Fas/dpd

[DONNIE]

Richard, you're too funny. I never expected an experiment would be in order for my question but I really do appreciate the response and the Saturday morning laugh I got out of it.

Donnie

[eljefe281]

Is it critical that all of the powder dissolves?

[chem geek]

I would say that it is not critical for the same reason that it is not critical to be precisely two scoops. I noticed that when I used more scoops (4) that more material did not want to fully dissolve. I tried measuring both ways -- soon after adding 4 scoops with a flat piece that did not initially dissolve and again measuring after 4 scoops but mixing more thoroughly over a longer period of time to more fully dissolve. I got a somewhat (1-2 drops depending on which trial I look at) lower result with more full dissolving, but I attribute that more to taking a lot longer and swirling which probably outgassed some of the chlorine (and raised its temperature closer to room temperature, though I'm not sure that mattered for this test). Just having the sample sit over minutes had it drop in measured chlorine.

It seemed to me that the most critical factor was doing the measurement rather quickly and not letting the sample sit for too long. The chlorine measurement was consistently lower the longer that the sample of water sat in an open container and the longer I swirled the sample. This was only a large effect (about from 3.2 ppm to 2.4 ppm) over the entire time of the experiment (I don't know -- maybe about 20 minutes or so for all the tests) and with maximum swirling.

Remember that the purpose of the DPD powder is to produce a color in the presence of ANY amount of chlorine and that more DPD powder will produce a darker (more intense) color for the same amount of chlorine (and conversely the same amount of DPD powder will produce a darker color when there is more chlorine) up to some point (with diminishing returns as most of the chlorine combines with the indicator). So the precise (especially the minimum) amount of powder is critical for the colorimetric DPD test, but not for the drop-based FAS-DPD test. The titration is simply consuming the chlorine so that when it all gets consumed there is none left to react with the DPD powder to produce any color. The titrant drops (FAS or Ferrous Ammonium Sulfate) remove the chlorine that is combined with the indicator to produce color, but it does not react with tightly bound chlorine aka combined chlorine. Adding the third reagent causes the chlorine that is bound tightly in combined chlorine to oxidize the third reagent (iodide) so that it (iodine) then becomes free to react with the DPD powder to produce color so that you can then add more drops to measure combined chlorine (by removing the iodine that is combined with the indicator to produce color). This link that describes the interference with non-chlorine shock (potassium monopersulfate) describes the chemistry involved.

So the bottom line is to quickly dump two scoops in, swirl briefly to mostly mix it, then add drops while swirling to see when the color disappears. If you know that there will be a lot of drops, then you could add a bunch without swirling and then start the swirling when you know you're close to the expected amount (but in general, you don't know what to expect so you would just swirl as you go). It's not important to be precisely accurate when it comes to chlorine. You want to be in the rough ballpark, but even if you are off by 20-30% it's not terrible assuming you maintain conservative chlorine levels (if you're off by 50% or more, then that could be a problem).

Richard

[fcfrey]

Thanks Richard,

I knew you would have a "Chemists" explanation for the test. I do try to maintain a constant temp for the sample with a water bath. I have to admit that I use a coffee stir stick I scrounged from Mickey Dees --- They work well and have a nice long handle and can be easily cleaned.

As a side note, Reagents are the least of my pool expenses, so I think I would rather be consistent not "cheap" and risk the results of the test. And yes, I agree it's not all that critical.

[DONNIE]

Thanks Richard. Like others have said before. We go through the motions of testing and adjusting but never really know whats going on with the chemicals themselves. I'm glad your here and (seem to) enjoy educating us.

Donnie