Pool Chlorine Testing - Palins D.P.D. Method

The Palins D.P.D. method for testing pool chlorine is a recent addition to the methods for testing free and total chlorine residuals and has the advantage that the reagent chemicals are in tablet form. This reduces the chances of contamination and the addition of the wrong amount of reagent. The test for free chlorine is also more positive and reliable.

Reagents required D.P.D. tablets (active constituent is diethyl-p-phenylene diamine).

• 1 bottle of No. 1 tablets
• 1 bottle of No. 3 tablets
• Colour disc range
• No. 3/40B 0.2 - 4.0 ppm chlorine
• No. 3/40A 0.1 - 1.0 ppm chlorine
• A filter is also supplied for use with the discs

Procedure

1. Fill one tube with water and place in left-hand side of comparator.
2. Take two tubes, rinse with sample water and leave about 1/4 " (no more) of water in one cell.
3. Add one D.P.D. tablet No. 1 to this tube and wait until tablet is broken up.
4. Fill tube to 10mL mark with water sample and mix by pouring backwards and forwards between the two tubes.
5. Place full tube in right hand side of comparator. Take reading and record as free chlorine.
6. Now add D.P.D. tablet No. 3 to the right hand tube.
7. Mix by pouring backwards and forwards with another tube.
8. Replace tube in right hand compartment. Take reading after two minutes and record as total available chlorine.
9. The difference between the two readings will show the combined chlorine content (or the total amount of chloramines present).
10. By the use of another D.P.D tablet, No.2, the monochloramine and dichloramine fraction of the combined chlorine can be estimated

For obtaining greater accuracy in reading, comparators can also be obtained which have larger cells. The colour change for the greater depths of solution are more easily recorded. On such type is the Nesslerise that has a cell holding 50mL of sample water.

Lovibond Comparator discs should not be used with fluorescent or uncorrected tungsten light, as they are match for daylight or its equivalent. When good daylight is not available, a special artificial daylight cabinet is available for supplying a source of white light equivalent to natural diffused daylight.

Amperometric Titrator

The most accurate method of determining chlorine in water is by means of measuring the flow of current in a buffered solution (Amperometric titration). The inherent disadvantages of the comparator methods such as the effects of temperature, turbidity, colour, iron, manganese and nitrites are eliminated by this method. The various forms of chlorine in water (free, total, monochloramines, dichloramines and trichloramines) can be estimated with extreme accuracy down to 0.01 ppm from the one sample.

The instrument needs a source of power to operate it. The time required to do the test is not much longer than the comparator method, once the instrument has been set up. However, greater care is needed in handling the instrument and keeping it in a clean condition free from contamination. The titrator is only valuable for research and development work or for troubleshooting.