Lab 9:

Chlorine Residual and Chlorine Demand

Reading Assignment

Read Chapters 7 and 8 in Simplified Procedures for Water Examination.

Chlorine Dose, Demand, and Residual

Most water treatment plants are required to disinfect the water, a process used to kill harmful bacteria.  The most frequently used method of disinfection is the addition of chlorine.  Here, we will briefly introduce three terms used during chlorination - chlorine dose, chlorine demand, and chlorine residual.  These three characteristics are related to each other using the following equation:

(Chlorine demand) = (Chlorine dose) - (Chlorine residual)

The amount of chlorine added to the water is known as the chlorine dose.  This is a measured quantity chosen by the operator and introduced into the water using a chlorinator or hypochlorinator. 

As the chlorine reacts with bacteria and chemicals in the water, some of the chlorine is used up.  The amount of chlorine used up by reacting with substances in the water is known as the chlorine demand.  If nothing reacts with the chlorine (as would be the case in distilled water), then the chlorine demand is zero.  However, in most cases the operator should count on some of the chlorine dose being used up when it reacts with substances in the water. 

The amount of chlorine remaining in the water after some of the chlorine reacts with substances in the water is known as the chlorine residual.  This lab introduces a test which can be used to calculate the chlorine residual.  The chlorine residual is the most important of these three values - dose, demand, and residual - because it represents the actual amount of chlorine remaining in the water to act as a disinfectant.

The test for chlorine residual is performed frequently at most water treatment plants.  Since regulations require a certain level of chlorine in water at the far ends of the distribution system, operators should be sure to test the chlorine residual in the distribution system as well as in the clear well. 

Introduction to Testing Procedures

The DPD Colorimetric Method introduced in this lab is one of several procedures which can be used to test for chlorine residual.  This method requires compensation for color and turbidity and can detect chlorine concentrations only as low as 10 ug as Cl2/L.  Standard Methods introduces several other procedures and explains which procedures are most effective under a variety of circumstances.  In every case, remember that chlorine is a relatively volatile substance and that samples should be tested as soon as possible after the water is collected. 

The chemistry involved in the DPD Colorimetric Method is relatively simple.  The buffer lowers the pH of the sample to 4 or less.  In this pH range, chlorine in the water is able to react with the added potassium iodide, replacing the iodine which is released into the solution as shown below:

2KI + Cl2 yields 2KCl + I2

When free iodine becomes present in the water, the indicator makes the solution change to a red color, with the intensity of the color equivalent to the amount of chlorine found in the solution. 


This procedure requires a piece of colorimetric equipment, some glassware, and titration equipment.  The colorimetric equipment must be one of the following:

  • Spectrophotometer, for use at a wavelength of 515 nm and providing a light path of 1 cm or longer.

  • Filter photometer, equipped with a filter having maximum transmission in the wavelength range of 490 to 530 nm and providing a light path of 1 cm or longer.

  • Standard potassium permanganate solutions
  • Phosphate buffer solution
  • N,N-Diethyl-p-phenylenediamine (DPD) indicator solution
  • Standard ferrous ammonium sulfate (FAS) titrant
  • Potassium iodide (KI) crystals
  • Chlorine-demand-free water
(For information on how to prepare these reagents, see p. 4-62 in Standard Methods.)

Laboratory Procedure

1. Calibrate the photometric equipment using the following procedure.  Note that this procedure uses potassium permanganate solutions.  You can use chlorine solutions to calibrate the equipment by following the procedure in Standard Methods on pages 4-63 to 4-64.

a. Set 100%T on the spectrophotometer or filter photometer using a distilled water blank, in accordance with manufacturer's instructions.  (Prepare the distilled water blank in the same manner as you prepare the sample for testing.)

b. Prepare a series of potassium permanganate standards covering the equivalent chlorine range of 0.05 to 4 mg/L.  (The procedure for producing the standards is explained in Standard Methods on page 4-64.)

c. Label empty flasks for each standard.  Place 5 mL of phosphate buffer and 5 mL of DPD indicator reagent in each labelled flask. 

d.  Add 100 mL of each prepared potassium permanganate standard solution to the appropriate flask and mix thoroughly.

e. Fill a photometer or colorimeter cell with the solution in each flask and read each standard at a wavelength of 515 nm.

f.  In the data section, plot a standard curve of mg/L equivalent chlorine versus %T. 

g. Return the cell contents to the appropriate flask and titrate with FAS titrant as a check on any absorption of permanganate by distilled water. 

2. Measure the chlorine content of the sample.

a. Pipette 0.5 mL of phosphate buffer solution into an empty test tube.

b. Add 0.5 mL of DPD indicator solution to the test tube.

c. Add 10 mL of sample water and read the color immediately.  Use the standard curve to determine the amount of chlorine in the sample.  Record this in the Data section as Reading A.

d. Continue by adding one very small crystal of KI (about 0.1 mg) to the test tube and mixing.  Read the color immediately.  Use the standard curve to determine the amount of chlorine in the sample.  Record this in the Data section as Reading B.

e. Continue by adding several crystals of KI (about 0.1 g) to the test tube and mixing.  Let the solution stand for about two minutes to allow color to develop, then read the color.  Use the standard curve to determine the amount of chlorine in the sample.  Record this in the Data section as Reading C.

f. Place a very small crystal of KI (about 0.1 mg) in a clean test tube.  Add 10 mL of the sample and mix.  In a separate tube, add 0.5 mL of the phosphate buffer solution and and 0.5 mL of the DPD indicator solution and mix.  Add the contents of the second tube to the first tube and mix.  Read the color immediately.  Use the standard curve to determine the amount of chlorine in the sample.  Record this in the Data section as Reading N.

3. Calculate the amount of each type of chlorine using the calculation methods listed in Table 2 in the Data section. 

For example, let's consider our calculations if the readings were A = 1.0 mg/L, B = 1.3 mg/L, C = 2.7 mg/L, and N = 1.9 mg/L.

First, the amount of free chlorine was shown by Reading A to be 1.0 mg/L. 

The amount of monochloramine is calculated as:

B - A = 1.3 mg/L - 1.0 mg/L = 0.3 mg/L

So the concentration of monochloramine is 0.3 mg/L. 

Since N is more than 0, the dichloramine concentration is calculated as:

C - N = 2.7 mg/L - 1.9 mg/L = 0.8 mg/L

So the concentration of dichloramine is 0.8 mg/L.

Finally, since N is more than 0 and there are monochloramines present, the amount of trichloramine is calculated as follows:

2(N - B) = 2(1.9 mg/L - 1.3 mg/L) = 1.2mg/L

So the concentration of trichloramine is 1.2 mg/L.


Standard Curve

Table 1
mg/L chlorine




Table 2
Type of Chlorine
Calculation Method
Free Chlorine

B - A

If N=0, then C - B;
If N>0, then C - N

If N=0, then 0 mg/L;
If N>0 and (B - A) = 0, then 2(N - A)
If N>0 and (B - A)>0, then 2(N - B)

Virtual Lab

There are two virtual labs introducing other methods of chlorine residual testing not covered by this page.  The first virtual lab introduces a titration method while the second virtual lab introduces a field testing method which can be used in the distribution system.  There is an assignment that needs to be completed concerning the virtual lab, so please print the assignment first and then answer the questions as you perform the lab. Once you have the assignment completed, log in and complete the assignment online to be entered directly into the database.


American Public Health Association, American Water Works Association, and Water Environment Federation.  1998.  Standard Methods for the Examination of Water and Wastewater.  American Public Health Association, Washington, D.C.

Chapter 6 and Appendix D: Methods Checklist: DPD Total Chlorine.  Drinking Water and Wastewater Operator Information Center.  Pennsylvania Department of Environmental Protection.