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Topic: acid stability of copper sulfide Cu2S / CuS  (Read 5972 times)

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Offline Schmittchen

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acid stability of copper sulfide Cu2S / CuS
« on: October 16, 2017, 11:35:29 AM »
Dear chemstry experts,

1. Consider an aqueous slurry of newly formed Cu2S. This suspension becomes acidified by addition of gaseous SO2 which - of course leads to formation of H2SO3. My question is: Is it possible to reach a pH value, so that  H2S becomes released from Cu2S? Please do not consider any redox interaction between the two S species S4+ vs. S2-. All I would like to know is if H2SO3 can have an acidity which can cleave Cu2S.

2. The same question as above please , just with CuS instead of Cu2S.

Apologies for the above - it appears to sound a a bit primitive. But I would like to stress that there is a real problem behind, the explanation of which would require just too much space & time.
Absolutely curious for your comments & with kind regards in advance
Schmittchen (Germany)

Offline Borek

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #1 on: October 16, 2017, 03:10:08 PM »
First step would be to estimate pH, then see if the protonation of S2- can produce H2S - taking into account solubility product of Cu2S. Doable, but not an easy task.
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Offline Schmittchen

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #2 on: October 17, 2017, 12:59:20 PM »
Dear Borek,
Thank you for the guidance.
The problem is, that unfortunately in the conditions I am considering it should be impossible to estimate pH. That is why I was hoping for some cue by calculation.
Example: While copper sulfide(s) become formed in an slightly acidic medium it also should be true that below a certain pH CuS / Cu2S become cleaved. Just wondering if someone knows the value of this - let me call it "stability border" of pH. Once this is known (maybe this even is in the literature, but so far I was completely unable to find these data) the 2nd question will be: Can a sulfurous acid be that acidic?
Hoping I am not asking too much let me again say thank you.
With kind regards
Schmittchen

Offline Borek

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #3 on: October 17, 2017, 04:24:50 PM »
No pH, no way of calculating anything.
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Offline Schmittchen

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #4 on: October 20, 2017, 04:25:55 AM »
Dear Borek,
Just disappointed. Being a chemistry fossil in his 112th  semester :( who forgot most of what he has been taught I really don’t think that I look & write like a spammer.

(1)   Please imagine a polyethylene film of – say – 30µm thickness. Among the physico-chemical parameters describing the properties of this film also is oxygen transfer rate (OTR) and water vapor transfer rate (WVTR). Both describe transport phenomena of small molecules in a polymer. PE has high OTR & WVTR, indicating that there is an easy diffusion of small molecules in PE.

(2)   Imagine our PE film is unilaterally covered (by standard methods) with elemental Cu, be it powder, be it by a sputtering technique.

(3)   Further imagine that this elemental Cu is covered by a 2nd polymer film, which is virtually sealed against diffusion, e. g. EVOH or something similar, possessing very small values for OTR and WVTR.

(4)   Imagine a scenario where on the PE side this sandwich becomes equilibrated with an atmosphere containing H2S. It is important to add here that we are considering a process over many months or even some years. H2S will follow the concentration gradient and diffuse through the PE film. Subsequently H2S will interact with Cu by chemisorption and/or formation of Cu2S / CuS. This process will come to its end as soon as one of the reactants becomes “eaten up”.       Do you agree until this point?

(5)   Now let’s change the above scenario by replacing H2S by SO2.  Again we will see diffusion of SO2 through the PE film. In parallel we will see diffusion of H2O molecules through the PE film.

(6)   The 1st molecule SO2, meeting a molecule H2O will form H2SO3. The 2nd molecule H2O passing this will lead to HSO3- + H3O+. Do you agree until this? In any case it is obvious that in the conditions described no pH measurements are possible.

(7)   From the literature (electrochemistry, corrosion science) it is known that elemental Cu in the presence of H2O interacts with SO2 in a complex way, ultimately according to the following molecular formula: 6Cu + SO2 --> 2 Cu2O + Cu2S. This is more than surprising (for me at least), because all my life I thought that SO2 is a potent reducing agent.

(8)   This said let’s return to the polymer sandwich scenario. In any case it is more than likely that we will see an interaction of SO2 and/or H2SO3 with our elemental Cu. According to the reaction formula given in (7) we also will get Cu2S.

(9)   Cu2S as well as CuS form (and are stable) in slightly acidic medium. On the other hand, however, at a lower pH both sulfides become cleaved, forming free H2S. This free H2S – once it is formed in the above scenario – would follow the concentration gradient and diffuse out of the PE film. I discussed the problem with a famous US geoscientist. He told me that in his opinion and derived from his experience with copper minerals the stability border of Cu2S / CuS should be around pH 1,5.

10.   So obviously the crucial question is: Can our H2SO3, putatively formed “in situ” from SO2 and H2O, reach an acidity capable of cleaving Cu2S / CuS or – for that matter – prevent formation of Cu sulfides, leading to formation of free H2S?

In summary let’s return to my question, asked in my 1st post: Is it conceivable that aqueous H2SO3 has a pH which can cleave Cu2S / CuS? Or is it possible that the pH of said H2SO3 is such that formation of Cu2S / CuS is impaired, in either way leading to the formation of freeH2S?

Borek, if you still think this thread is of public interest I have no issues in publicly discussing it. However, I still think we should switch to a private channel. Being sort of an admin it should be easy for you to enable this.

Thank you and curiously looking forward to your comment.
Schmittchen

Offline Arkcon

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #5 on: October 20, 2017, 08:09:47 AM »
Hi there, Schmittchen:, with a better explanation, we can see that your topic is very advanced, and very specialized.  Its better suited to this sub-forum.  However, experts are hard to come by, so there may be more of a wait.  At any rate, there's nothing "analytical" about your problem.

I understand worrying about "spamming", but we really don't feel that way about people who are trying to learn.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Borek

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #6 on: October 20, 2017, 05:01:03 PM »
What you described is very different from "aqueous slurry" mentioned in the first post. In aqueous solutions the line of attack is rather obvious, it is just a fancy equilibrium that can be solved using standard methods, all needed equilibrium constants being easy to locate. In the case you have described now things are much more complicated. The only approach I can think of is to use thermodynamic data to estimate the end result, but that's not an area I feel comfortable with.

Commercially available 6% solutions of the sulfurous acid have pH around 2. It is possible to prepare much more concentrated solutions (up to 12%, or 1.5 M at least), with pH below 1. Sounds like even in water you would need quite high partial pressures of gaseous SO2 to keep pH below 1.5, but it also means that the possibility of the sulfurous acid being strong enough to give pH values lower than 1.5 can't be completely neglected.
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Offline Schmittchen

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #7 on: October 21, 2017, 04:51:23 AM »
Borek,
Glad to see you are still in this thread despite moving it to a different forum.
Yes, "aqueous slurry" is pretty different to the actual scenario. On the other hand, however, not too distant IMO....
Cant we use the "aqueous slurry" as a model for an attempt to get information on what is giong on in the "sandwich" scenario? Speaking of models: Cant we even use H2SO4 (instead of H2SO3) as a model?

All I want to know is if 
i. an aqueous slurry of Cu2S or CuS becomes cleaved at which pH. If we ask this way the nature of the acid anion should be (more or less) insignificant. All we need is the information at which pH the copper sulfide(s) become cleaved with liberation of H2S. Do you agree?
ii. Or, the other way round: If we have an acidic aqueous solution of Cu1+ or Cu2+ (well, the former seems to ber a bit difficult because of the notorious small solubility of Cu1+ componds as far as I know) and we add H2S to this - at which pH no CuS forms?

Once this question can be answered there admittedly remains the more difficult 2nd question, if these conditions will be reached in the "sandwich scenario". However, it is felt nevertheless that in that case we are a bit closer to an answer, arent we?
Have a relaxed weekend & with kind regards
Schmittchen

Offline Borek

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Re: acid stability of copper sulfide Cu2S / CuS
« Reply #8 on: October 21, 2017, 07:16:56 AM »
In general it is mostly an equilibrium problem - we have Ksp values, we know two Ka values for H2S, basically it is about solving system of equations for a given pH and seeing how much copper can get dissolved. We will definitely need to define some metrics to establish what we consider to mean "stable" (if the volume is large enough sulfides will always dissolve in the end, no matter what the pH is), but for water solution the line of attack seems to be rather obvious.
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