disinfection

Inactivation is a gradual process involving a series of physicochemical and biochemical

steps. Inactivation is described by the equation:

Nt/N0 = e-kt

Where:

N0 = number of microorganisms at time = 0

Nt = number of microorganisms at time = t

k = a decay constant (1/time)

t = time

Lo

g (

N /N

)

Time

Shoulder

Ideal, first order

Rapid, initialinactivation

Ta iling o ff

0

t

o

-x

Ideally, inactivation follows first-order

kinetics (blue line), but often non-ideal

behaviors occur resulting from clumping

of cells or multiple hits of critical sites

before inactivation

Concentration and Contact Time

Effectiveness of chlorination depends primarily on the concentration used and the time

of exposure

Disinfectant effectiveness can be expressed as a C ▪ t value where:

C = disinfectant concentration

t = time required to inactivate a 99% of the population under specific

conditions

The lower the C ▪ t, the more effective the disinfectant

In general, resistance to disinfection is in the following order:

vegetative bacteria < enteric viruses < spore-forming bacteria < protozoan cysts

Sampling and Sample handling and preservation

As a general guide, the World Health Organization (WHO, 2006) recommended that one

sample per 1000 persons served should be examined each month for water supply system

serves up to 100,000 persons

Preservation of the sample is not practical. Because biological activity will continue after

sample has been taken, changes may occur during handling and storage.

If analysis is to be carried out within 2 hours of collection, cool storage is not necessary.

If analysis can not be started within 2 hours of sample collection keep the sample at 40C.

Do not allow sample to freeze

Do not open the sample bottle before analysis

Begin analysis within 6 hours of sample collection.

Total residual chlorine measurement

methods

N, N diethyl-P-

Phenylenediamine DPD

Amperometric titration Iodometric

Environmental significance: active chlorine (free and combined) should be determined at

each stage in the treatment process of drinking water and in the water mains in order to

guarantee bacteriological impeccable water.

U.S. EPA-approved analytical methods for chlorine

Cl2 + 2 I- → I2 + 2 Cl-

How would you know this reaction occurred?

I2 complexes with starch to form a blue I2-starch complex.

So you can use starch as the indicator for the presence of I2

Determination of residual chlorine

Titration Method

Well, not if you are titrating Cl2 with I-

It will turn blue immediately and stay blue…

So, we do a little backhanded trick. We don’t titrate the Cl2, we titrate the I2 !!!!

I2 + 2 S2O32- → S4O6

2- + 2 I-

This will eliminate the blue color as the iodine gets converted back to I-

All 3 reactions are relevant to the titration.

Convert all of the Cl2 to I2:

Cl2 + 2 I- → I2 + 2 Cl-

Indicator reaction:

I2 + starch → I2-starch (blue complex)

Quantititative Titration:

I2 + 2 S2O32- → S4O6

2- + 2 I-

Example: 25.00 mL of a waste water sample is tested for residual chlorine using the iodometric titration. 10.00 mL of 1 M KI is added to the waste water followed by titration with 0.02450 M Na2S2O3. The starch endpoint is reached after addition of 18.54 mL of thiosulfate. What is the residual chlorine concentration in the waste water?

Determine the net overall stoichiometry of both reactions and then i2M1V1 = i1M2V2

Or do it in steps, working your way back through the reactions.

18.54 mL * 0.02450 M Na2S2O3 =

0.4542 mmoles Na2S2O3 = 0.4542x10-3 moles Na2S2O3

0.4542x10-3 mol Na2S2O3 * 1 mol S2O32- * 1 moles I2 =

1 mol Na2S2O3 2 mol S2O32-

0.2271 x10-3 moles I2 * 1 mol Cl2 = 0.2271x10-3 mol Cl2

1 mol I2

0.2271x10-3 mol Cl2 = 9.084x10-3 M Cl2

.025 L

You could have also just kept it in mmol and then used mL in the last step.

Cl2 + 2 I- → I2 + 2 Cl-

I2 + 2 S2O32- → S4O6

2- + 2 I-

2 mol S2O32- * 1 mol I2 = 2 mol S2O3

2-

1 mol I2 1 mol Cl2 1 mol Cl2

i2M1V1 = i1M2V2

2 * M1 * 25.00 mL = 1 * (0.02450 M) (18.54 mL)

M1 = 9.0846x10-3 M Cl2

9.0846x10-3 Mol X 70.9 g Cl2 X 1000 mg = 644 mg Cl2/l

Remember free chlorine is not just Cl2 it includes HOCl and OCl- also.

The digital colorimeters is used as the following:

A DPD-1 (free chlorine) or DPD-3 (total chlorine) tablet or powder is added to a vial of

sample water that causes a color change to pink.

The vial is inserted into a meter that reads the intensity of the color change by emitting a

wavelength of light and automatically determine and display the color intensity (the free or

total chlorine) digitally.

N, N diethyl-P- Phenylenediamine DPD Method: