Confused by FAS-DPD explanation on Taylor website.

[HT2020]

hi guys,
Maybe I'm dense but I'm not understanding a critical concept on the Taylor website.  They are explaining why the FAS-DPD test may be better than test strips or color gradation type testing. In their first paragraph they specifically state that this test allows users to "measure both free and combined chlorine".  However, in their concluding paragraph they state that "the second part of the test" is what determines combined chlorine (ie: counting the number of drops needed to turn colorless).  If so, then what part of the test gives you the Free chlorine level??

"The latest trend among service professionals and a significant number of health officials is testing chlorine with FAS-DPD. This variation of the traditional DPD method allows users to measure both free and combined chlorine levels as low as 0.2 ppm.

A buffered DPD indicator powder is added to a water sample and reacts with chlorine to produce the pink color characteristic of the standard DPD test. Ferrous ammonium sulfate (FAS) is then added drop by drop until the pink color completely and permanently disappears, signaling the endpoint of the reaction. To get the reading, the number of drops used to cause this color change is multiplied by the appropriate factor for the size of the water sample.

The second part of the FAS-DPD test determines the amount of combined chlorine present (i.e., mono-, di-, or trichloramines) by the number of drops needed to turn the sample from vibrant pink to colorless again."

[chem geek]

• You get a pool water sample -- 25 ml if you want 0.2 ppm per drop resolution or 10 ml if you want 0.5 ppm per drop resolution.
• You add a scoop (or two, for the larger sample) of DPD powder.
• If there is any Free Chlorine (FC), the sample will turn pink/red when you swirl the sample to dissolve the DPD powder.
• You then add FAS-DPD drops, swirl to mix, and count the number of drops you add until the sample turns clear.  That's your Free Chlorine (FC) level and is the first part of the test.
• You then add R-0003 reagent (potassium iodide).  If there is any Combined Chlorine (CC) present, the sample turns pink/red.
• You then add FAS-DPD drops, swirl to mix, and count the number of drops you add until the sample turns clear.  That's your Combined Chlorine (CC) level and is the second part of the test.

You multiply the number of drops you counted in each step by 0.2 (or divide by 5) if you used a 25 ml water sample or you multiply by 0.5 (or divide by 2) if you used a 10 ml water sample.

The advantages to the FAS-DPD test are:

• It's more accurate with a higher resolution to within 0.2 ppm if you use a 25 ml sample.  For higher levels, the error is within 10% of the reading.  Even with a 10 ml sample, it's accurate to within 0.5 ppm (or 10% of the reading, whichever is larger).
• It's easy to see when the sample turns clear and to simply count the drops until that happens -- no comparisons with a standard are needed.
• The test does not bleach out as standard DPD tests do where standard DPD is not accurate above 5 or 10 ppm depending on the manufacturer.  With the FAS-DPD, you can test up to 50 ppm.  If you see a flash of pink when you add DPD powder, you just add more powder until the color holds.

[HT2020]

Thanks chemgeek.  That's a very good explanation.  Test strips seem to be good if you're ballparking the FCL level within 2-3ppm.  That may be accurate enough for most people.  However, the other day I had wildly conflicting readings from the strip and the test tube kit (3 on the strip and 7 on the tube) and felt that both modalities were too dependent on color gradations to be accurate.  I think this should be a more accurate way of doing it.  I assume that I can simply purchase the necessary reagents and do this without purchasing a whole other kit?

[chem geek]

Yes, you can buy the TFTestkits FAS/DPD Chlorine & CC's test (which uses Taylor reagents) or the Taylor K-1515-A where the former has more volume of reagents so is more economical.