Post on 09-Mar-2018
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: