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Test for Residual Chlorine

Water Analysis for Chlorine

The amount of residual chlorine left in the chlorinated water after the required contact period, can be experimentally determined by using any of the following tests:

  • Orthotolidine test
  • DiethIy-p-Phenylene-Diamine (DPD) test
  • Chloroscope

Orthotolidine Test

In this test, 10 ml of chlorinated sample of water is taken after the required contact period, in a glass tube. To this 0.1 ml of orthotolidine solution is added. The color formed is observed after 5 minutes. The formation of yellow color normally indicates the presence of chlorine (either combined or free) in the water. The more yellow the color, the greater, is the chlorine residual. The amount of residual chlorine can be ascertained by comparing the colour developed in the glass tube with the standard colors already kept in the laboratory. This test, is therefore, very simple and does not require much technique or time. Under normal conditions, a lemon yellow color is satisfactory for public water supply. The orthotolidine test will normally gives the total residual chlorine present in water. However, it may be adjusted so as to give separately the quantities of free residual as well as combined residual of chlorine. The free residual chlorine forms the yellow color during the first 5 seconds of the addition of orthotolidine, while the combined residual chlorine goes on forming the colour for about 5 minutes. Hence, the colour after 5 seconds will give the quantity of free residual chlorine, and the colour after 5 minutes will give the free and combined chlorine. The difference in value between the two values is the combined chlorine. The orthotolidine test, however, is not accurate, because the impurities such as iron, manganese, nitrate etc., are likely to cause a false yellow colour, and indicating wrong and increased chlorine residual.

Orthotolidine Arsenite test

To overcome this problem Orthotolidine – Arsenite (OT A) test is carried out. Sodium arsenite is added to the chlorinated sample of water. This will dechlorine the sample and orthodolidine is then added. The colour formed (Xl) now is only the intensity of colour caused by interfering agents like nitrates, iron, manganese etc. Now another sample is taken in another test tube and orthotolidine solution is added first, and just after 5 seconds, sodium arsenite is added. The sodium arsenite will arrest the colour to be formed by the combined chlorine. Hence, the colour observed at the end of the experiment (X2) will be due to the free residual plus due to the interfering colour causing compounds of iron, manganese etc. Then, a third test is conducted on a third sample of water. In this, only the orthotoldine solution is added to the given sample of water and the colour is noted after 5 minutes. The noted colour (X3) will evidently be due to the free and combined chloeine plus due to interfering colour causing copounds.

Now the different chlorine residuals can be easily determined as follows:

Free residual chlorine  = (X2) (X1)

Combine residual chlorine = (X3)  (X2)

In the orthotolidine test as well as in the OT A test, the temperature of water is to be controlled to room temperature.

Diethly-p-Phenylene-Diamine (DPD) Test

This test is widely used nowadays. The testing is carried out in a colour comparator, so as to easily compare the developed colour with the standard colours. The comparator is provided with a special dulling screen with a disc which must be fitted to the right hand viewing window of comparator. The comparator discs will be available in two different chlorine concentration ranges;

0.1 to 1.0 mg/L of chlorine and 0.2 to 4.0 mg/L of chlorine.

In order to carry out the test, 10 mL of water sample is taken in a tube, and it is placed in the left compartment of the comparator. A reagent tablet (DPD chlorine tablets) is placed in another tube to which 1 cm depth of water is added and the tablet is allowed to dissolve. More water is now added to this until the tube contains 10 mL. Now the comparator disc is rotated till the colours match. The residual chlorine amount can be directly read from the window in the lower right corner of the instrument.

Chloroscope test

This is similar to DPD test with comparator instrument, called chloroscope. Orthotolidine is added to the sample of water to produce colour and that colour is matched with standard colours by rotating the disc through the viewing window.

20 Responses to “Test for Residual Chlorine”

  1. Hunter says:

    Helpful information Thanks!

  2. dhanu says:

    thank a lot for this info!

  3. Ken of Kiti, Larnaca says:

    I never knew one had to wait 5 minutes before looking at the colour of the test sample…

    We learn something everyday!

  4. Glo says:

    Thank you for the information. You answered my question on whether or not orthotolidine measured total or free chlorine residual.
    orthotolidine measure Total Chlorine, Combine chlorine and Free chlorine in water analysis

  5. Dr.Ankit,dehradun says:

    Thanks for the information about the water treatment ,It helps me

  6. Charles says:

    Thank you

  7. Jerry Conley, Plumber says:

    So why do the test on residential water if it is not accurate….found a bottle in my well house along with phenol red for ph…??? what is a good test for residual chlorine on a well


    DPD Colorimetric Methods :
    N,N-diethyl-p-phenylenediamine (DPD) is added to a water sample and, let it allow the reactions, a chemical reaction result is produced that is Red colour. The colour intensity correlates to the sample water residual chlorine concentration.
    A spectrophotometer is used to measure this intensity of the red color.
    The sample’s color is compared to DPD- specific color wheel to determine chlorine concentration.
    This is the easiest method to use for analyzing residual chlorine.

  8. Edward,water operator says:

    Thanks i found my answer!!!

  9. Mohammed Ali says:

    it was helpful but i want to know more about chemical structure and the mechanism of the Reactions

  10. mashimaro says:

    thank you a lot for this information.

  11. nurboo says:

    clear information… helpful

  12. ian says:

    I get weird results when testing PH and for chlorine using phenol red solution and “OTO”

    The colours I get for the ph test is almost indigo after 2 mins.

    For the chlorine test the colour is red….

    I think I have very high iron content plus copper and manganese. Could this be the reason????

  13. Water Treatment says:

    Hello Mr.Mark
    Before adding phenol red, most test kits advise to add another reagent first-a Neutralizer. The neutralizing reagent will rid the test sample of any chlorine or bromine (or any other alternative). If chlorine or bromine is present in high amounts, the addition of phenol red will turn the sample a dark purple.Try with this to find pH and inform us.

  14. sudhanshu says:

    thanx a lot

  15. mahfouz says:

    thanks alot

  16. Rex Butler says:

    Is the leftover reagent from the test considered hazardous waste or can it be poured down the drain? I’m speaking specifically about the reagent for removing manganese as an interference to the total residual chlorine concentration.

  17. jayesh says:

    how much bleaching powder will be reqired as per concentraction of 0.5mg/litre for 10000ltrs of water
    secondly in one litre how much gram is there

  18. Chris says:

    OTO – should not be used for free chlorine analysis due to poor results (see the ASTM method on Residual Chlorine analysis). DPD is the accepted method. Surprised to see OTO still being used for free chlorine when it is so inaccurate! Also it is a known carcinogen.

  19. r.anandaraj says:

    i need the o-tolidine concentration

  20. pawar santosh says:

    Thanks for helping

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