Dpd Results Increase Over Time - Normal?

[canTub]

New to the forum, loving all the info.

I've been using Dichlor/Bleach since I bought my tub 6 months ago, no issues.

It's getting cold here in the North so yesterday I switched to Bromine without draining (tub @ 1000 TDS) to lower my daily maintenance.

I'm doing 3-part so I added a bromine bank (sodium bromide), shocked (Bleach and MPS) and now have a BCDMH floater running.

So, not a big fan of the new smell, but it's early so I my have something wrong (here comes the question)...

When I use a Taylor DPD test for Bromine, after the 5 drops of R0001/R0002 it basically shows 0ppm however over the course of 30-60 seconds, it gradually increases (darker color) to >10ppm. Is this normal? I don't remember this happening with Chlorine (color was always immediate).

Test strip shows 0-1ppm.

FAS-DPD looks like I'm getting 10ppm (but my powder is quite old and questionable). It also seems to take a while for the DPD powder to turn my sample pink (where it was immediate with Chlorine).

Is it normal for Bromine to take a while to color in tests?

[canTub]

So I did a quick check with Dichlor mixed in a drinking glass.

With a K-1001 DPD kit, I get an immediate color change to ~4ppm with my Dichlor sample. Sitting for 5 minutes on the counter, the color remains constant.

I then take my hot tub (bromine) and sample the same way. Immediately I see no color change (0ppm). However over the course of 5 minutes, it gradually goes well off the chart (deep purple).

Is this normal?

How long are you supposed to let a bromine sample sit before reading values?

[chem geek]

Bromine tests only check for Total Bromine because bromamine also reacts with the DPD dye (technically it's the monobromammonium ion NH₃Br⁺ that is in equilibrium with monobromamine NH₂Br and that may form Br⁺ that probably reacts with the DPD dye). However, just as with the chlorine DPD test, the DPD dye can bleach out if the chlorine or bromine level is too high. Perhaps waiting has something consuming the bromine so that as the level drops then less DPD is bleached out or perhaps the clear bleached-out form of DPD is unstable so eventually a color forms.

For the FAS-DPD, you would add more powder if your initial scoop only resulted in a flash of pink. In your case, it sounds like it did have a stable color. So your measurement was probably OK. When you say you are getting 10 ppm, is that after converting to bromine units or is that still in chlorine units? That is, what sample size did you use (10 ml or 25 ml) and how many drops of FAS-DPD reagent did it take before it turned clear?

[canTub]

Chem geek,

I did do the 2.25x.

With FAS, I put the powder in (10mL) and it takes about 30s of swirling before it starts to turn pink. The longer I wait, the deeper the pink color.

Once "somewhat pink" it takes 9-10 drops to clear (0871 Cl reagent), so about 10ppm.

If I let the sample sit for a while, it starts turning pink again. The longer I wait, the darker it goes.

I honestly don't know how long to wait for a sample to sit - my tub could be 0ppm or 10ppm. No idea.

Update:

So I did another 10mL sample, waiting about 2-3 minutes after adding powder. Took about 13 drops to clear and seems to be stable. So it looks like my ppm has been off the charts since converting from Chlorine (I'm sure my cover has enjoyed that).

I find it odd how it takes time for the color to appear - I would have thought the FAS-DPD test could handle the high levels?

[chem geek]

Try adding more DPD powder in the FAS-DPD test. That is what you are supposed to do if you don't see color -- did you see a "flash of pink" when you added the powder? It definitely sounds like your bromine level is very high and just as when chlorine is very high this will bleach out DPD. At least with the FAS-DPD test you can add more powder and still do the test. That's what I suggest you do. You should not wait for the color to develop.

[canTub]

I'm down to about 7ppm today.

There's still something odd with the testing - it still "ramps" the color, both in DPD color and FAS-DPD tests.

I added 3 scoops of powder and it behaves the same, 0 color and gradually goes dark (no quick flash indicator of bleaching).

[chem geek]

Mmmmm. Well, I don't understand what's going on in that case. It's behaving as if the bromine is reacting very slowly with the DPD. If you had been using a lot of tabs, then I suppose a high DMH level from the tabs could potentially tie up the bromine so that it reacted more slowly, but you haven't been using tabs for very long and your bromine is mostly from oxidizing bromide from the bromide bank. We know that high CYA levels can slow down chlorine reacting with phenol red in the pH test (though such slowdown doesn't matter for DPD since chlorine is still released quickly from CYA and apparently reacts quite quickly with DPD) and what you are seeing is a similar behavior with bromine and DPD -- I just don't know the cause.

You said you switched from using Dichlor-then-bleach to using bromine without a refill, which is perfectly OK to do, but maybe there's something in the water interfering with a proper (i.e. fast) reading of bromine with DPD or FAS-DPD. I'm hard-pressed to come up with what that might be, however. It will be interesting to see if this problem persists after your next water change assuming you start off with bromine right away.

[canTub]

Well I had a party last night and added no Bromine. My level should have been -20ppm (figure of speech) with the dirt we put in the pool but this morning I tested and it actually got worse. Same rise of color but the FAS-DPD came out as 15ppm.

So enough was enough, I drained/flushed today and went straight bromine, standard 3 step w/ the addition of borates.

Everything now looks great and the DPD turns pink/purple immediately.

As you said, something in my Di/Bleach much have been interfering/skewing the readings.

For the record (if it helps anyone) I was using Purox 12% bleach with my remaining original Beachcomber products.

"Care Free Boost" appears to be standard Dichlor and "Care Free" appears to be MPS. "Soft" may have been the culprit - I assumed it was Boric Acid but there were no details on the container.

Anyways, thanks for the input, things are good now.

Next step is to find a decent Boric Acid supply in Canada. The little Canadian Tire tin is going to put me in the poor house.

[chem geek]

If you were using the Care Free which is MPS, then maybe that was causing some issues. It normally measures as Combined Chlorine (CC) but high CC can bleed through to FC over time which is what you were seeing. However, with a bromide bank, the MPS should have oxidized bromide to bromine so there shouldn't have been MPS left, but maybe your bromine got used up so you just had MPS. Again, strange. I don't think it's the "Care Free Boost" which is Dichlor. If it's the "Soft", then it's probably not the boric acid in it (if that's what it is) since people have used boric acid without seeing the strange test results you were getting.

You can ask your druggist if they have boric acid. If you have a Walmart, then they may have 20 Mule Team Borax, but then you'd have to add acid as well. Quadra Chemicals has boric acid sometimes sold as Optibor. It's also in spa products such as Proteam Gentle Spa or in larger quantities in Proteam Supreme Plus or in BioGuard Optimizer Plus.