FAS-DPD and Speedstir -- strange results

[tpspeier]

I've filled in the data from the for the first two rows. When I did steps #10 and #12, I incorrectly used fresh pool water instead of aged pool water. I'll refill my jug and perform those tests on the aged water tomorrow.

[chem geek]

Maybe the stirring is causing the chlorine to outgas. Yes, your suggestion of trying turning off/on Speedstir between drops to better simulate the amount of manual swirling would be a good test. If having the Speedstir not swirl as much is making a difference, then swirling is the culprit, though I would have the expected that if you took longer with the Speedstir (so longer time swirling) you'd have an even lower result than you've been seeing.

If swirling was causing more outgassing, I'd expect the pH to rise so one other test you can try (you already tried TA and found them to be the same) is the pH test. See if the longer you swirl the higher the measured pH using the Speedstir.

[JimK]

Maybe the stirring is causing the chlorine to outgas. ......

I was wondering the same thing. I had a friend that had several large aquariums and would aerate tap water to remove CL before using for the fish (she prefered not to use chems if possible).

[PoolDoc]

Maybe the stirring is causing the chlorine to outgas.

How, unless the pH is below 6.0?

I checked; the R0871 FAS solution shows a pH of 2.71. It's possible that when FC > 10 ppm (20 drops or 1.0 ml) that the acidity of the FAS overcomes the potassium phosphate buffer, and pushes the sample solution pH to 5.0 or below. If so, then Cl2 will be present in solution, and could be off-gassed under agitation.

I'll add a NEW test to the spread sheet, using 1 drop of R-0006 Base Demand Reagent, add to the water sample after adding DPD powder. I don't know if this will change the accuracy of the test, but it should reduce any off-gassing of Cl2.

http://www.taylortechnologies.com/MSDSS/0870.PDF
DPD powder => 60% potassium phosphate (pH ~7.2?)

http://www.taylortechnologies.com/MSDSS/0871.PDF
pH = 2.71

http://www.taylortechnologies.com/MSDSS/0006.PDF
Soda ash solution, pH => ~1.0

[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.