Difficulty discerning FAS-DPD color changes

[thechairperson]

Hi- I started my first SLAM a couple days ago and it's going well. However, I'm color blind and find the FAS-DPD test to be quite difficult. My wife was helping me with it but is now out of town, and it's pretty much a useless test to me. I don't think I'd be able to discern the exact point it goes clear, although I could have a pretty good estimate that it's within a certain number of drops. I guess target FC for non-SWCG pools is within a 5 drop range, so I should be able to tell if I'm in the right range. But that would still hurt my ability to adjust chlorine accurately. Is there an alternative?

Also, when I put the R-0780 powder into the water sample, it's never quite as pink a it is in this example video. Maybe the FC is mostly used up by the time I test it?

 

[jblizzle]

The absolute color does not matter, it is the color change you are looking for.
You always add drops until the last drop causes no discernible color change and that last drop does not count, this is true for FC, CC, TA, and CH tests
The FAS-DPD is by far the easiest and best chlorine test.

If your FC level is very low, then the pink will not be very bright.

Are you using 10ml of water so that each drop is 0.5ppm? That might be easier to see.

 

[thechairperson]

Yes, I'm using 10ml. But I was going to ask about doubling that as it is a bit easier for me to discern colors when there is more surface area of the color in the tube.

My biggest problem is that I struggle to discern between light pinks and whites. So I can definitely see the color changing at times, but discerning the final change is very challenging.

 

[jblizzle]

You can use 25ml of water and then each drop is 0.2ppm ... that is going to require a lot more reagent though. AND I think the color change will be more gradual so the end point might be ever harder to see.

Do you use a SpeedStir? Wondering if different lighting might help ... although with the underlight on the SpeedStir I find it can be harder to see the last change too as the light pink seems to be washed out.

 

[thechairperson]

I do use the SpeedStir but haven't been using it with this test. I find that with only 10ml of water I can't see enough water once it's stirring it around, since the magnetic stirrer is in there and it creates a whirlpool in the middle. Tests that have 25ml work a lot better for me. I'll give it a try with 25ml and see how it works out for me. When my wife is back home we can drop back down to the lower amount.

 

[jblizzle]

Look at the vial from the side not the top?

 

[thechairperson]

When using 25ml of water, do I need to double the amount of the powder I put in at the beginning? And then double the reagent before the CC test to 10 drops?

Also, say you have 5ppm of FC when doing a 10ml test, which means it took 10 drops of the reagent to go from pink to clear. If I go beyond that and add a couple more drops to make sure it is no longer changing, does that affect the CC test after it?

 

[jblizzle]

You may need a little more powder for the 25ml test, more so if your FC is very high for a SLAM.
You would only need 5 drops for the CC test still.

I don't think extra drops in the FC portion of the test would affect the CC portion of the test ... but let's get confirmation from @JoyfulNoise

 

[thechairperson]

Great, thanks for the help! For what it's worth, the pool is coming along. You all have been such a big help. I had never done anything with one before and I think I'll be done with the SLAM in a couple days. Here's last Friday to this morning.

 

[jblizzle]

Great progress!!

 

[Wobblerlorri]

Try running the speedstir at a speed that does not cause a whirlpool, you don't need it that fast. Adjust the speed so the magnetic stirbar just does stir everything. It will spin, but relatively slowly. There will not be a column of air, just the liquid. It might take a bit longer, but you might be able to see the color change, especially with a 25 ml sample.

 

[mknauss]

The Speedstir I have is on or off. No adjustment.

 

[Wobblerlorri]

Ah. That's a problem.

 

[Richard320]

Great progress!!

You might be to the point where some DE in the sand filter would accelerate things. The link to the how-to is in the SLAM Process article.

 

[JoyfulNoise]

You only need 5 drops of the R-0003 regardless of sample size. There’s more than enough iodide added by the reagent to react with any chloramine present.

If you add excess R-0871 titrant to the sample after the FC Test is complete, you could adversely affect the oxidation of the DPD dye in the second step. So NO, you don’t want to add excess FAS titrant.

 

[thechairperson]

Thanks everyone. That's all helpful.

@Richard320 I had been thinking about adding some DE. I think I'll give that a try. The guide says to avoid "gardening DE". Is there anything in particular to look for when purchasing DE?

 

[Wobblerlorri]

You want to specifically get swimming pool DE. HD has it sometimes over by the pool supplies.

 

[Richard320]

Thanks everyone. That's all helpful.

@Richard320 I had been thinking about adding some DE. I think I'll give that a try. The guide says to avoid "gardening DE". Is there anything in particular to look for when purchasing DE?

It'll say something like "filter aid" on the package and it'll be in the pool supplies. And it'll be way bigger than you want or need.

There's also cellulose DE replacement that comes in smaller bags, but it's also more expensive per pound.

Either works.