Problems for modeling electrochemical oxidation of bromide

[xcui]

Hello, all,

I tried to model the electrochemical oxidation of bromide in aqueous phase through ACT using the dataset of thermo.com.v8.r6+.dat. The results showed that Br- wasn't oxidized until the pH dropped below zero no matter what Eh value I chose. However, from my experience, I know that the bromide can be oxidized electrochemically at the pH around 6. I have attached the results file. Could anyone please help take a look to see whether there is anything wrong.

 

Thanks,

XC

Act2_output.txt

[Brian Farrell]

Hi,

 

Keep in mind that these are predominance diagrams. Only the most stable species will appear under given chemical conditions. That being said, I am able to produce a diagram on axes pH 0 to 14, Eh -.75 to 1.25 that shows several redox states of Br using thermo.com.v8.r6+.dat, as long as I turn the water stability limits off. The bromate and perbromate ions are extremely oxidized, and much less stable than Br- under normal conditions of interest (especially perbromate).

 

If you're trying to model the actual process of the redox reactions, rather than just the equilibrium state, you might consider suppressing the Br species which you know are unlikely to form. If you know that one of the oxidized Br species can form at pH 6, you might decouple that redox pair and experiment with a kinetic rate law in React relating the two different oxidation states.

 

One more point, you have several additional anions included in the "in the presence of" field (Cl-, F-, SO4--). These will not react with the Br at all, so they are not going to contribute to the diagram. There must be some cation that is in your fluid, which might react with the Br in some way.

 

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

[xcui]

Hi, Brian,

Thanks for the help. Could you please give more details about how to turn off water stability limits? The only clue I can find in the manual is that this might be related to pressure setting. Should I just set the lowest pressure allowed by the software or should I revise the thermo.com.v8.r6+.dat.?

 

It's great if bromate is not stable because the major reason for me to model bromide oxidation is to make sure no bromate would be formed. However, based on the literature, I found that Br2 can disproportionate into HBrO in water, which can then disproportionate into Br- and bromoate almost spontaneously.

 

As for the additional anion in presence, I just found that chloride has a significant effect on electrochemical oxidation of bromide. I don't know exactly how, but the sample did smell like bleach after the electrochemical treatment.

 

Thanks,

XC

[Brian Farrell]

Hi XC,

 

Larger pressure expands the stability limits of water. In a normal Eh-pH diagram, the top line indicates where the water is so oxidizing that the water is unstable and forms O2(g). Similarly, the bottom line represents where the water is so reducing that H2(g) is formed. Increasing the pressure allows more O2(aq) or H2(aq) to build up in solution before the water decomposes to form gas. To view an Eh-pH diagram assuming water will not decompose to O2(g) or H2(g), simply type water_limits, off or right-click the diagram, click View..., then uncheck water limit.

 

Keep in mind that Act2 will solve for the predominant species at equilibrium in the system considered, which is dependent upon your input. This includes the thermo data and what you type into the Basis pane. When interpreting these diagrams, always consider the possibility that your real life system will not reach chemical equilibrium during your time scale of interest, and may instead be in some metastable state.

 

Brian

[xcui]

Hi, Brian,

I finally got all the species involved. The diagram is beautiful. Thanks for your help.

 

XC