FAS-DPD Keeps Going Pink

[kristof]

I upgraded to a Taylor K-2006 this season. I 've used walmart kits and strips in previous years.

I ran though my first round of testing with the K-2006 today and I have a few questions.

Here's the FAS-DPD procedure given in my kit:

1 - Add two spoons of R-0870 (DPD- Powder) to a sample and swirl till mixed.
2 - Add R-0871 (FAS-DPD Titrating Reagent) dropwise counting drops until color changes from pink to colorless.
3 - Add 5 drops of R-0003 (DPD Reagent #3). Swirl to mix. Solution will turn pink if there is combined chlorine.
4 - Add R-0871 (FAS-DPD Titrating Reagent) dropwise counting drops until color changes from pink to colorless.

My problem is that while doing step 2 my solution would go crystal clear but if I waited 20-30 seconds the solution would turn deep pink again. I reran the test thinking something was wrong. It happened again but I had no idea what to do. I saw two options:

#1 When dropping and swirling do it fast! As soon as the solution goes crystal clear stop adding drops. If the solution turns pink again ignore it.

#2 Once the solution goes crystal clear wait a minute. If it turns pink then continue adding and swirling.

So which is it?

There's room for interpretation in the Total Alkalinity and Calcium Hardness tests too. In both cases, my color changes occurred over a series of drops. It was 2-3 drops between the beginnings of change and a full, deep final color. Where do you stop? When none of the initial color is left or when the final color is full and deep?

I have to comment on the amount of reagent provided for Cyanuric Acid testing. Unless I'm missing something, there's only enough for six tests! I would think that a $70 test kit would provide at least twice that if not more.

There was a pleasant surprise in the K-2006 though. The little booklet titled "Pool & Spa Water Chemistry" turned out to be a fun and informative read!

 

[PaulR]

In the FAS-DPD test, if the pink is gone for 10 seconds you're done with that part of the test. If you let it sit, yes it will turn pink again; that doesn't matter. Do the R-0003 part right away so you don't get mixed up.

For TA and CH, what you're looking for is a color change; the precise colors don't matter. Keep doing drops until the color stops changing. The last drop that caused a change is your drop count.

See a link to some alternate instructions in my sig.
--paulr

 

[JasonLion]

The FAS-DPD test needs to be done fairly quickly, especially when there is a high CC level. Typically, when CC is zero and you do the test reasonably quickly, the sample will remain clear for more than a minute. But if the CC level is very high, or you do the test too slowly, the time before it turns pink again can get shorter. If you do it too slowly, the results will be incorrect.

For TA and CH, you don't count the final drop, the one that didn't cause any further change.

 

[kristof]

The FAS-DPD test needs to be done fairly quickly, especially when there is a high CC level. Typically, when CC is zero and you do the test reasonably quickly, the sample will remain clear for more than a minute. But if the CC level is very high, or you do the test too slowly, the time before it turns pink again can get shorter. If you do it too slowly, the results will be incorrect.

For TA and CH, you don't count the final drop, the one that didn't cause any further change.

Thanks, that clears everything up for me.

I do have a high CC level. I'm doing the opening shock and there's a lot of organic matter in the pool this year. It's eating chlorine like candy.

 

[Craig]

I had the same exact issue this year. Test sample kept turning pink quickly and chlorine was burning up as fast I was putting it in (ok a little exagerating). I started a thread upon opening how much chlorine did you use and came to the same conclusion on the test.

Another point to consider is you probably have ammonia in the pool. I believe that was the reason for my unusually amount of chlorine consumption. Over the winter CYA can converted to ammonia as a result of bacteria in the water.

Good luck and just maintain your shock levels unitl it holds overnight. Took me about 5 days.