Different FAS/DPD Results with Same Water Sample

[MarkAZ]

I was in the process of shocking, using the TF-100 test kit, and started an overnight FC loss test with an initial evening reading of 15ppm using FAS/DPD. The next morning bright and early I filled up my separate sample bottle, did the test, and the FC was at 13ppm. I dumped out the tested water, but then thought I should also test the CC, so I re-did the FC test (with the same water sample from my sample bottle) and got a reading of 15ppm. Being suspicious, I did the test a 3rd time and got a reading of 16ppm. Each time after adding the 0870 powder my solution was dark pink so I believe the tests were done properly.

I noticed the R-0870 DPD Powder I have has a varied texture to it (see pic). There is very fine particles and various sizes of larger particles. Seems this would make it difficult to get a predictable mix of those when taking a scoop. Is this normal? Could this be the cause of the inconsistent tests? If not, any other ideas? Thx.
[attachment=0:2q0rfqcu]R-0870 DPD Powder-cropped75.jpg[/attachment:2q0rfqcu]

 

[257WbyMag]

Your DPD powder looks fine. It can even get somewhat darkish and yet, still be ok to use.

My guess is either your sample size varied a bit or where you drew the water for the sample was not properly mixed. Did you just add chemicals to the pool? Was the pump on for at least 30 minutes prior to drawing a sample? Did you draw the sample from the same spot as the others? Did you wipe the tip of the bottle dropper between tests? Just a few things to look at.

 

[harleysilo]

from: extended-test-kit-directions-t25081.html

seems like it could be several things, drop application and +-10% error in test


Notes
Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling.
The exact amount of R-0870 powder used is not critical. The goal is to add more than you really need rather than using too little. Using too little R-0870 powder can throw off the results of the test. You need to use enough to bind to all of the chlorine that is present. Adding extra, within reason, has no effect. At very high FC levels it is likely that you will need to use more than the normal amount.
If left sitting on the counter, the sample will turn pink again one or two minutes after the test is completed. This is normal.
When measuring high FC levels, or measuring FC when the CC level is relatively high, it is important to move through the test quickly. Drops should be added about once per second, or slightly faster, swirling the entire time. You can slow down a little at the end of the test, to give you time to watch for the end point.
Over time, it is common for a cloudy residue to build up on sample tubes used for FAS-DPD chlorine testing. The residue can be removed by filling the sample tube with bleach, letting it sit for several minutes, rinsing, and then wiping throughly with a paper towel.
There isn't normally any reason to do this, but if you need more precision you can do this test with a 25 ml sample of pool water and multiply the number of drops by 0.2. In most situations the added precision is useless and simply wastes reagent.
R-0870 is DPD powder. The indicator in the powder turns pink when bound to chlorine. R-0870 powder gets darker over time and eventually starts to clump up. If it is clumped up, you should crush it back into a slightly lumpy powder before using it.
R-0871 is FAS-DPD titrating reagent. It should be a clear colorless liquid. If it turns a dark yellow color, it has gone bad.
R-0003 is DPD Reagent #3. It should be a clear colorless liquid. If it turns yellow, it has gone bad.
The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.

 

[jblizzle]

Do you use a SpeedStir? That certainly helps mix the water and take the variability of swirling it for yourself out of the equation.

 

[duraleigh]

The exact amount of powder used is essentially irrelevant as long as the sample in the cylinder turns pink. There is usually some residual powder that doesn't dissolve and that's irrelevant, too.

When you are new to the test, some error usually creeps in but you get more precise over time. I am not saying that's what happened but often that is the case.

Getting the same amount of sample water in the cylinder is fairly important. That said, if you got 11ml the first time, go ahead and put 11ml again. Your results will be off buy about 10% but you will be CONSISTENT which is the single most important factor.

That bottle of powder is white, not blue like it looks in the pic, correct?

 

[MarkAZ]

My guess is either your sample size varied a bit or where you drew the water for the sample was not properly mixed. Did you just add chemicals to the pool? Was the pump on for at least 30 minutes prior to drawing a sample? Did you draw the sample from the same spot as the others? Did you wipe the tip of the bottle dropper between tests? Just a few things to look at.

Thanks for the reply. This was the first test of the morning and the pump had been running all night. Had not added chemicals since the night before.

I don't think sample size is the problem, as I have a separate plastic squeeze bottle that I use for all the tests. This way I can collect about 4 ounces of sample pool water into this bottle and then use that to fill the various test cylinders, etc. It's nice because I can add as little as one drop at a time to get exact sample amounts. In this particular case the 3 FC tests were done with water from the same sample bottle.

I didn't wipe the tip of the bottle, and I also think the size of test chemical drops could have varied a bit. This may be a key

 

[MarkAZ]

from: extended-test-kit-directions-t25081.html

seems like it could be several things, drop application and +-10% error in test

Much appreciated. Thanks for reminding me about the extended directions. I had seen these before but forgot about them. And I think both of the things you mention are good possible culprits.

 

[MarkAZ]

Do you use a SpeedStir? That certainly helps mix the water and take the variability of swirling it for yourself out of the equation.

Hey neighbor . I don't have the SpeedStir . But with the powder I've been using the spoon itself to stir - am able to get most if not all of the powder dissolved (vs. swirling).

 

[MarkAZ]

That bottle of powder is white, not blue like it looks in the pic, correct?

Yeah, it's a mix of white and light gray (no blue).

 

[MarkAZ]

Thanks for the all the quick replies everyone!

After reading them, I think my best bet at improving the consistency of the tests is to pay more attention to the way in which I drop the testing chemicals. My drop sizes may have been different, as I was not always holding the bottle completely vertical and may have put more pressure on the bottle at times.

Also, the +-10% test accuracy is pretty important to know as well. So an FC of 10 could just as easily be a 9 or an 11.

Thanks again.

 

[anonapersona]

Do you use a SpeedStir? That certainly helps mix the water and take the variability of swirling it for yourself out of the equation.

Hey neighbor . I don't have the SpeedStir . But with the powder I've been using the spoon itself to stir - am able to get most if not all of the powder dissolved (vs. swirling).

Try using the two spoons of powder as described in the instructions. When FC is high, one heaping spoon may not be enough. How much is "enough"? Well, if ALL the powder dissolved, you may not have saturated the solution. Maybe you needed a bit more powder. You ought to have a tiny bit of powder undissolved, then you know that there was more than enough. More than enough is fine, not enough is not.

 

[jblizzle]

I don't know how high the FC needs to be to require 2 scoops. I have used 1 flat scoop and gotten a lovely deep pink up around 23 ppm, but I still saw a crystal or 2 on the bottom.

The SpeedStir helps while doing the test, not just mixing it. You should be continually swirling the tube and water while putting in and counting drops.

 

[MarkAZ]

OK, I'll make sure I have at least a couple undissolved crystals to ensure I've saturated. Thanks anonapersona and jblizzle.