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."