DPD test turns pink then brown but never colorless

[techieprof]

I feel sure this question has been answered, but I've searched and read many posts and not found one that quite matches what I'm seeing. My DPD test turns pink then goes to a color I'd call taupe or brown but never clear. I am definitely using the right reagents, and they don't show the color changes that would indicate they've gone bad. I have tried using an extra scoop of R-0870 with the same results. I wonder if I can't add R-0871 fast enough and whether a speed stirrer would solve the problem, but if that's not likely the issue, I'd rather not make the purchase. This is a new problem. I was previously able to get to clear.

Pool status details: My pool has terribly high CYA. I estimated at 270ish by diluting. When I took over pool care for our family, I discontinued di- and tri-chlor and have been adding gallons and gallons of liquid chlorine, overshooting what the pool calculator recommends since it hasn't raised FC as much as predicted. (My pool size estimate and/or CYA estimate may be off.) Because we want to have the pool resurfaced within the next year and we can get a one time break on the water cost, I'm trying to manage it at high CYA and take advantage of the rainy season (18" per month over the summer) to drain a few inches after each rain and let the rain refill. I've taken out about a foot this way so far. I need to keep the FC really high to keep the algae away, which I am not doing very effectively. Sadly, my Dolphin also just died in a lightning storm along with three other expensive items, contributing to my frustration about spending so much money and effort on the pool just now.

FC: That is the question! OTO is bright yellow, not orange or brown. DPD is a mystery as it never goes clear.
CC: I added the last of the MPS we had on hand, so I couldn't test just now even if the DPD test were cooperating. I have often gotten readings around 4. (Only one box of four MPS packs was ever bought for this pool, not a regular or continuing treatment.)
pH: 6.8 (has come down after rain, raising again with Borax)
TA: 70
CH: 275
CYA: 270ish

So, do I buy a speed stirrer? Is another issue more likely the explanation? Thanks in advance! The help and encouragement of people on this site are so appreciated!

 

[Richard320]

There's probably some kind of interference from metals or something.

If the mixture goes bright Barbie pink and there are a few grains of undissolved powder, you have enough. When the pink disappears is when it should be going clear. I'd use that as the FC.

I'd also work on reducing that CYA level. I too started out with 200+ CYA, and I didn't have the option of draining due to water restrictions, so I had to make it work. It can, but it's not easy and I don't recommend it. Maintenance is a lot easier with reasonable CYA. In Florida, you probably have to worry about floating the pool, but you should be able to lose a foot at a time or so with no problem.

 

[ctrav]

My 2 cents is that a speed stirrer is essential to testing! You can get one here: tftestkits.net

 

[techieprof]

There's probably some kind of interference from metals or something.

The test used to go clear, and the only things I've added are plenty of rain, bleach, borax and backing soda plus that sole pack of MPS. Would rain or MPS introduce something that could cause this?

I have just rigged up a manual speed stirrer - group of magnets I can spin next to a test cylinder with a double-ended magnet in it. All built from toys and tape. It does take one hand to spin it every few seconds, but it works well. However, this didn't solve the problem.

Thanks for your input Richard320 and ctrav!

 

[Texas Splash]

Curious …. you mentioned you added an extra scoop of powder. SO how many and what type of scoops did you add? Two hefty scoops, or 2-3 level scoops? Just trying to see if in your somewhat unusually high CYA/FC scenario, if enough powder is still being used.

Using the small spoon shaped end of the dipper, add one heaping dipper, or two level dippers, of R-0870 and swirl to mix. If the sample turns pink for a moment and then turns clear again, or if it turns brown, add another dipper of R-0870.

 

[techieprof]

I'm starting to think I need to replace R-0870 and/or R-0871. R-0871 is not yellow though, and some say R-0870 never goes bad. The kit has recently been stored in the house, but previously it was stored on a shelf in full shade outside. The reagents are several years old.

Texas Splash, normally I would add one heaping scoop. Because I hope my FC is high, even when it's not quite all dissolved, I have tried adding two like that. Just for kicks, I tried adding a third heaping scoop once. No more will dissolve. I have never gotten "Barbie pink" but I do start with pink. The brown does not show up with the R-0870 (although with so much undissolved after 3 heaping scoops, just the swirling crystals do darken the suspension) but appears when adding the [still clear, not yellow] R-0871 dropwise. Replace one or both reagents?

I made a happy discovery this afternoon that after almost two weeks of lots of rain and draining after each rain and enjoying the free refills from the sky, my CYA is down somewhere around 130 with more rain in the forecast. I confess that I was using my starting CYA value in my initial post, not expecting it to change as quickly as it seems to be. I hope I will soon be able to read it without diluting for a more accurate measure.

 

[Bama Rambler]

I had some old powder do that once. It was several years old and had turned dark. I decided to try it for the heck of it, and it wouldn't turn clear. It's probably time to replace it.

 

[techieprof]

Thanks, Bama Rambler. There is comfort in knowing someone else has seen this before.

 

[techieprof]

Just confirming R-0870 can go bad. A new container is testing as it should with the old R-0871.