TF-100 Titration Test for FC

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alanrmeadows

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May 5, 2010
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This morning I did 4 consecutive tests and got the following readings: 16.5, 15, 20, 19. Do any of you have a way of making this test more accurate? In my opinion, the size of the scoop and the dissolving rate of the powder have to be the variable that causes my readings to vary somewhat wildly. To my eye, it looks like the "heaping scoop" is the same size on each test, but who knows. I do know my water sample is identical each time and I'm wiping the end of the titration bottle after 5 drops.

Thanks,
Alan
 
Actually, the amount of powder has little to do with it. As long as there is enough powder to turn the solution pink, the test will perform correctly.

Having test results that far apart is not normal, for sure, but I can't think of an obvious reason why it is happening to you.

Wiping the dropper tip doesn't hurt anything but that really is only applicable to reagent R-0009 when you are doing the TA test.
 
I'm confused myself...I'm getting the water sample from the exact same spot in the pool everytime. It seems to me that a smaller amount of powder turns the solution "less pink," which I would think would turn clear much easier? Is this logic not applicable to this test? That sure seems to be the case with me.

Thanks,
Alan
 
Is this logic not applicable to this test?
Click to expand...
Not quite. A smaller amount of powder will result in less intensity of the pink but the endpoint of the test should be the same.

That's why "a heaping scoop" (which is a pretty innaccurate measure) is sufficient for this test.

It is also not uncommon for the powder to not dissolve completely......makes no difference to the endpoint of the test.

My assumption is your pump is running when you take the sample from below the surface and/or your pool water is thoroughly mixed.
 
alanrmeadows said:
So this is probably a stupid question...I'm a new pool owner...should the pump not be running while doing these tests?

Thanks,
Alan
Click to expand...
There are no stupid q's. :wink:
Yes, the pump should be running and should have been running for at least an hour before taking a water sample to make sure the water has mixed.
 
Now I see that I read his question wrong...YES, my pump actually runs 24 hrs., so my water should be thoroughly mixed, but I still can't get a consistent reading with the FC titration test.

Thanks,
Alan
 
The only other thing that I can think of is that you are testing with a slightly different amount of water each time you test. Your FC is high right now since you are shocking. With such a high FC, even tiny variances in sample size can make enough of a difference that your results are skewed.

You must ensure that the sample lies perfectly at the 10 ml mark each time using the bottom of the meniscus as a reference point. I have attached a picture for you to understand what I am talking about. This is just a generic picture and is not saying that you should use the amount indicated on the test tube in the photo. Your sample size for testing FC should be 10 ml.

Some people use a dropper or a syringe to make achieving to proper sample amount a little bit easier.
 

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That's the problem with using small sample sizes. The bigger the sample size the less significant the error. But with a bigger sample size you use more reagent. 10% error with 10 mL is 1 mL, that's an easy error to make, whereas a 10% error with a 100 mL sample is 10 mL, less likely to happen. Try changing the sample size (try 25 or 50 mL) and adjust the equivalence factor accordingly.
 
I was just like you in the beginning and would get various results. I found that trying to put the stuff in and mix by hand was hard. IF you have the money and can afford it get this little gadget and it will help a lot.

http://tftestkits.net/SpeedStir-Magneti ... r-p56.html

I have found when I do my FC test, I alwasy try to do a huge scoop and usually my test are spot on each time.

Maybe someone can expand on this as well, would the size of the drop affect the test?
 
The Taylor reagent bottles dispense 42 uL per drop, or if you put it another way, 24 drops are required to dispense 1 mL of reagent. I don't know what size drops the TF bottles dispense, if they are different you may get different results (when compared to Taylor).
 
I just thought they could be a different size that's all, I'm just trying to figure out why the results were not repeatable: sample size, drop size, different reagent lots (age), different conditions (lighting), time between sample and test, time of day, dead spot in circulation, I don't know. Apparently the same sample taken from the same spot in the pool require between 30 and 40 drops (22, 30, 40 and 38) of FAS to reach endpoint. Lots of things to check. Didn't mean to say that the TF100 drops are smaller, just looking at all the possibilities.

Doh! Somehow I thought he was comparing his readings with Taylor's, I'm sorry, it's with a single test kit. :oops: I'm just back from a meeting where we discussed discrepancies in testing and testing methods (nothing related to here), I guess I was still on that "mode", sorry.
 
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