Thanks James for the clear explanation. I've continued to be curious why, if I wait a few minutes, it starts drifting back towards pink?
CYA not considered in Pool Math?
- Thread starter DougJack
- Start date
Correct, the HOCl and OCl- are the only molecules that react with organics and other oxidizable things.
However, as the HOCl and OCl- get used up, the chlorine atoms bonded to CYA are released so that you always have some free FC to oxidize and sanitize.
That can be due to combined chlorine or from oxygen in the air oxidizing the sample.
Oxygen can oxidize N,N-diethyl-p-phenylenediamine (DPD).
The oxidized form of DPD is called Würster dye.
The amount of Würster dye that forms is proportional to the amount of oxidizer present.
The reagent for combined chlorine is iodide and this gets oxidized by combined chlorine or oxygen to iodine, which oxidizes the DPD into magenta Wurster dye.
If you have added the iodide, the oxygen in the air reacts very quickly due to how easily iodide is oxidized into iodine.
The oxidized form of DPD is called Würster dye.
The amount of Würster dye that forms is proportional to the amount of oxidizer present.
The reagent for combined chlorine is iodide and this gets oxidized by combined chlorine or oxygen to iodine, which oxidizes the DPD into magenta Wurster dye.
If you have added the iodide, the oxygen in the air reacts very quickly due to how easily iodide is oxidized into iodine.
| Oxidizing Reagent | Oxidizing Potential |
|---|---|
| Ozone | 2.07 |
| Hydrogen Peroxide | 1.77 |
| Permanganate | 1.67 |
| Hypochlorous Acid | 1.49 |
| Chlorine Gas | 1.36 |
| Hypobromous Acid | 1.33 |
| Oxygen | 1.23 |
| Bromine | 1.09 |
| Hypoiodous Acid | 0.99 |
| Chlorine Dioxide | 0.95 |
| Hypochlorite | 0.94 |
| Chlorite | 0.76 |
| Iodine | 0.54 |
The oxygen dissolved in the sample can react with the DPD.
8.735 ppm Oxygen = 0.00027297M.
0.00027297M Oxygen (O2) is equivalent to 19.355 ppm chlorine (Cl2).
Dissolved oxygen (O2) in water does not react with DPD (N,N-diethyl-p-phenylenediamine) quickly.
It does so very slowly and this is one reason that the sample can turn pink after turning colorless.
What would cause positive results using DPD when no chlorine is present in the sample?
Aug 6, 2022Knowledge
Title
What would cause positive results using DPD when no chlorine is present in the sample?
Summary
Positive interferences to Chlorine methods
Answer
Other oxidants such as bromine, iodine, ozone, chlorine dioxide, or hydrogen peroxide can react with DPD and cause false positives.
The most common interferent is oxidized manganese, which can be corrected for by treating the sample with potassium iodide and sodium arsenite.
Sunlight can react with the DPD indicator during the 3-minute reaction time for total chlorine; keep the sample covered during the reaction time if testing outdoors.
Aug 6, 2022Knowledge
Title
What would cause positive results using DPD when no chlorine is present in the sample?
Summary
Positive interferences to Chlorine methods
Answer
Other oxidants such as bromine, iodine, ozone, chlorine dioxide, or hydrogen peroxide can react with DPD and cause false positives.
The most common interferent is oxidized manganese, which can be corrected for by treating the sample with potassium iodide and sodium arsenite.
Sunlight can react with the DPD indicator during the 3-minute reaction time for total chlorine; keep the sample covered during the reaction time if testing outdoors.
Potassium monopersulfate (MPS or potassium peroxymonosulfate) is a non-chlorine oxidizer that will interfere in the total chlorine DPD test.
Some oxidizers like MPS and combined chlorine can slowly oxidize the DPD reagent resulting in a slow return to pink after going colorless.
I am not sure how much dissolved oxygen reacts with the DPD dye.
https://www.taylortechnologies.com/it/page/113/shocking-interference
Some oxidizers like MPS and combined chlorine can slowly oxidize the DPD reagent resulting in a slow return to pink after going colorless.
I am not sure how much dissolved oxygen reacts with the DPD dye.
https://www.taylortechnologies.com/it/page/113/shocking-interference
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O2 + 2H2O + 4e- --> 4OH- = +0.4 V
If this is the primary reaction of dissolved oxygen, then the dissolved oxygen probably does not react much in the test even if iodide is added.
If this is the primary reaction of dissolved oxygen, then the dissolved oxygen probably does not react much in the test even if iodide is added.
Hi James, one reason to tell the bot who it is, is that it structures the response it will give in a more technologically sound manner. I want to avoid a fluffy answer.
I understand that more clearly now. I am now starting to to try to understand the dynamics involved. The reactions both ways are fast, but not instantaneous or there would be very little movement in FC with the recommended additions (my current opinion)
Be careful with the AI stuff. The answers tend to just assume that the truth is found by finding conclusions that have the largest quantity of web data and assuming those are accurate, but that’s a faulty assumption to build an algorithm on. Not much better than a thorough google search.
I really agree with that point of view. I find it a useful tool that is helpful to me to find some but not all, critical elements of an approach, even if I think I know a subject well. I almost always find a flaw if I question more deeply as above. But its response often contains a hint of what will become important in my next set of questions.