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NOTE: As I've been finding new informations about the question, I've been editing the question to keep it up to date. If some comment doesn't make sense just check the Edit History.


DPD (N,N-diethyl-p-phenylenediamine) is used in free chlorine colorimetric analysis since it reacts with Hypochlorous acid and Hypochlorite ions acquiring a pinkish color that can be quantitatively related to the concentraction of those substances if it's inside the working range.

Even though the analysis method is very well documented, I haven't found much about the reaction itself and its mechanism, and since it involves lots of subjects I was recently studying (like the relationship between π-systems and color in organic molecules) I thought it would be interesting.

First taking a look at the indicator molecule:

DPD

There are two amine groups: a primary and a tertiary one. I don't have software here to run a Molecular Orbital analysis, but I'd guess the amines lone pairs plays a big role in the HOMO (probably the tertiary amine a little more since it's more basic). The carbons in the aromatic ring probably also influence the energy and shape of the HOMO since they are very nucleophilic due to the electron density added by both nitrogens lone pairs to the π-system. I'd guess the biggest contributors for the LUMO would be the hydrogens connected to the primary amine and the ethyl groups connected to the tertiary one, since they are more positively charged due to induction.

The reagent is mostly used in its salt form, as N,N-diethyl-p-phenylenediamine sulfate for example.

According to some sources[1] [2], the final products of the DPD oxidation by the free chlorine are the following ones:

Final Products

The first one (the free radical), being called Würster Dye, is responsable for the pinkish/magenta color that is used to measure the amount of free chlorine in the solution. The second one (Imine) is colourless, and is favored on more strong oxidizing conditions, reason why the DPD-Method can't be used in very high concentrations of free chlorine.

I decided to test this new information and ran a DPD-Analysis on very concentrated sodium hypochlorite and then on wastewater dosed with sodium hypochlorite.

Concentrated Sodium Hypochlorite:

Sample initially yellowish and photometer baseline set before reaction. After adding DPD the color seemed to be a slightly more bright yellow. Measurement of free chlorine was about 0.35mg/L.

Analysis #1 Before

Analysis #1 After

Chlorinated Wastewater:

Sample initially colourless and photometer baseline set before reaction. After adding DPD the color changed to a light pink, as expected by the method. Measurement of free chlorine was about 1.50mg/L.

Analysis #2 Before

Analysis #2 After

The test confirmed the information on both cited articles that excess free-chlorine would result in the colourless Imine product being favored.

As for the mechanism, I'm still unsure of how it goes. First I thought about an homolytic cleavage of the HO-Cl bond and the N-H bond of the tertiary amine. Later the hydroxyl radical would combine with the hydrogen radical to form water and the chlorine radical would combine with another chlorine radical to form $\ce{Cl2}$. But that doesn't make sense because we would end up replenishing the free-chlorine as the reaction goes. So instead of the hydroxyl radical combining with the hydrogen one, maybe the chlorine radical is the one combining resulting in the release of $\ce{HCl}$. But then we end up with a hydroxyl free radical. It could recombine with another hydroxyl radical forming $\ce{H2O2}$ which would further decompose to $\ce{H2O + O2}$ but it sounds a little odd for some reason. Despite sounding odd it's still the best I could come up with, so I draw the mechanism below:

Würster Dye Mechanism

As for the Imine mechanism, would the following be correct?

Imine Mechanism

I'm not sure about neither, and I'm very confused as how a radicalar reaction takes place instead of the heterolytic one if there's no external influence to favor the formation of a free radical (I always thought we would need UV or a free-radical catalyst to favor a homolytic fission).

As for the HOMO-LUMO gap getting smaller, I guess it makes sense. When we think for example, about the combination of two hydrogen radicals, there's a lowering of the occupied orbital energy. So maybe the formation of a free-radical raises the energy of the HOMO but doesn't raise the LUMO energy of the DPD making the gap between them smaller, reaching visible light spectrum.

Hydrogen MOs

So the questions: Are any of those mechanisms correct? If not, what is happening on the DPD/Free Chlorine reaction? Is my reasoning on the HOMO/LUMO gap getting smaller right?

Side-note: The original article that described the DPD method for the first time apparently was this: PALIN, A.T. 1957. The determination of free and combined chlorine inwater by the use of diethyl-p-phenylene diamine.J. Amer. WaterWorks Assoc.49:873. But I'm not sure if there's an open version of it. If there is I couldn't yet find it.

[1] CHLORINE ANALYTICAL TEST ELEMENT AND A STABILIZED N,N-DETHYL-P-PHENYLENEDLAMINE SOLUTION - Patent Application Publication, Pub. No.: US 2015/0050195 A1 - https://patentimages.storage.googleapis.com/ec/5e/50/485b22cea62cdc/US20150050195A1.pdf

[2] 4500-Cl CHLORINE (RESIDUAL)* - http://edgeanalytical.com/wp-content/uploads/Inorganic_SM4500-Cl.pdf

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  • $\begingroup$ do you have any evidence for the final product being what you drew? I was wondering about dimerisation via the first chloro compound you drew ... the resulting hydrazine dimer being oxidised up to a N=N compound ... anyone else? $\endgroup$ – user1945827 Jun 15 at 15:21
  • $\begingroup$ @user1945827 No evidence, I haven't found much information on the reaction, so all the mechanisms and products on the question are mostly deductions. I've just stumbled into an article that I'll examine better, but it indicates a reaction of a protonated DPD salt (which aligns with the fact DPD packages are sulfate salts) with actual $\ce{Cl2}$ to form a radicalar product called "Würster Dye". I'll update the question as soon as I get to read the article enough to have better conclusions. $\endgroup$ – IanC Jun 15 at 20:21
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    $\begingroup$ The mechanism after "As for the Imine ..." is improbable. Loss of hydride from a protonated amine?? Just chlorinate the -NH2 once and then eliminate HCl. As to the structure of the red dye, it apparently remains a mystery. $\endgroup$ – user55119 Jun 16 at 15:04
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    $\begingroup$ This reference may be of interest, link.springer.com/chapter/10.1007/978-3-662-12512-0_24 I haven't translated it with sufficient rigour to say one way or the other. $\endgroup$ – user1945827 Jun 17 at 8:44
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    $\begingroup$ Hi ... I think you should still explorer the 'Wurrster dye' and as a competing hypothesis the dimerization which I would guess happens with the starting material. You need to gather more evidence, either through the literature or empirically, for example this paper pubs.acs.org/doi/abs/10.1021/ja01249a026 discusses Wurster dyes as does this - both useful. BTW, Alexandra Elbakyan's work may be of use to you. $\endgroup$ – user1945827 Jun 18 at 10:16

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