ENGINEERING SCIENCE DEPARTMENT
Water and Environmental Technology Program
Name _________________
SPRING TERM
Aquatic Chemistry II, WQT-134
Laboratory #8
Chlorine Demand STM 2350 (Applied Water and Spentwater
Manual Ch 22)
Purpose: The purpose of this lab is to understand the oxidant powers of chlorine. Oxidants (i.e.,
chlorine) are added to water supplies and wastewater primarily for disinfection. Other beneficial
uses include slime removal, oxidation of undesirable inorganic species (e.g., ferrous ion, reduced
manganese, sulfide, and ammonia) and oxidation of organic constituents (e.g., taste and odorproducing compounds). Oxidant demand refers to the difference between the added oxidant dose
and the residual oxidant concentration measured after a prescribed contact time at a given pH
and temperature. Oxidant requirement refers to the oxidant dose required to achieve a given
oxidant residual at a prescribed contact time, pH, and temperature. The chlorine demand of a
sample of water is the difference between the amount of chlorine applied to the sample and the
amount of free, combined, or total chlorine residual remaining at the end of the contact period.
The amount of free chlorine residual is directly related to the amount (dose) of chlorine added.
Lets look a the chlorine breakpoint curve below.
In Zone I- all the chlorine you will add to your sample is destroyed by reducing agents such as
iron, manganese, clay and silt. The chlorine added may get reduced to chloride. In Zone I, we
really aren’t disinfecting, we are getting rid of compounds that inhibit disinfection. Now lets say
you dose (add) a little more chlorine to your sample to reach (Zone II), here chlorine comes into
contact and bonds with organics and ammonia; we call these combined chlorine residuals.
These new compounds, chloroorganics and chloramines can be considered weak oxidizers
(disinfectants) but they are not as powerful as the almighty free chlorine. Unfortunately, some
chlorine may react with many organic compounds to form stable chlorinated organics, such as
trihalomethanes (THM), many of which may be carcinogenic. These are called disinfection
byproducts (DBPs) and are driving changes in disinfection practices. To get rid of some of these
THM and DBPs you may dose more chlorine into your sample to reach (Zone III), where
chloroorganics and chloramines are partially destroyed. Chloramines are broken down and
converted to nitrogen gas which leaves the system. Finally, in (Zone IV) you have reached
breakpoint. However, when breakpoint is obtained there will be some stable chloroorganic
compounds (DBPs), including some carcinogens, that remain. Nevertheless, beyond this point,
an increase in chlorine dose will usually produce a proportionate increase in free chlorine
residual. Free chlorine residual is pH dependant and includes hypochlorous acid or
hypochlorite ion (which is 1% as effective as hypochlorous acid). Free chlorine residual is one
of the most powerful oxidants (disinfectant) available in water treatment. That’s the object or
goal, to disinfect. In conclusion, oxidant reactivity is influenced by temperature,
pH, contact time, and oxidant dose.
Factoids/Conversions
 Surface Water Treatment Rule (SWTR), water treatment plants must be designed to meet
a required residual concentration and contact time before
 Free chlorine residual is usually specified to be < 2.0 mg/L
 The contact time is then determined depending on the pH, temperature and required
reduction of Giardia lamblia
 DWS for total trihalomethanes 0 mg/L


DWS for chlorine and chloramines, Maximum Residual Disinfectant Level Goal
(MRDLG) = 4 mg/L
DWS for chlorine dioxide MRDLG = 0.8 mg/L, chlorite MCL= 0.8 mg/L
Advantages
 Two forms of free chlorine residual hypochlorous acid (pH <7) and hypochlorite (pH>7)
 Chlorine disinfection is widely accepted in the US.
Disadvantages/Interferences
 Pay special attention to interferences caused by oxidation products such as MnO2,
NH2Cl, and NHCl2.
 If the ammonia or organic nitrogen content of the water is significant, combined chlorine
may form
Sample collection: Collect 1 liter sample with no headspace. Most reliable results are obtained
on fresh samples that contain low amounts of suspended solids. If samples will be analyzed
within 24 h of collection, refrigerate unacidified at 4°C immediately after collection. To preserve
for up to 28 d, freeze unacidified samples at −20°C. Warm chilled samples to desired test
condition before analysis.
SUPPLIES/SAFTEY

PLEASE WEAR LAB COATS, SAFETY GOGGLES, AND GLOVES AT ALL TIMES FOR
THIS LAB PROCEDURE. USE THE FUME HOOD WHEN NECESSARY
Lab Materials:
1. Sample of water
2. the Hach DR 100 Colorimeter
3. the Hach pH meter with buffers
4. 10 500 mL Erlenmeyer flasks
5. 1 L beaker
6. Standard 100 mg/L chlorine solution (as Cl2)
7. Standard chlorine solutions
8. Phosphate buffer solution
9. DPD free chlorine powder pillows
10. DPD total chlorine powder pillows
REAGENTS/STORAGE Mandatory Use of Gloves and Goggles!:
Hach DPB Free Chlorine Powder Pillow
Hach DPB Total Chlorine Powder Pillow
Standard chlorine solutions 1mg=1ml
Dilute 1 ml of commercial CLOROX bleach (5.7% sodium hypochlorite) to exactly 100 ml of
DI water. Transfer 0.86 ml of this solution to a 500 ml volumetric flask and bring to volume
with DI water. The final solution should be 1 mg=1ml chlorine.
Dilute 10 ml of commercial CLOROX bleach (5.7% sodium hypochlorite) to exactly 100 ml of
DI water. Transfer 0.86 ml of this solution to a 500 ml volumetric flask and bring to volume
with DI water. The final solution should be 1 mg=10 ml chlorine.
Dilute 100 ml of commercial CLOROX bleach (5.7% sodium hypochlorite) to exactly 200 ml
with DI water (~28,500 mg/L). Transfer 1.75 ml of this solution to a 500 ml volumetric flask
and bring to volume with DI water. The final solution should be 1 mg=100 ml chlorine.
SUMMARY PROCEDURE
Chlorine Breakpoint/Demand STM 2350 (Applied Water and Spentwater
Manual Ch 22)
Total Chlorine Breakpoint/Demand Measurement: The sample is divided into subsamples
and each is dosed with the standardized oxidant (chlorine) solution to yield a series of increasing
doses. After the appropriate contact time, chlorine residual, pH, and temperature are measured
and the demand/requirement determined by difference between initial and final concentrations.
When free residual is first detected the sample has achieved breakpoint.
“Breakpoint occurs when you get a measurement of free residual chlorine (pink color after
adding a pocket pillow)”
Chlorine Demand STM 2350 (Applied Water and Spentwater
Manual Ch 22)
Part 1 Chlorine Dose of Samples To Determine Breakpoint
1. Calibrate the pH probe and record the pH________________ and Temp_____________
of the unknown sample.
2. Dispense 5, 100 mL of your sample aliquots of the sample into clean 150 mL Erlenmeyer
flasks capacity to permit mixing with a stir bar.
3. While mixing the samples, add 1 ml of 1.0 mg/L chlorine standard to first sample, 1 ml of
10 mg/L chlorine standard to second sample, and 1 ml of 100 mg/L chlorine solution to a
third successive 100 ml samples in the series (and record the time). The amount of chlorine
added to the first portion should leave no chlorine residual at the end of the contact period,
especially if low demands are being studied (you are making a chlorine breakthrough curve
for your sample).
4. Dose the samples according to a 10 minute staggered schedule that will permit the
determination of chlorine residuals at the predetermined contact time (use 10 minute contact
time).
5. At the end of the 10 min. contact period determine the free and combined available residual
chlorine using DPD total and free chlorine powder pillows on an aliquot from each portion
(Fill out data sheet).
6. Measure the pH for the lowest and highest chlorine dosages to ensure that pH effects are
negligible.
7. If you do not reach breakpoint use a stronger dose of the 100 mg/L standard.
Part 2 Free Chlorine Analysis using the Hach Pocket Colorimeter II:
1. Fill a 10-ml cell with sample (the blank). Cap
2. Press the POWER key to turn the meter on. The arrow should indicate the low range
channel (LR)
3. Remove the meter cap. Place the cell with the diamond mark facing the keypad. Cover
with meter cap.
4. Press the ZERO/SCROLL the display will show “---” then 0.00. Remove the blank.
5. Add the contents of one DBP Free Chlorine Powder Pillow to the sample.
6. Cap and shake for 20 seconds.
7. Place sample in the sample holder and cover.
8. Press Read/Enter record results in mg/L. Decant and rinse vial.
Part 3 Total Chlorine Analysis using the Hach Pocket Colorimeter II:
1. Fill the 10 ml-cell sample with 10 ml of sample.
2. Add the contents of one DBP TOTAL Chlorine Powder Pillow to the sample.
3. Cap and shake for 20 seconds, wait 3-6minutes.
4. Place sample in the sample holder and cover.
5. Press Read/Enter record results in mg/L.
Part 4 Free Chlorine Analysis using the Hach DR 100 Colorimeter:
6. Fill Open light shield and turn the RIGHT SET control fully clockwise.
7. Insert 1 cm cell into the LEFT SET position of the sample well and press down firmly.
8. While holding the ON button down, adjust the LEFT SET control to align the meter
needle with the arrow at the far left of the scale arc.
9. Remove the cell holder and close the light shield
10. Fill 2.5 cm sample cells to the 10 ml mark with water to be tested.
11. Add the contents of one DBP Free Chlorine Powder Pillow to one 2.5 cm sample cell.
Cap and shake for 20 seconds. If chlorine is present a red color will develop. A yellow
color indicates to high a concentration and dilution may be necessary. Immediately
analyze, less then 1 minute.
12. Cap sample cell, open light shield, and place sample in cell. Close light shield.
13. Hold the On button down, adjust RIGHT SET control for a reading of zero mg/L.
Remove sample
14. Place the next sample into the cell.
15. While holding the ON button down, allow the meter to stabilize. Record the mg/L free
chlorine from the upper (2.5 cm) scale arc. If sample contains no free chlorine but some
combined chlorine, the meter may drift to a higher value. If the meter stabilizes within a
few seconds only Free chlorine is present.
Part 5 Total Chlorine Analysis using the Hach DR 100 Colorimeter:
16. Take a sample of water to be tested by filling a clean 2.5 cm cell to the 10 ml mark
17. Add the contents of one Total DBP Chlorine Powder Pillow to one 2.5 cm sample cell.
Cap and shake for 20 seconds. If chlorine is present a red color will develop. A yellow
color indicates to high a concentration and dilution may be necessary. Immediately
analyze, less then 6 minutes.
18. Cap sample cell, open light shield, and place sample in cell. Close light shield.
19. Open the light shield, turn the RIGHT SET control fully clockwise and place the 1-cm
sample holder in the LEFT SET position tightly in the sample well.
20. While holding the ON button down, adjust the LEFT SET control to align the meter
needle with the arrow at the extreme left of the scale. Remove the cell holder.
21. Place sample in cell
22. Hold the ON button down while adjusting the RIGHT SET control for a readinf of 0
mg/L. Remove the sample.
23. Hold the ON button down until the meter stabilizes. Read and record the mg/L TOTAL
chlorine from the upper (2.5 cm) scale arc.
Part 6 Data Analysis plotting the Chlorine Demand Curve in Excel
1. To make a graph in Excel go to Insert and then Chart or the Chart Icon
2. Now select the X Y (Scatter) Chart Type with data/line points Chart subtype and click
Next
3. Now we need to select our data to make a graph. Our data are in columns (hit column
button). Click in the data range and then highlight the columns (of data only=no text) for
Concentration Dose (X axis) and Chlorine Residual Concentration (mg/L) (Y axis).
Note: your raw data should be ='Raw data'!$E$6:$F$9. Click Next
4. Now label your Chart Titles as Chlorine Dose, X axis: Concentration Dose (X axis), and
Y Axis: Chlorine Residual Concentration (mg/L) (Y axis). Click Next then Finish
5. Voila you have made a graph in Excel (see below: Note your graph will have the data).
Department of Engineering Science
Water & Environmental Technology
Environmental Chemistry II, WQT 134
Chlorine Breakpoint/Demand
Analyzed by
Date
Facility Name
Sample Location
Sample pH
Sample Temp
Flask ID
Blank
Chlorine Dose
(mg/L)
Free Residual
Measured (mg/L)
Total Chlorine
(mg/L)
Chlorine Demand
(mg/L)
0
Calculations: When reporting chlorine demand, include dose, contact time, sample temperature,
sample pH, and analytical method:
Demand = Dosage - Residual
BREAKPOINT:____________________________
Chlorine Demand :__________________________