FAS-DPD and Speedstir -- strange results

[chem geek]

tpspier, please look at the Taylor video at this link where you should click on the "Pool / Spa" under "Liquid Kits" on the left and then click on the "To Test (Free and Combined) Chlorine using FAS-DPD [updated 3/5/10]" under the "K-2006 Complete™ Kit with FAS-DPD:" under the video area. Look at this video and notice how they stir. Do you call that "a very light swirl between adding drops" or "swirling more aggressively (almost shaking) between drops" or something in between. One should definitely not be doing just a light swirl. It should be a fast back-and-forth to cause a thorough swirl and mixing. It shouldn't be so vigorous as to cause splashing or getting air bubbles into the sample.

I still have a concern that too vigorous a stirring might be outgassing chlorine. A simple way to verify that is to take two samples where for one you do vigorous stirring BEFORE adding any DPD powder of titrating drops while for the other you do not. Then measure the chlorine in each of these using a consistent technique -- say, moderate swirling. If you measure a difference, then vigorous swirling may in fact be changing the chlorine level so would not be accurate. If there is no difference, then it says that thorough mixing during the test is important so swirling should not be light.

[JimK]

Wow, they use alot of powder in that demo!

I've found the suggestion here to use one scoop works just as well as long as CL isn't really high.

[singingpond]

I still have a concern that too vigorous a stirring might be outgassing chlorine. A simple way to verify that is to take two samples where for one you do vigorous stirring BEFORE adding any DPD powder of titrating drops while for the other you do not. Then measure the chlorine in each of these using a consistent technique -- say, moderate swirling. If you measure a difference, then vigorous swirling may in fact be changing the chlorine level so would not be accurate. If there is no difference, then it says that thorough mixing during the test is important so swirling should not be light.

I tried measuring 3 slightly different ways this morning, all with 10 ml water sample:

1) the way I usually do, with moderate swirling between drops -- 4.5 ppm FC

2) agitating the water sample, before adding powder, for 2 minutes (sloshing the water around as much as I could without actually losing any out of the open tube); then adding powder; then doing the habitual moderate swirling between drops -- 4.0 ppm FC

3) no pre-agitation of the water, but doing more aggressive swirling after each added drop -- 4.0 ppm FC

So, at these relatively modest chlorine levels, both of the approaches that involved more agitation of the water (either pre-test or during test) gave somewhat lower results. It wasn't a big difference, but maybe that difference would be relatively greater if the total chlorine levels were higher (as I think they were in the original post in this thread)?

[chem geek]

The chlorine loss rate would be a percentage so 0.5 out of 4.5 is somewhat over 10% though your resolution is only 0.5 ppm when using a 10 ml sample so this isn't really a fair test. With the higher levels of FC you were seeing 17 vs. 22 drops, a difference of more than 20%. Still possible, however. I just find it surprising that the swirling of the Speedstir is that vigorous. It's certainly very helpful at getting a better reading in the CH test where it requires much more thorough mixing and where doing it manually can take a much longer time having to swirl between each drop.

[tpspeier]

chem_geek: I watched the Taylor video, and I would say that my original swirling (not the aggressive style) was about the same as in the video. One difference between my swirling and the video is that I am not yet able to swirl and drop at the same time (my coordination isn't there yet). I would add one drop, then swirl, then another drop, then swirl, etc. I noticed in the video that the presenter was manually swirling the entire time while adding drops (which is similar in behavior to the Speedstir). When I use the aggressive style swirling, the water keeps moving between swirls while adding the next drop.

Last night, I performed test #10 from PoolDoc's list. This involved using the Speedstir to mix the DPD powder in each of two samples, and using the Speedstir while dropping one sample and manually swirling the other sample (with my original style swirling). The Speedstir took 16 drops and the manual took 20 drops. I also tried chem_geek's suggestion of agitating the water before adding any reagents. I ran the Speedstir on the water sample for about two minutes before adding the DPD powder. I mixed the DPD manually and swirled manually after each drop. This took 20 drops. At least with this test, it appears as if the difference in results is due to the stirring method while adding drops. I'll perform tests #12 and #14 from the PoolDoc's list, but I need to order more reagent.

[JimK]

chem_geek: I watched the Taylor video, and I would say that my original swirling (not the aggressive style) was about the same as in the video. One difference between my swirling and the video is that I am not yet able to swirl and drop at the same time (my coordination isn't there yet). I would add one drop, then swirl, then another drop, then swirl, etc. I noticed in the video that the presenter was manually swirling the entire time while adding drops (which is similar in behavior to the Speedstir). When I use the aggressive style swirling, the water keeps moving between swirls while adding the next drop.

Last night, I performed test #10 from PoolDoc's list. This involved using the Speedstir to mix the DPD powder in each of two samples, and using the Speedstir while dropping one sample and manually swirling the other sample (with my original style swirling). The Speedstir took 16 drops and the manual took 20 drops. I also tried chem_geek's suggestion of agitating the water before adding any reagents. I ran the Speedstir on the water sample for about two minutes before adding the DPD powder. I mixed the DPD manually and swirled manually after each drop. This took 20 drops. At least with this test, it appears as if the difference in results is due to the stirring method while adding drops. I'll perform tests #12 and #14 from the PoolDoc's list, but I need to order more reagent.

This is interesting.

I've always added drops while continuously swirling much like in the Taylor video.

[nefretrameses]

Doc or Chem geek: Just wondering if there was ever any resolution to this.

Santa brought me a speedstir and with pollen starting to fall in my pool, it's time to ramp up my testing program.

[chem geek]

No resolution as far as I know. It's very strange because your experiments would seem to indicate that it's the amount of stirring while adding drops that is making the difference. Agitation beforehand didn't affect the results. Did you ever contact Taylor about this? I know Ben mentioned you call them, but I didn't see any response about that.