1 Rinse test tube with sample leaving a few drops of sample in the tube.
2 Crush the DPD No 1 tablet in two or three drops of the water sample until the tablet is thoroughly dissolved.
3 Add the 10 ml test solution, mix and seal the tube with the cap.
4 Gently invert the tube to remove any bubbles from the inner walls of the tube
5 Select Phot 7 on Photometer.
6 Take photometer reading in usual manner – see Photometer instructions.
7 The result represents the free chlorine residual in milligrams per litre (mg/l).
Stop the test at this stage if only free chlorine determination is required.
8 If it is desired to measure combined or total chlorine residual, continue the test on the same test portion. Select ‘Follow On’ from the screen options to continue the test program.
9 If shock treatment chemicals are present in the pool, add one DPD Oxystop tablet, crush and mix to dissolve. Stand for one minute before proceeding.
10 Add one DPD No 3 tablet, crush and mix to dissolve.
11 Stand for two minutes to allow full colour development.
12 Take photometer reading in the usual manner.
13 The result represents the total chlorine residual as milligrams per litre(mg/l).
14 The combined chlorine residual is obtained by subtracting the free chlorine residual result from the total chlorine residual result :- ie Combined Chlorine = Total Chlorine – Free Chlorine
Notes A too high chlorine level (above 10 mg/l) can cause bleaching of the pink coloration formed in the DPD test and give a false negative or lower than expected result. If a colourless or pale pink test solution is obtained then a high level of chlorine may be present, check for the possibility of bleaching by repeating the test on a sample diluted with chlorine-free water.