Correct way to interpret fas-dpd test.

[d.sebens]

So when I test I am using the 25ml mark with two scoops.  When doing titrations, it will turn clear but if I let it set it will start to go a little pink again.  About two drops more worth so about 1 ppm.  What is the accurate way to do this test?  Should I be doing drops until the first time it clears or if it turns back pink keep adding drops until it stays clear?

[waterbear]

Stop when it first clears, add the R-0003, and if it turns pink titrate again until it clears for your CC reading.  The pink color often reappears on both the FC and CC tests if you let the sample sit. If you are using OTO or DPD testing the color will often darken if you let it sit and not read it immediately. Also, use a 10 ml sample (which usually needs only 1 scoop of DPD powder) so each drop is a .5 ppm equivalent instead of using the 25 ml sample and a .2 ppm equivalent. There is no reason to know FC or CC to a precision of .2 ppm IMHO. (You would still use 5 drops of R-0003 with a 10 ml sample since this reagent converts any remaining chlorine compounds (combined chloramines), if present,  after the FC has been neutralized into iodine compounds that are then titrated.

[d.sebens]

So I was just following the quick instructions of 2 scoops for 25ml is .5 and two scoops of 10ml is 1.25. So if I do one scoop at 10 that is close enough for .5?

[d.sebens]

Also sorry, I’m testing bromine if it makes a difference. 

[waterbear]

5 hours ago, d.sebens said:

So if I do one scoop at 10 that is close enough for .5?

You only need to add enough DPD powder to get a pink color that does not fade out immediately. If the color does not hold add another scoop.

The DPD is merely and indicator showing that a halogen (chlorine, bromine, or iodine) is present. The number of drops needed to neutralize the halogen by the FAS (Ferrous ammonium sulfate) is counted and multiplied by the chlorine or bromine equivalent, IF you are testing bromine you are done. IF you are testing chlorine you then add R-0003 (potassium iodide) to convert the combined chloramines (which don't register) into iodine compounds that do and  if you get a pink color again which indicates that chjloramines are present then you titrate again to determine the ppm of combined chlorine in the same way.

5 hours ago, d.sebens said:

Also sorry, I’m testing bromine if it makes a difference. 

In a word, no. the only difference is the different titrant that is a different strength to get the bromine equivalents and that R-0003 is not needed since the first titration gives total bromine. As far as DPD powder, as long as it turns pink and stays pink until you start titrating you are good.

Also, the chlorine test can be used to test bromine (ANY chlorine test in fact). All you have to do is multiply the 'chlorine' results by 2.25 to get a total bromine reading. (doubling it is close enough in most cases. Doesn't matter if it's FAS-DPD, DPD, or OTO testing.)