FAS-DPD Extra Drop in FC Portion

jseyfert3

Bronze Supporter
Oct 20, 2017
949
South-Central WI
In another thread I stated my thought that adding an extra drop after the sample went clear in the FC portion of the FAS-DPD test would cause an incorrect reading in the following CC potion of the test. @JoyfulNoise stated this was not the case, and if I was curious to post the question here and he'd answer it.

I am curous why an extra drop in the FC won't throw off the CC portion. To me, it would seem logical that it would, similar to how drops of R-0012 can be added at the start of the CH test if a fading endpoint is observed, but need to be accounted for at the end when they are normally added.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,222
Tucson, AZ
It because of the chemistry of how the DPD dye works and what the FAS reagent does. I'll try to put in simple terms so as to not have to revert to a lot of diagrams or chemical formulae.

The DPD indicator has three forms - an unoxidized amine form, an oxidized Würster configuration and an additional oxidized form as an imine structure. This is what happens to DPD in the presence of an oxidizer like chlorine -

Screen Shot 2020-07-19 at 7.04.49 PM.png (sorry for the tiny pic)

The colorless reduced form (amine) reacts with chlorine and forms the pink colored Würster configuration. There is also a very small amount of oxidized imine created which is dependent on pH and the concentration of the oxidizer and this is the origin of DPD indicator getting bleached out to clear when the chlorine is too high. At normal testing pH and FC levels, the amount of imine created is not worth worrying about. Combined chlorine compounds, which for the most part should be monochloramine and a small amount of dichloramine, do not initially react with the DPD indicator and so none of the dye is oxidized to the pink color by the CC's. CC's are weak oxidizers but, given enough time, they can cause the DPD dye to turn pink which is the origin of "chloramine breakthrough" when someone lets the sample sit for too long and it starts to turn pink.

The FAS titrating reagent is ferrous ammonium sulfate and it is a reducing agent, that is it will make something that is oxidized become "un-oxidized" or reduced. In the case of the DPD indicator, when you add FAS titrating reagent you are converting the Würster compound from it's oxidized form (pink) back to its colorless amine form. So, by getting the concentrations of FAS right and making sure the volumes dispensed are accurate, the amount of FAS added is directly proportional to the amount of chlorine that oxidized the DPD in the first place.

So how are CC's measured?

The R-0003 reagent is nothing more than potassium iodide (KI). When you add iodide to an acidic, aqueous solution containing a weak oxidizer like monochloramine, the iodide (I-) gets converted to the triiodide anion (I3-). Triiodide is a strong enough oxidizer to react with the DPD amine dye and convert it into the pink, oxidized Würster dye. That reaction happens completely and then adding the FAS titrating reagent reduces the pink form to clear again. So now the second volume of FAS added is directly proportional to the amount of triiodide that caused the amine to turn pink which in turn is directly proportional to the original amount of CCs in the water sample.

So why doesn't FAS react with CC's?

Well, CC's are weak oxidizers and do not react quickly with the FAS reagent at low pH. Given enough time, and perhaps a rise in pH, then there would be some cross reaction of FAS with CC's. But, given how quickly the test is performed and the low pH of the sample after adding the R-0870 powder, there is no loss of CCs. A single, extra drop between the FC and CC parts of the test will have very little influence on the reaction of CC's with iodide and then iodide with the DPD dye. So, when you follow the protocol, the test is fairly accurate. And of course this is all based on the caveat that you are, to the best of your ability, following the test protocol and not intentionally trying to overdo it. Certainly if one added 10 extra drops of FAS titrating reagent beyond the pink/clear endpoint and then let it sit for a few minutes before doing the CC test, you would expect inaccurate results.
 

jseyfert3

Bronze Supporter
Oct 20, 2017
949
South-Central WI
Maybe it's cause it's late, maybe it's the beer, but while it makes sense to me that the CC's don't react with the FAS it still doesn't make sense why if there was an extra drop of FAS added before the R-0003 why once the R-0003 and the dye was oxidized why the "extra" FAS floating around would not immediately reduce the dye by the volume of this one extra drop?

Also, to hijack my own thread, you mentioned the CC's will eventually breakthrough causing a sample to turn pink after the FC portion of the test. I've noticed a sample will slowly turn pink (over many minutes) after completing of the CC portion of the test. Is this oxygen in the air oxidizing the dye?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
17,222
Tucson, AZ
Maybe it's cause it's late, maybe it's the beer, but while it makes sense to me that the CC's don't react with the FAS it still doesn't make sense why if there was an extra drop of FAS added before the R-0003 why once the R-0003 and the dye was oxidized why the "extra" FAS floating around would not immediately reduce the dye by the volume of this one extra drop?
I'm sorry, I misunderstood your question earlier. I thought you were asking why the FAS doesn't affect the CC's in a water sample.

Sure, if you overdo it an add extra FAS titrating reagent, you'll mess up the test but there's a couple of things that make this unlikely. For one, FAS is not very stable in solution so while it is reducing the the oxidized DPD dye back to the clear amine, it can also degrade on it's own a bit. Second, our eye's are not really that good at knowing exactly when the pink/clear transition occurs. So adding one extra drop is unlikely to affect any results by the full extent of that drop and you'll likely not see any color difference when it does. If you had a highly sensitive photometer set to 530nm then you could probably see a difference but your eyeballs just aren't that good. Finally, these drops are not super accurate and so there is always a +/- 1 drop error in everything you do. Adding an additional drop is unlikely to make any difference at all....which of course means that if you are really interested in looking for CC's, you should not be using a 10mL water sample. A 25mL water sample will give you a +/-0.2ppm resolution and any actionable level on CCs is really at 0.6ppm or higher (I actually wouldn't be concerned at all with a CC of 0.8-1.0 as long as it trended downward over time). So again, using a 25mL water sample and an additional drop on the FC side of the test really isn't a big deal. For a 10mL water sample with a resolution of only 0.5ppm, it might make seeing CC's a bit harder.

And finally, this old bit of wisdom reigns - this is a SWIMMING POOL not solid-fuel rocket boosters or Manhattan project uranium enrichment....being off by a drop or two in any test is not going to summon the apocalypse and bring about the end of the world. Keep it in perspective and don't sweat the small stuff....it's really not worth the brain cells.

Also, to hijack my own thread, you mentioned the CC's will eventually breakthrough causing a sample to turn pink after the FC portion of the test. I've noticed a sample will slowly turn pink (over many minutes) after completing of the CC portion of the test. Is this oxygen in the air oxidizing the dye?
Yes, dissolved oxygen can cause the dye to go pink. It is very sensitive to chemicals that can cause oxidation.
 
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Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,569
NW Ohio
Yes, dissolved oxygen can cause the dye to go pink. It is very sensitive to chemicals that can cause oxidation.
Which is the key to a classic pool store scam: The mark The pool owner comes in with their water to test. They've been adding chlorine but just can't get it to register, and the pool store FAS-DPD test shows zero. Mr Honest-pool-store-guy tells them that there is chlorine there (you can't just add gallons of chlorine and not have chlorine, where could it possibly go?), but it's being suppressed. To solve this, they need to buy a dozen pounds of non-chlorine shock. As proof they take a pinch of MPS and add it to the FAS-DPD and *poof* the solution turns a deep red. The chlorine that's been there the whole time has been liberated, much like the cash that has been in the pool owner's wallet this whole time will soon be.