DPD powder affecting FC reading

[lefizzet]

So I've read a lot of threads about this and it seems that the general consensus is that you can add too little powder but adding too much does not affect your results. When I follow the directions from the TF 100 kit and use 1 heaping scoop, my FC reading is always lower than if I add two scoops. I also noticed that 1 heaping scoop always produced a light shade of pink (while there were still some undissolved particles) whereas 2 scoops always produced a much darker and vibrant shade of pink (with undissolved particles). So my question is which method is correct? should I always add enough to get the vibrant pink color or just 1 heaping scoop?

Also, does it affect the test if I stir the solution with the scooper? Sometimes the DPD powder clumps up after adding it into the water so I just stir it around a bit with the little scooper to make sure it gets dissolved. Thanks.

I tried using some test strips that I have from my hot tub and the FC reading is always 0. Should I just disregard the accuracy of those strips?

 

[chem geek]

Welcome to TFP!

We did a test a while back where people tried different numbers and sizes of scoops and the results were either the same or within one drop of R-0871 in the test. Adding more does have the intensity of the pink be darker (more saturated) but the point at which it turns clear is still the same. The differences are whether one is able to visibly see a faint pink well. If not, then using more powder helps to make that last distinction from pink to clear more obvious. So if for your vision you need to use two level scoops (or one heaping scoop), that's fine though it will use more DPD powder faster.

Usually one scoop is sufficient for a 10 ml water sample. It's usually when using a 25 ml water sample where two scoops or one heaping scoop are needed for better clarity of the transition. Nevertheless, the Taylor instructions say "Add 2 dippers R-0870" which of course makes no sense to be the same for the two sample sizes since it will look a lot weaker in 25 ml. I generally test using a 25 ml sample size and use the equivalent of two level scoops (I usually just use one heaping one) and the endpoint is almost always very distinct.

As for clumps, I would just swirl the tube and not put the scooper into it. You want the scooper kept dry and clean. You can always crush the clumps before you add them to the water sample. I don't find powder to clump much after it's added -- it's clumped if it was originally clumped.

As for the test strips, yes disregard them as they are obviously not accurate.

 

[lefizzet]

Welcome to TFP!

We did a test a while back where people tried different numbers and sizes of scoops and the results were either the same or within one drop of R-0871 in the test. Adding more does have the intensity of the pink be darker (more saturated) but the point at which it turns clear is still the same. The differences are whether one is able to visibly see a faint pink well. If not, then using more powder helps to make that last distinction from pink to clear more obvious. So if for your vision you need to use two level scoops (or one heaping scoop), that's fine though it will use more DPD powder faster.

Usually one scoop is sufficient for a 10 ml water sample. It's usually when using a 25 ml water sample where two scoops or one heaping scoop are needed for better clarity of the transition.

I would just swirl the tube and not put the scooper into it. You want the scooper kept dry and clean.

As for the test strips, yes disregard them as they are obviously not accurate.

Thanks!

 

[chem geek]

Another thing to keep in mind is that if you have significant CC then it can bleed through into the FC test if you do not do that test quickly enough. So after you are done getting to clear (colorless) measuring FC, you may find it turning pink again. If so, then that's CC bleed-through. It's not that common because we usually don't have much CC in our pools, but in some situations we do so one needs to do the FC part of the test without much delay and stop when first colorless and ignore the bleed-through -- just move on to the CC part of the test at that point.