FAS-DPD not working for higher Cl levels

[rhythm]

I had this going on another thread, but there were two issues going on simultaneously and to avoid further confusion I wanted to post my latest results on this topic now that I've tried new reagents.

Basically, I'm having trouble getting the titration to "finish" properly when performing the FAS-DPD test for chlorine when the chlorine levels are high (I'll be more specific in a moment). When checking the FC level after shocking with liquid chlorine I have noticed that after a certain point the sample would go from pink to a "pale brown/salmon-ish" color, and would just remain like that no matter how many more drops I added.

Purchasing new reagents (both the powder and titrating solution) didn't change anything :grrrr:

Tonight I checked the FC level with the old reagents and it titrated just fine...at 15 ppm (30 drops). I added a gallon of chlorine, which should have raised my FC to ~25 ppm. After a couple of hours with the pump running, I took another sample and attempted to titrate. I expected to put in around 50 drops. At about 43-44 drops, the sample turned its "salmon" color and never changed. So I tried the new reagents and got the exact same result.

So in the past couple of days I've had successful titrations at 15 ppm FC and 13 ppm FC, and unsuccessful titrations (permanent salmon color, never going colorless) with expected FC levels of ~33 ppm and ~25 ppm.

My experience seems to agree with what the Taylor people say (this test is good up to 20 ppm FC), but I know that everyone agrees that the test should work for FC levels up to 50 ppm. Before I first shocked a couple of days ago, my other levels were:

FC 2-3
CC 0.5 or less
pH 7.4
TA 90
CYA 40-45
CH normally ~350, haven't measured in a few months

So I guess I'm stumped. The good news is that 99% of the time I'm checking FC levels in the 4-8 ppm range so the test should be no trouble as usual. If anyone can think of anything else I should try in order to solve this mystery, please let me know.

Thanks!
Greg

 

[chem geek]

Did you try adding more of the R-0870 DPD powder initially? It should have flashed pink and not held color as well with just one scoop. With more DPD powder, the transition may be a bit more distinct. For the FAS-DPD test, Taylor says the following:

Chlorine levels > approx. 25 ppm may bleach out indicator or cause sample to develop a brown color; to prevent, add more DPD powder or dilute sample with DI water as necessary and retest. Other halogens and oxidized manganese may cause positive interference.

I did not see where they say that 20 ppm is the max. for the FAS-DPD test.

 

[rhythm]

Hi chem geek,

It's strange...I know that higher FC levels can bleach out the indicator, but after adding one dipper the sample turned pink without a problem. I actually added another partial dipper full after the first one. The sample holds its pink color for quite a while, then turns that brownish/salmon color after 43-44 drops of the titrant. The issue definitely seems associated with higher levels of FC, as I didn't have any issue when testing below 20 ppm.

The 20 ppm max thing was something that a phone technician at Taylor told me when I called them to ask about this. I haven't seen it in print, either.

There seems to be some connection with their advice about levels over 25 ppm and the brown color, as that's kind of close to the color I got after titrating for a long time. But the sample had no problem turning pink initially. I wonder if adding more DPD powder at the start of the test would have prevented this, even though the sample turned pink right away.

Thanks,
Greg

 

[Guest]

I have personally titrated samples with FC as high as 45 ppm. you do need to add additional DPD powder, however.