FAS-DPD and Speedstir -- strange results

[chem geek]

Chlorine at pool/spa pH (i.e. in the 7-8 range) does not outgas as molecular chlorine (Cl2), but rather as hypochlorous acid (HOCl). The Henry's Law constant for Cl2 is 0.093 M/atm while for HOCl it is 660 M/atm so chlorine gas is 7000 times more volatile (from an equilibrium perspective). However, in pool water with an FC/CYA ratio of around 10% and pH near 7.5 and 350 ppm chloride, the HOCl concentration is over 3 million times higher than Cl2. At these concentrations, the equilibrium pressure of HOCl is roughly 500 times higher than Cl2. It is HOCl that outgasses at pool/spa pH, not Cl2. (A saltwater chlorine generator pool will have the Cl2 outgas 8.5 times faster than my example, but that's still over 50 times slower than HOCl).

Now that said, the aeration to remove chlorine from tap water is with chlorine that has no CYA in it so is roughly 10 or more times higher in concentration and it usually takes quite some time to lower the chlorine level. Even so, it's a possibility and one that could readily be tested on its own by using just using the Speedstir on its own before doing the test and doing so for different lengths of time. If the measured chlorine level declines as a function of the stirring time, then the loss from outgassing during stirring may our culprit.

[tpspeier]

I was able to get to the bottom of this issue, and I am relieved and embarrassed to admit that this is a case of user error. The short explanation is that I was not manually swirling the samples enough after each drop of titrating agent.

Here is the long explanation ....

My 8-year old daughter has taken an interest in helping me take care of the pool (yay!). She wanted to perform the FC test by herself. She did a really good job getting exactly 10 mL of water, and adding the DPD powder. However, the number of titrating agent drops was way higher than I expected it to be based on the amount of bleach I had added the night before. I had watched her perform the test, so I knew that she swirled between each drop. I then repeated the test myself and it resulted in several drops less than her test. It occurred to me that she gave a very light swirl between adding drops. I repeated my test, this time swirling more aggressively (almost shaking) between drops. This resulted in even fewer drops than my test before. I then got out the Speedstir and tried again. The Speedstir showed the same results as the "aggressive" manual swirling. I have been able to reproduce the same results between both methods the last two days.

I see how often many of you post of these boards, and I know your are busy. Thank you for your help with this issue. I am sorry that it ended up being user error. I will definitely always use the Speedstir from now on, and would recommend it to all new pool owners to help eliminate inconsistent results.

[singingpond]

I've been following this thread with interest. Glad you figured out a reason for the discrepancy!

However, the reason you discovered (a difference on how hard you swirl the sample) makes me wonder which result is actually the more 'correct' one. If the aggressive stirring result gives the truer report on the actual chlorine levels, that would suggest that a lot of us doing the manual swirl are getting results that are too high. I certainly don't 'almost shake' between drops.

I suppose as long as one uses the same technique every time, the differences between readings from day to day are still meaningful. However, one really wants correct readings, not just repeatable readings.

I'm going to try your experiment (doing the FAS-DPD manual measurement on two identical samples, just varying the amount of swirling between drops), to see how large a difference I can get.

edited to add:

It does seem worthwhile to look at the other thread you linked earlier:
http://www.poolforum.com/pf2/showthr...-Fas-dpd/page2

A comment from chem geek in that earlier thread:
> 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). <

It still leaves the question, though, as to whether the Speedstir result is closer to correct than the typical manual result, or whether it's the other way around.

[PoolDoc]

It would be worth knowing how 'swirling' affects the test . . . and which result is the valid one. So, please continue!

Thanks!

[JimK]

I've been following this thread with interest. Glad you figured out a reason for the discrepancy!

However, the reason you discovered (a difference on how hard you swirl the sample) makes me wonder which result is actually the more 'correct' one. If the aggressive stirring result gives the truer report on the actual chlorine levels, that would suggest that a lot of us doing the manual swirl are getting results that are too high. I certainly don't 'almost shake' between drops.

I suppose as long as one uses the same technique every time, the differences between readings from day to day are still meaningful. However, one really wants correct readings, not just repeatable readings.

I'm going to try your experiment (doing the FAS-DPD manual measurement on two identical samples, just varying the amount of swirling between drops), to see how large a difference I can get.

I was wondering the exact same thing. I also don't swirl/shake agressively. I would think if my results were inaccurate that I would have noticed an algae issue (I have not) since I tend to keep my CL level at the lower end of the Best Guess recommended level.

I guess the way to find out is to add a known level of CL to water known to be CL free, then test using both methods.

[nefretrameses]

I have also followed your test with interest. Thank you for doing all that work. As others have stated, most of us are manually stirring, but not aggressively. I suppose the technique shown in the Taylor demo videos would be "correct". It would be nice to know which is more accurate, typical manual stirring or speedstir/aggressive stirring.

[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.