5ml FC test verses 10ml


LifeTime Supporter
Jun 6, 2013
I know this has been debated before but just want to get a clearer answer in my mind. Right now I have a CYA of 70ppm and keep my FC at a 8ppm target (Use the TF-100). I want to use less of the regent so instead of 10ml I want to use 5ml of water sample. Can I use 5ml and count each drop as 1ppm? I tested until it was slightly pink still and figured the reading was slightly highier than that value. The next drop turned it clear. Thanks in advance!


Gold Supporter
Silver Supporter
Jul 13, 2013
I'm no expert but I believe that's fine as long as you're not doing an overnight test to check for loss of FC


LifeTime Supporter
Jun 29, 2013
Yes, you can. Obviously, it's not as precise. I wouldn't try to guess intermediate, slightly pink values, though. If it turns clear at drop 8, then read it as 8 ppm. If it turns clear at 9, then 9 ppm. If you want something more granular, go back to a larger sample.


TFP Expert
LifeTime Supporter
Jun 7, 2011
Midland TX
You can run a 5ml in a pinch, but you lose accuracy. I would not suggest it, unless you are waiting on new reagent. A reduced sample size always sacrifices too much for a long term solution.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
The accuracy loss comes from the +/- 1 drop and from the estimate of the water volume. The one drop goes from 0.5 ppm FC to 1 ppm FC. An error of, say, 1 ml out of 5 ml is 20% error while out of 10 ml it is a 10% error. So for a given FC level, the proportional error from volume measurement of the sample is doubled and the absolute error from drop size relative to sample size is also doubled. When measuring 8 ppm FC, the error goes from 8*0.1 + 0.5 = 1.3 ppm to 8*0.2 + 1.0 = 2.6. So it is still linearly proportional in error.