Fas/dpd

[fcfrey]

Carl and Richard
Please don't yell but I do have to disagree.

When I ran several tests and adding enough to turn it "nicely pink" gave different results than if you use the "prescribed 2 scoops". I called Taylor and they told me that their test requires 50 milligrams of DPD powder which is 2 of their blue scoops. I get very consistent results using this method.

IMHO it's like baking a cake add too much flour it gets tough, too many eggs will make it fall.

Do what you want but I think the manufacturer of the test should know what is best. Even though they do want to sell reagents.

[DONNIE]

But how much is a scoop? A heaping scoop? A level scoop? A scoop that just fills the depression on the spoon?

[fcfrey]

I used a "mini" electronic scale (for reloading ammunition), which measures from 50 grams down to .001 Gram. I found that if I squashed the filled scoop against the inside of the container and then slide it up out of the powder, it gives a packed, nearly level, scoop which is consistently 25 milligrams. Obviously I use 2 scoops (50 milligrams)

Please keep in mind that I use the small blue scoop supplied by Taylor not the larger scoop that is in Ben's kit.

I think, and am sure most every one will agree, the most important part is being consistent with your methods. You will get consistent results which is more important than if you are .1 or .2 ppm off in the result.

In cooking and pool chemistry I admit to being a bit anal about following the directions. That way the recipe comes out right!!

[chem geek]

Out of curiosity, I tried my own experiment and used 1, 2 and 4 scoops and got within 1 drop of the same results with quick consecutive tests, but I did notice variation with time -- mostly declining. There were several factors that could have contributed to the decline with time (mostly that I used a single container that was filled with pool water which was probably warming up and outgassing and getting partly contaminated even though I rinsed the measuring test tube with tap water between tests -- and yes, tap water has chloramine so that didn't help), but I kept going back to different scoop amounts and saw no variation consistent with the number of scoops. If I swirled longer I usually got a slightly lower result than expected (accounting for the slower decline with time). However, I did not try less than one scoop for a "barely visible" test.

I think the main reason that the 50 mg is recommended is that the DPD test is colorimetric in that the intensity of the pink is related to the chlorine concentration AND the amount of DPD powder, so even in the FAS-DPD test the "endpoint" of light pink to clear will be a little dependent on this absolute intensity. Taylor probably assumes a certain typical chlorine level for this test. If you only used one scoop and had very low chlorine levels, you probably wouldn't see much pink to start with so the transition to clear would be hard to catch. Also, higher chlorine levels will bleach out the DPD powder and that could be as low as 12 ppm if only one scoop were used. Two scoops is probably the minimum that works well for this test, but more scoops would certainly not be a problem as even the Taylor instructions (see this link) say to add more scoops if the pink goes away right away (which happens with > 25 ppm FC if you start with two scoops).

Anyway, it's probably best to stick with the Taylor recommendation and at least now we know that a compacted "flat" scoop fill is the accurate amount, but I'm with Carl that it's not critical to get the scoop measurement exactly right. I am not disputing your results, but it seems that there are lots of factors for variation that outweigh at least 1,2,4 scoops. Maybe more (or less) scoops than that starts to give a problem, but I didn't want to use up all my chemicals!

Richard

[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