FAS-DPD Chlorine Test

The FAS-DPD chlorine test is used to find the level of Free Chlorine in the pool water. It uses a powder to show the presence of free chlorine in the sample pool water followed by liquid drops to determine the total amount of free chlorine in the sample of pool water. It is very important to know the amount of free chlorine in the pool water for swimmer safety and water cleanliness.

1. Rinse the sample tube with pool water.
2. Fill the sample tube with pool water to the 10 ml mark. The top of the sample will be curved. This curve is called a meniscus. The bottom of the meniscus should be level with the 10 ml mark.
3. Using the small spoon shaped end of the dipper, add one heaping dipper, or two level dippers, of R-0870 and swirl to mix. If the sample turns pink for a moment and then turns clear again, or if it turns brown, add another dipper of R-0870. If the sample remains clear the entire time, your FC level is probably zero, however it is best to verify that with an OTO chlorine test, since the FAS-DPD test is prone to the occasional false zero.
4. Swirling constantly and counting the number of drops as you go, add R-0871 one drop at a time. Continue adding drops until the color changes to clear. The pink color may become extremely faint before it goes away. Make sure that the sample goes completely clear.
5. Multiply the number of drops by 0.5 to get your FC level.
6. Add 5 drops of R-0003 and swirl to mix. If the sample remains clear, your CC level is zero.
7. If the sample turns pink again, add R-0871 one drop at a time, swirling constantly and counting the number of drops as you go. Continue adding drops until the color changes to clear. The pink color may become extremely faint before it goes away. Make sure that the sample goes completely clear.
8. Multiply the number of drops by 0.5 to get your CC level.
9. Dispose of the sample safely. It is best to pour it down the drain with the water running. Do not add it back to the pool.
10. Rinse the sample tube with tap water and store for next time.

Notes

• Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling.
• The exact amount of R-0870 powder used is not critical. The goal is to add more than you really need rather than using too little. Using too little R-0870 powder can throw off the results of the test. You need to use enough to bind to all of the chlorine that is present. Adding extra, within reason, has no effect. At very high FC levels it is likely that you will need to use more than the normal amount.
• If left sitting on the counter, the sample will turn pink again one or two minutes after the test is completed. This is normal.
• When measuring high FC levels, or measuring FC when the CC level is relatively high, it is important to move through the test quickly. Drops should be added about once per second, or slightly faster, swirling the entire time. You can slow down a little at the end of the test, to give you time to watch for the end point.
• In rare cases the sample may turn cloudy. If this happens the test is still valid. You add drops until the sample turns cloudy white, i.e. the red/pink is completely gone, rather than waiting for it to turn clear.
• Any MPS in the water will count towards the FC reading. Taylor R-0867 Deox Reagent can be used to eliminate this interference.
• Over time, it is common for a cloudy residue to build up on sample tubes used for FAS-DPD chlorine testing. The residue can be removed by filling the sample tube with bleach, letting it sit for several minutes, rinsing, and then wiping throughly with a paper towel.
• There isn’t normally any reason to do this, but if you need more precision you can do this test with a 25 ml sample of pool water and multiply the number of drops by 0.2. In most situations the added precision is useless and simply wastes reagent.
• R-0870 is DPD powder. The indicator in the powder turns pink when bound to chlorine. R-0870 powder gets darker over time and eventually starts to clump up. If it is clumped up, you should crush it back into a slightly lumpy powder before using it.
• R-0871 is FAS-DPD titrating reagent. It should be a clear colorless liquid. If it turns a dark yellow color, it has gone bad.
• R-0003 is DPD Reagent #3. It should be a clear colorless liquid. If it turns yellow, it has gone bad.
• The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.