Home >> Science & Nature >> Science

How to Monitor Residual Chlorine in the Presence of Sodium Metabisulfite

The treatment of wastewater using chlorine is a complicated process consisting of the elimination of toxic chemicals, the removal and destruction of harmful microorganisms and the production of drinkable water. Monitoring the amount of residual chlorine left in the treated wastewater is critical to safeguarding the environment. Following chlorination of the wastewater, residual chlorine is eliminated by the addition of dechlorinating agents such as sodium bisulfite and sodium metabisulfite. These reducing agents remove residual chlorine and reduce the hazardous effects of chlorine on aquatic life. The use of test strips based on the DPD Colorimetric Method of chlorine analysis is the simplest and most widely used technique for monitoring residual chlorine in effluents.

Things You'll Need

Filter
Wastewater
Bottles
Chlorine test strips

Instructions

- 1
  Filter a sample of processed wastewater into a plastic storage bottle. Record the location, time and date of the sample.
- 2
  Dip a free and total chlorine test strip into the water by placing the end with the small test pads into the sample. Ensure that the test strips you choose are appropriate for the chlorine concentration range you expect to be present in the water sample. These test strips are based on the well known N,N-diethyl-p-phenylenediamine (DPD) chemistry, which forms red complexes when reacted with residual chlorine. When the test pads, which are impregnated with the DPD, react with residual chlorine in the sample, a red colored product is produced. The amount of red complex produced is an indicator of how much residual chlorine is present in the water sample.
- 3
  Allow the test strip to remain in the solution for approximately 60 seconds before removing. Use the same amount of time in the water sample for each test. Most test strips come complete with instructions and color charts that explain results; be sure to use these resources when performing your tests.
- 4
  Allow the test strip to develop its color before comparing it against the color chart on the package. This should take about one minute.
- 5
  Match the pattern of the pad colors to the color chart in order to determine the concentration of the residual chlorine in the water sample. The darker the color pads, the higher the concentration of residual chlorine. Based on the concentration range the test strips monitor, a darker resulting color indicates that the concentration of dechlorination agents added to the effluent is insufficient.

Science