FAS-DPD test sample never going clear

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rhythm

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Mar 1, 2008
124
Lake Forest, CA
water sample cloudy after adding DPD powder

I know I've seen this mentioned in the past, but I can't find any relative posts right now....

Normally when I add DPD powder to my pool water sample, the sample stays pretty clear (I can very easily see some of the loose DPD granules swirling around near the bottom as I'm mixing the sample, for instance). But just recently I've noticed that the sample is getting quite cloudy after I add the powder.

Anyway, is this cloudiness indicative of anything that I should be concerned about? Something is certainly different. In the past I've had pretty low CYA levels, and recently I did bring the CYA up to a more "normal" level (around 40 ppm). Is the CYA making the difference?

Thanks,
Greg
 
I shocked my pool tonight to what I would figure would be about 30 ppm, and after a couple of hours I tested for the Cl level with a 10 ml sample. As expected, the sample was a very deep red at first, then a very dark pink for much of the testing, but after about 40 drops, the sample began turning a bit of a dull brown/plum type color as it began to fade a bit. After 50 drops, same. After 60 drops, same. When I got close to 70 drops, it seemed apparent that nothing was going to change at this point. The sample was not getting any closer to clear, and it had been a while since it looked anything like the "normal" pink or red shade that I always see when testing. Do I have bad DPD powder or titrant? Or is there something in the pool water that may be causing this?

Thanks,
Greg
 
Re: water sample cloudy after adding DPD powder

I'm doing the FAS-DPD test for chlorine. After adding the DPD powder and swirling, the sample is pretty cloudy. This may not be anything to be concerned about, but normally in the past the samples have been pretty clear, so I was wondering if the cloudy water after adding the DPD powder means anything that I should care about.

I mentioned the CYA because in the past my CYA has been quite low (~20 ppm), and I recently brought it up to a more useful level (40-45 ppm), so I was wondering if the CYA I recently added is what's making the water sample turn cloudy when I add the DPD powder (the sample appears clear before adding the powder). In other words, is it normal for the sample to turn cloudy with the DPD powder if the CYA level is where I have it now?

Greg
 
I added 3 gallons of liquid chlorine. Since my pool/spa combo is approximately 12,000 gallons I figure that 1 gallon of the liquid chlorine probably raises my chlorine level ~10 ppm. Prior to adding the 3 gallons, I had:

FC 3
CC 0.5 or less
pH 7.4
CYA 40-45
TA 90
CH normally around 350 (haven't checked in a few months)

The DPD powder and FAS-DPD titrant are just over a year old (well, that's when I bought them). I have always stored the kit inside the house in a cabinet (no excessive light, heat, or moisture), so it's not subject to harsh conditions.

Update: This morning I performed the test again with similar results. The sample was pink after adding the powder, but not as deep and dark as last night. After adding between 40-50 drops, the sample was no longer pink, but was that same pale amber/brown color (almost seemed very pale orange for a while also), and stayed that way until I quit after nearly 100 drops. I'm not quite sure about this, but it also looked like the sample was actually a bit darker pink after the first 8-10 drops (then it began to get more pale again later).

In any case, I've been doing this test for a couple of years now, and I've never had any issue reaching a totally colorless endpoint. So I think I'm going to pick up some new reagents today on my way home from work. This seems pretty strange.

Thanks,
Greg
 
I called Taylor about this and the gal I spoke to had no idea why the test might be discoloring and never reaching a point where the sample goes clear.

But she told me something that is contrary to what I thought: she said that the FAS-DPD test is meant to work on water samples with 0.2 - 20 ppm of chlorine. Now I thought that it should work for samples with up to 50 ppm of chlorine! Is she wrong, or should I expect problems if I have over 20 ppm cl in the water?

Thanks,
Greg
 

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What Taylor "promises" and what actually works are two different things. In practice the test works up to 50+ ppm, even though Taylor doesn't "promise" that it will.

I merged the two threads. The topics overlapped and were creating confusion separately.
 
JasonLion, thanks for the reply and confirming what I thought about the 50 ppm usable limit for this test. I guess I'm going to try some new reagents and see what happens.

Thanks for merging the two threads. I wasn't sure if I should do one thread with both questions or not. The sample going cloudy after adding the DPD powder may be a completely separate issue than the issue I'm having with my sample not titrating properly. I guess I'm wondering if it's normal at all for the sample to cloud up after adding the DPD powder.

Thanks,
Greg
 
Yeah, that's new to me, too, Greg. I have, in fact tested FC up in the 60's using the FAS/DPD and it has worked without a problem each time.

I would have to suspect that either the R-0870 powder or the R-0871 has somehow been contaminated but that seems pretty unlikely given the way they are packaged.
 
Dave, it's pretty weird for sure. I've tested water with some pretty high chlorine levels too, and never had a problem. Both reagents are getting near the bottom 10%, so I'm just going to go ahead and pick up some new R-0870 and R-0871 today on the way home from work and see what happens.

I have actually seen the cloudiness once or twice in the past, but still had no trouble titrating, so my two issues may not be related at all. In any case, I'll see tonight what happens with some new reagents.

Thanks,
Greg
 
50 ppm for the FAS DPD, although practical, requires either 100 or 250 drops depending on the precision you want. Doable, but not practical.

- What's the CH of the sample ? I'm thinking it may be some form of calcium precipitating (the powder is mostly phosphate buffer)

- Try testing tap water
 
CaOCl2, my water is pretty hard....around 350 ppm, but it's been that way since I've had the pool and I've never experienced a titration problem with the FAS-DPD test before. I have seen the cloudiness once or twice, but that's about it.

As far as testing for 50 ppm, I don't think I'll ever have my Cl levels that high, but since that's the "usable" limit of doing this test, it seems to rule out that my ~30 ppm level should be a problem for this test.

I just brought home some new reagents and I'm going to check things out again tonight.

Greg
 
I've noticed this 'clouding' quite a bit myself. The color clearly goes from red, to pink, to white, but the water isn't clear, it's just cloudy white. It seems to happen randomly to me and I've just figured it was me adding too much powder sometimes (since the directions call for a heaping scoop). The test still seems accurate and the FC makes sense with what I'm expecting, just cloudy.
 
EskimoPie, perhaps you're right about more powder than necessary sometimes making the sample a bit cloudy. I have used two level dippers (going by the Taylor instructions) many times when one would probably have done the trick.

The bigger issue I have here (this thread is a bit confusing because I have two separate issues going on) is that some of my recent titrations, with high Cl levels, have never completed. In other words, the color of the sample never became clear, or colorless (as opposed to clear instead of cloudy). I'm working on that tonight with some new reagents and I'll post results later.

Thanks for the input.

Greg
 
Ignore the cloudiness. I have seen it time and again and it means nothing. The color change you are looking for is from pink to no pink or colorless (a sample can be colorless but not clear. Turbidity is not a color!). I thnk the word 'clear' might be confusing here. The Taylor instructions for this test never use the word "clear", they use the word "colorless", meaning lack of pink. Perhaps saying, until the pink color disappears" might be a better way to word this.
http://www.taylortechnologies.com/produ ... umber=5811

They also say to add additional DPD powder if the initial pink color does not hold (when testing high FC).

I suspect that CaOCl2 is right and that the phosphate buffer is responsible for the cloudiness. BTW, using one of his magnetic stirrers really helps eliminate this problem in many cases! It is well worth the money!
portable-magnetic-stirrer-from-apollo-pools-t1489.html
If you don't have one yet you should really consider adding one to your testing equipment.
 

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