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pH profile considering buffer effect
Posted Jul 24, 2015, 3:09 a.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions Version 5.0 6 Replies
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Dear COMSOL users,
I am pretty new to COMSOL and I would like to simulate an electrochemical reaction where OH- ions are formed in the reaction and the whole reaction occurs in a buffer solution of pH 8.4.
The equilibrium reactions are:
1) B(OH)3 + H2O -> B(OH)4- (Sodium borate)
2) H2O -> OH- + H+
3) Ag2S + H2O --> 2Ag + OH- + SH-
I know that there exists a model of COMSOL that handles this problem. (model_crevice_corrosion_fe)
So I only changed the equilibrium reactions of this model to the appropriate reactions and didn't changed any values for the reactions (only to try out if this model is working).
The problem is, that the changing of the equilibrium reactions results in an error.
Is it possible that the reaction 3) is the reason for this problem?
Thank you for your answers,
Patricia
I am pretty new to COMSOL and I would like to simulate an electrochemical reaction where OH- ions are formed in the reaction and the whole reaction occurs in a buffer solution of pH 8.4.
The equilibrium reactions are:
1) B(OH)3 + H2O -> B(OH)4- (Sodium borate)
2) H2O -> OH- + H+
3) Ag2S + H2O --> 2Ag + OH- + SH-
I know that there exists a model of COMSOL that handles this problem. (model_crevice_corrosion_fe)
So I only changed the equilibrium reactions of this model to the appropriate reactions and didn't changed any values for the reactions (only to try out if this model is working).
The problem is, that the changing of the equilibrium reactions results in an error.
Is it possible that the reaction 3) is the reason for this problem?
Thank you for your answers,
Patricia
6 Replies Last Post Jul 28, 2015, 2:27 p.m. EDT
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Posted:
9 years ago
Hi Patricia:
I have had trouble getting equilibrium reactions to converge in the past. When you say that you did not change the numbers but changed the equations, there are 4 equations in the original model and you have only 3, so probably the numbers did not match and there was no actual solution. Did you try the model with your equilibrium constants yet? Sometimes, it is the smallest of things like a sign somewhere which can make it hard to achieve equilibrium. Can you post your model, it is easier to diagnose it that way.
Regards,
Sri.
I have had trouble getting equilibrium reactions to converge in the past. When you say that you did not change the numbers but changed the equations, there are 4 equations in the original model and you have only 3, so probably the numbers did not match and there was no actual solution. Did you try the model with your equilibrium constants yet? Sometimes, it is the smallest of things like a sign somewhere which can make it hard to achieve equilibrium. Can you post your model, it is easier to diagnose it that way.
Regards,
Sri.
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Posted:
9 years ago
Hey Sri,
I have to tell you that I have changed reaction Nr. 3 to
3.) S^-2 + H2O --> SH- + OH- to get easier reactions.
I attached my file to this post.
Hopefully you can find my mistake in the model.
Thank you for your answer in advance,
Patricia
I have to tell you that I have changed reaction Nr. 3 to
3.) S^-2 + H2O --> SH- + OH- to get easier reactions.
I attached my file to this post.
Hopefully you can find my mistake in the model.
Thank you for your answer in advance,
Patricia
Attachments:
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Posted:
9 years ago
Hi Patricia:
In the Fe Corrosion example, there were a total of 5 reactions, 4 equilibrium reaction and 1 electro-chemical reaction. You said your electro-chemical reaction produces OH-, so is reaction 3 an electro-chemical reaction, or is there a 4th reaction which is your electro-chemical reaction?
In your porous electrode reaction node you have S2- -> 2e- + S as your electro-chemical reaction. Is that the right one? If it is correct, there is a sign error in there. According to COMSOL sign convection, you have to use a +sign for the S2-
I think the polarization numbers you are using may not be realistic for your case. It would be useful to have some experimental data to compare your simulation to and to use a input data for your model.
Regards,
Sri.
In the Fe Corrosion example, there were a total of 5 reactions, 4 equilibrium reaction and 1 electro-chemical reaction. You said your electro-chemical reaction produces OH-, so is reaction 3 an electro-chemical reaction, or is there a 4th reaction which is your electro-chemical reaction?
In your porous electrode reaction node you have S2- -> 2e- + S as your electro-chemical reaction. Is that the right one? If it is correct, there is a sign error in there. According to COMSOL sign convection, you have to use a +sign for the S2-
I think the polarization numbers you are using may not be realistic for your case. It would be useful to have some experimental data to compare your simulation to and to use a input data for your model.
Regards,
Sri.
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Posted:
9 years ago
Hi Sri,
thank you for your quick answer.
Your assumption is right, the 3rd reaction is my electrochemical reaction.
But the real electrode reaction is the first one I posted.
I thought I need an equilibrium constant also for this reaction.
So the true electrode reaction is this:
3.) Ag2S + H2O --> 2Ag + SH- + OH-
And I have still 3 reactions, two equilibrium and one electrochemical (Nr. 3)
What would be right sign for Ag2+ for the ríght sign convection?
Okay I will try to figure out the right polarization numbers.
As next step, I will load some current profile of my reaction into Comsol.
Regards,
Patricia
thank you for your quick answer.
Your assumption is right, the 3rd reaction is my electrochemical reaction.
But the real electrode reaction is the first one I posted.
I thought I need an equilibrium constant also for this reaction.
So the true electrode reaction is this:
3.) Ag2S + H2O --> 2Ag + SH- + OH-
And I have still 3 reactions, two equilibrium and one electrochemical (Nr. 3)
What would be right sign for Ag2+ for the ríght sign convection?
Okay I will try to figure out the right polarization numbers.
As next step, I will load some current profile of my reaction into Comsol.
Regards,
Patricia
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Posted:
9 years ago
Hi Sri,
sorry I had to change my assumptions again, because we aren't sure about the real reaction yet.
I now consider about these 4 reactions (Nr. 1 electrochemical reaction):
1.) Ag2S + 2 e- → 2 Ag (reduction to silver)
2.) S2- + H+ → HS-
3.) B(OH)3 + H2O → B(OH)4- + H+
4.) H2O → H+ + OH-
I did not change the i_loc data for my system, I think the problem is due the equilibrium reactions and the charge numbers.
Regards,
Patricia
sorry I had to change my assumptions again, because we aren't sure about the real reaction yet.
I now consider about these 4 reactions (Nr. 1 electrochemical reaction):
1.) Ag2S + 2 e- → 2 Ag (reduction to silver)
2.) S2- + H+ → HS-
3.) B(OH)3 + H2O → B(OH)4- + H+
4.) H2O → H+ + OH-
I did not change the i_loc data for my system, I think the problem is due the equilibrium reactions and the charge numbers.
Regards,
Patricia
Attachments:
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Posted:
9 years ago
Hi Patricia:
While you are correct that the equations are important to get right, you also need realistic values of polarization. If the current vs. potential curve (i_Fe) you are using says that the current at a certain potential value is higher than what the electro-chemical reaction (Ag2 + 2e- ----> 2Ag) can provide given the concentration of Ag2S you are using, you will never get the model to converge. For example, your "concentration boundary condition" gives the Ag2+ concentration as 1mM. If you convert all the 1mM into current, will you be able to satisfy the current required by i_Fe vs. V_polarization?
Are you sure about the concentrations of the ions you are using for your boundary conditions? You have 2.5 M Na+, 0.001 M Ag, 3M B(OH)4, 0.0003 M B(OH)3 etc.. Since you are solving a stationary problem and not a temporal problem, please make sure you concentration BC represents your source of ions. For a stationary problem, the initial condition almost has no meaning, especially after the 2nd iteration. For example, the file you sent did not have any Ag sources in your boundary.
The concentration of Ag2+ and S2- are different at time zero. Is that correct? For charge neutrality, should not the concentration of Ag2+ and S2- be the same? I was able to get your model to converge when I change the surface area of reaction from 2/w to a smaller number (1 m^-1). This means that you should use the low value of surface area and slowly ramp it up to the 2/w value.
Sri
While you are correct that the equations are important to get right, you also need realistic values of polarization. If the current vs. potential curve (i_Fe) you are using says that the current at a certain potential value is higher than what the electro-chemical reaction (Ag2 + 2e- ----> 2Ag) can provide given the concentration of Ag2S you are using, you will never get the model to converge. For example, your "concentration boundary condition" gives the Ag2+ concentration as 1mM. If you convert all the 1mM into current, will you be able to satisfy the current required by i_Fe vs. V_polarization?
Are you sure about the concentrations of the ions you are using for your boundary conditions? You have 2.5 M Na+, 0.001 M Ag, 3M B(OH)4, 0.0003 M B(OH)3 etc.. Since you are solving a stationary problem and not a temporal problem, please make sure you concentration BC represents your source of ions. For a stationary problem, the initial condition almost has no meaning, especially after the 2nd iteration. For example, the file you sent did not have any Ag sources in your boundary.
The concentration of Ag2+ and S2- are different at time zero. Is that correct? For charge neutrality, should not the concentration of Ag2+ and S2- be the same? I was able to get your model to converge when I change the surface area of reaction from 2/w to a smaller number (1 m^-1). This means that you should use the low value of surface area and slowly ramp it up to the 2/w value.
Sri