The Determination of Chlorine Concentration in Water

Purpose: In this experiment you will measure the amount of dissolved chlorine present in water samples. Bring a water sample, including pool water, from home if you wish (make sure it is in an airtight container, like Tupperware, and is kept as cold as possible).

Background: Chlorine is often added to drinking water supplies to kill harmful microorganisms. Chlorine is not only an effective disinfectant, but it also reacts with ammonia, iron and other metals and some organic compounds to improve overall water quality. There is a limit to the use of chlorine, however, as negative results are possible with the addition of too much chlorine. Bad tastes or odors in water are often enhanced. Excess chlorine can be harmful to fish and other aquatic animals when the water contains nitrogen compounds. Finally, the formation of chloroform and other suspected carcinogens is possible. It is very important for water suppliers to monitor closely the levels of chlorine present in the water for which they are responsible. Practically, water systems look for a level of chlorine remaining in the water after treatment to be 1 mg/L or less for minimizing adverse effects while maintaining disinfectant properties. Above 1 mg/L, odor and taste often become problematic.

The method of determining chlorine in this experiment relies on a color indicator, DPD (N,N-diethyl-p-phenylene-diamine). In the presence of chlorine, DPD reacts rapidly to form a red color, the intensity of which is an indicator of chlorine concentration. The higher the absorbance, the higher the chlorine concentration.

DPD + Chlorine Red color measured at 515 nm

By creating a series of standards of known chlorine concentration, it is possible to construct a calibration curve to measure the chlorine concentration in unknown samples.

Procedure: The procedure for this experiment involves the use of the spectrophotometer as outlined in earlier experiments. The chlorine present in the water samples will react with the DPD indicator forming a red colored compound which is allowed to develop for a set period of time and then measured with the spectrophotometer at 515 nm. It will be necessary to first mix several known standards and measure their absorbance. These will be used to generate a calibration curve. Unknown water samples can then be prepared and their absorbance used to determine their chlorine concentration. A detailed procedure follows.

Preparation of the Standard Curve:

  1. Take four flasks or beakers capable of holding at least 100 ml and add to them the following quantities of standard solution and deionized water. Use pipets to measure the volumes of standard solution in each case. The water can be measured with your graduated cylinder.

    Volume Standard (ml)

    Volume DI Water (ml)

    Chlorine Conc. (mg/L)














  3. To each of these solutions add three (3) DPD tablets and stir to dissolve. You should notice the red color developing as the tablets dissolve. If you do not see the color notify your instructor.
  4. Allow the color to develop for 10 minutes and read the standards in the spectrophotometer set at 515 nm.
  5. While the color is developing you can prepare the unknown samples or calibrate the spectrophotometer.


Preparation of the Unknown Samples:

  1. The unknown samples must be prepared exactly as the standards were for the test to be accurate. 100ml of each unknown needs to be measured into the graduated cylinder and transferred to a beaker or flask.
  2. Before doing the next step, test the odor of each unknown as demonstrated by your instructor. Do you detect any "chlorine smell"? Report your observations in the Results section.
  3. Into each unknown solution place three (3) DPD tablets and stir to dissolve.
  4. Allow the color to develop for 10 minutes and read the absorbance on the spectrophotometer.
  5. If you brought a water sample from home, it is possible that the water is very high in chlorine, especially pool water, and will need to be diluted for the test to work. If the color of your unknown is darker than the darkest standard, consult your instructor for further instructions.
  6. All solutions from this experiment can go down the drain and any opened DPD tablets can be thrown in the trash can. Any unopened DPD tablets can be returned to the experiment box for later use.


Analyzing the Solutions:

  1. Turn the spectrophotometer on and allow it to warm up for at least 15 minutes.
  2. Set the wavelength to 515 nm.
  3. With the sample chamber empty, adjust the meter to read 0% transmittance with the left-hand knob.
  4. Insert a cuvette with deionized water. Close the cover and set the meter to 100% transmittance with the right-hand knob.
  5. Measure the absorbance of each of the four standard solutions and the unknown water samples. Record the absorbances along with the chlorine concentration listed in the previous table. These values will be used to construct the standard curve.


Results: The Results section for this experiment should include a calibration curve of absorbance vs. mg/l chlorine based on the values obtained for the four standards. Plot the points and draw the best straight line possible through them. Remember to use a full graph and use correct scale. Using the calibration curve, determine the concentration of chlorine in the unknown samples and report these values.

  1. These samples were taken from a water treatment facility that has a history of varying chlorine levels. Based on these samples, does it appear they have the problem under control? Why or why not?
  2. If you brought a sample from home, or shared someone else’s sample, how did the chlorine level compare to the unknown samples?
  3. Which of the samples would you prefer to drink if there were no known problems with microorganisms? What about if there was a past history of microorganism contamination in the water?
  4. In the sample(s) where there was a relatively high chlorine level, were you able to detect any odor? Would odor be a good indicator of low chlorine levels? Why or why not?